*OT* Variable Pitch Propellers *OT*

[PRB]

This feature of modern airplanes has always been interesting to me. I'm reading a book now called “The Flight of the Mew Gull”, by Alex Henshaw. He flew this airplane in a record setting speed run from England to Capetown in 1939, and flew in the British air racing circuit before that. Not only is this book a great adventure story, it's also a fascinating peek into the engineering of fast airplanes in the 1930s.

One of the interesting things about a fixed pitch prop is that as you go above its optimum airspeed, the air loads on the prop actually slow down the engine! This is amazing. It's why the need for a VP prop was realized in the first place. And it makes sense. If you imagine a fixed pitch prop rotating at some RPM, there is a certain airspeed at which the air mass will pass through the plane of the rotating prop such that the “AOA” of the wind across the propeller blades is 0, meaning the prop is slicing through the air with minimum resistance. I would think that the optimum airspeed for a given pitch/RPM combination is slightly less than this “zero-AOA” speed. At speeds above and below this optimum, the air loads on the prop evidently “bog down” the engine and prevent you from maintaining maximum engine revolutions.

During the air racing scene in the 1930s, a lot of time and effort was spent adjusting the prop's pitch angle on the ground in order to achieve fast speeds, then testing it in flight. Once adjusted, however, that's where it stayed for the entire flight. Then some engineer put an air bladder in the propeller hub, and when the air loads built up on the prop, pressure was exerted on this bladder, which bled air out, and moved a prop pitch adjusting mechanism. But there was no pitch setting for the pilot to muck about with, and it presumably only worked once, for once the air is gone, there was no way to put the prop back into course pitch. Then they came up with electrically driven pitch controls, with only two positions “take off” (coarse) and “fast” (fine).

The “constant speed” propeller system came about in the late 1930s (I think?) Now we're talking about a system that automatically adjusts the prop's pitch to maintain the RPM dialed in by the pilot. Space age stuff, right there.

Isn't this fun? :jump:

[SkippyBing]

I would think that the optimum airspeed for a given pitch/RPM combination is slightly less than this “zero-AOA” speed.

But at zero-AoA you're not going to be generating much thrust either. It's more likely to be where the prop is experiencing a relative airflow of around 15 degrees. Note the reason prop blades have twist on them is to maintain a fairly consistent relative airflow across the whole blade, allowing for the increase in relative speed from the hub to the tip.

Apparently the RAF being run to a budget in the 1930s only had two position props, i.e. you could change it in flight from a take-off setting to a cruise setting. Fortunately around 1939/40 some bright spark pointed out if you drilled a couple of holes in the hydraulic manifold you could make it work over the whole range between those two points!

[TeaSea]

More to it of course....speed is one thing, efficiency is another.

To extend range, a variable pitch prop is an absolute necessity. Think of it as a transmission in a car. You need lots of power initially, lots of grab at the air. Once in cruise, you don't need near as much, and as pointed out, the angle of the prop becomes a hindrance.

This is a factor in Helicopters also, since one can only adjust the rotors so far...which physically limits you to a certain degree of speed.

The Eurocopter x3 and the Osprey are attempts to get beyond that limitation.

Space age stuff, absolutely!

[lucas81]

I think that the experiments with the variable pitch started from the early 30's. A good example can be Polish competition plane RWD-9 which had 2-position prop pitch set on ground in the German VDM propeller. One setting served for the competitions which demanded very high acceleration and very short take off, the second one was for the competitions where the high top speed was very useful.
Another one, the PZL.37 "Los" bomber had variable pitch set manually by the pilot. The angle ranged from 23 to 30 degrees if I am not wrong. So, it still was not contant speed prop and could not be feathered. The early Messerschmitt Bf-109's had manual prop pitch as well, which, I suppose, required some attention from the pilot to avoid overreving the engine on too high RPM vs prop blade angle. In Spring 1940 the automatic prop pitch was introduced to the E-1's and E-3's.
On the other hand, the Hamilton-Standard company introduced the constant speed props from the late 30's and these became a standard.

Lucas

[fliger747]

With the Jelly poppers the fact that one of the airfoils (the retreating blade) eventually will be standing still to the airflow and cause instability, loss of lift and so forth limits speed severly.

For an aircraft even with a constant speed prop, at some point in say a steep dive, the airloading will drive the engine and shaft speed will actually increase!

Cheers: T

[allcott]

I think that the experiments with the variable pitch started from the early 30's. A good example can be Polish competition plane RWD-9 which had 2-position prop pitch set on ground in the German VDM propeller. One setting served for the competitions which demanded very high acceleration and very short take off, the second one was for the competitions where the high top speed was very useful.
Another one, the PZL.37 "Los" bomber had variable pitch set manually by the pilot. The angle ranged from 23 to 30 degrees if I am not wrong. So, it still was not contant speed prop and could not be feathered. The early Messerschmitt Bf-109's had manual prop pitch as well, which, I suppose, required some attention from the pilot to avoid overreving the engine on too high RPM vs prop blade angle. In Spring 1940 the automatic prop pitch was introduced to the E-1's and E-3's.
On the other hand, the Hamilton-Standard company introduced the constant speed props from the late 30's and these became a standard.

Lucas

I think variable pitch propellers started much earlier than that - around 1910 on airships. I think you're thinking about constant speed propellers, which are similar, but different.

[fliger747]

Some bush aircraft of the 20 and 30's had ground adjustable pitch props. The pilot/mechanic got out his wrench and moved the fixed pitch to a new value, depending for instance how much low speed thrust was needed for that short strip....

T

[allcott]

Ground adjustable have been around as long - longer actually - than heavier-than-air aircraft. That's why they were made of wood - they could be `adjusted` by filing, sanding and shaving, or cutting a new one. The science of propellers was years ahead of the aircraft they hung them on.

The definition of variable pitch usually means in-air-adjustable by the pilot. It wasn't needed in early aircraft because the speed range was too low to need to worry. But as aircraft started to get faster the need increased, driven mainly by air-racing between the wars.

[TeaSea]

Good observation.

[Roadburner440]

I would like to point out on rotorcraft that the aerodynamic limits of the blades are different as the rotor blade is used for different purposes than a propeller. I am not a fixed wing guy, so I cannot help with that. With aircraft propellers though as you generate forward airspeed they are not the load bearing structure of the aircraft (the wing is) so as you approach their stall point, or max efficiency it is not such a big deal. Most props spin at a couple thousand RPM, and speeds at the tips can sometimes reach the speed of sound. As far as a rotor goes the rotor disc itself is the primary load carrying structure of the aircraft, and also the blade chord profile is thicker than a regular prop. For this (following is for the H-60, so disregard for other communities) the 258RPM puts the rotor tips at about .7 Mach static. As you increase airspeed though at some points the rotor blade tips can reach to speed of sound as well just like the prop. Now the interesting part to that is because the rotor disc is what carries the aircraft you can create an imbalance at high speed with the retreating blade known as retreating blade stall. Some of the things they have put in the design of rotor blades is really interesting. There is a slight twist as you look spanwise down the blade, but hardly noticably unless you are right up on it. Along with sweeping the tip cap trailing edges 20 degrees to reduce sound/improve efficiency. The japanese have taken this one step further and put what I refer to as reverse winglets on the end of the blades to improve efficiency further. I included a link to a photo as I think those tip caps are pretty cool.

Other than that is about all I have to add. I find the whole prop discussion pretty interesting in itself. I am hoping to go work on some turboprops birds later in the year finally. It is always a double edge sword when you talk about adjustabilitiy in aircraft. Cause the added weight/complexity of the prop hubs and drivetrains add huge maintenance/reliability implications. The adjustable blade pitch does allow fixed RPM's, and a wider flight regime. As far as aerodynamics and origins behind it though I am clueless.

http://img2.blogs.yahoo.co.jp/ybi/1/3d/1e/gala8357/folder/961405/img_961405_46940845_1?1230870669

[fliger747]

Multiengine prop planes also have a great need for the ability to feather to meet engine out climb and controllability issues associated with engine failure.

With the advent of turboprops which use internal mass flow for cooling rather than relying on external air velocity, reverse became a more practical device, though some piston installations (such as DC6) were so equipped. Such props becme complicated in design and operation as several pitch locks and additional controls are necessary to provide sufficent safety and reliability.

The early RR Dart powered F-27 (and similar YS-11) had only a ground fine to use prop the flattened prop disk to provide aerodynamic braking. Later model aircraft I flew such as the DHC-7 and Lockheed L-382G (Herk) had truly reversable pitch props.

Dang, my Supercub on wheels, skis or floats has truly spectacular takeoff acceleration with it's large and finely pitched Borer prop, but the cruise performance is equally less exciting. Some 180 hp units do have constant speed units, at additional weight and perhaps still inferior prop efficency for takeoff. But the cruise is much improved.

Airplanes are tradeoffs, optimized for some specific mission. Less "good" away from that design parameter.

T

[warchild]

fliger?? Didnt the B-337 have some those incredibly complex props attached to the Waspmajors they used? and wasnt that one of the main problem area that existed with that plane??

[fliger747]

Brakes of the era were really not very good, and have lagged behind heavy and high performance aircraft till the advent of the current carbon brakes as used on the 747-400 and contemporaries. Hence the interest in reverse thrust as an aid in stopping. Yes the 337 did have rather complex props, but probably wasn't the major issue with the plane. The turbo compound engines reached about the limit of maintainable sophistication and were right at the reliability boundary.

Today, the somewhat less sophisticated "C" series R2800 powered aircraft (DC6, C46) are still flying whereas the R3350 and R4320 aircraft fell out of commercial favor more than half a century ago.

Off to fly the dreamlifter.... later: T

[SkippyBing]

The japanese have taken this one step further and put what I refer to as reverse winglets on the end of the blades to improve efficiency further. I included a link to a photo as I think those tip caps are pretty cool.

The link's not working, have you got another one!?

[PRB]

On a related note, I'm now reading Spitfire, a Test Pilot's Story, by Jeffrey Quill. Good book, BTW. In it he speaks of the Spitfire's first flight:

“...Given the serial number K5054, it was fitted with a special fine-pitch propeller to ensure a safe take-off run and to minimize swing due to propeller torque during take-off...”

This I don't get. How would the pitch setting of the propeller affect torque? Surely the only things affecting toque are rpm and the size of the prop, no?

[SkippyBing]

Torque is the turning force imparted by the propeller, if you think of it like a paddle wheel on an old river boat the coarser the pitch the more rotational force the prop will impart to the air stream. So using a fine pitch prop will reduce the problem, at the cost of limiting your top speed if you've only got one pitch setting! On variable pitch aircraft it's less of a problem although aircraft like the Sea Fury could flip you on your back if you suddenly applied full throttle at low speed even with the prop in it's fine position.

As an aside I know it was a problem on the later Seafires with the Griffen engines as one gear oleo would be noticeably more compressed than the other at take-off power, hence the move to a contra-prop on the last few models.

[PRB]

This is interesting. Torque is a rotational force. The power imparted by the engine against the crank shaft is torque. And Newton's third law says the airframe will experience a rotational force in the opposite direction.

A “fine pitch” prop, in the context of Quill's statement above, means the propeller blades are turned "sideways" to the wind, the “opposite” of a feathered prop, right? This would mean the engine would have to develop less power to turn the prop at a given RPM, which would equate to less torque. But it would also mean less thrust produced by the propeller, which does not equate to a “safer take-off”, which was the other reason Quill stated the fine pitch prop was used for the first flight.

I think my understanding of prop pitch may be backwards... If you can only attach a fixed pitch prop to your plane, and you desire a propeller better at take-off performance, at the expense of speed at altitude, then I thought you would want a prop angled such that more “bite” is taken from the air, i.e., one with blades angled at 45 degrees (more or less) right?

[fliger747]

Paul:

A propeller is an airfoil and like a wing has an optimum AOA for maximum lift over drag. Rather than thinking of pitch of a static propeller one must think of the propeller as a corkscrew. The angle of the relative wind is the important factor. As the aircraft speed increases, the angle of the relative wind to the prop at any given rotational speed changes. So for a fixed pitch prop there will be just one true airspeed that is optimal. At low airspeed the AOA will be too high and more drag than lift (thrust), at too high an airspeed the AOA will be quite low and little lift will be produced.

T

[SkippyBing]

But it would also mean less thrust produced by the propeller, which does not equate to a “safer take-off”, which was the other reason Quill stated the fine pitch prop was used for the first flight.


It depends on what you call a safe take-off! As long as it's producing enough thrust to get airborne, with a safety margin, that's probably safer than having too much thrust.
Additionally with the prop fined off the engine should accelerate and decelerate faster allowing easier control of aircraft speed. I seem to remember* when I've flown aircraft with constant speed props it was set in the fine position (max RPM) before entering the circuit for this reason as it makes the go round safer.

*I think it was last in 2002 unless you count helicopters.

[warchild]

Whats confusing me about this whole conversation is what i'm seeing in fsx as to opposed what i believe is seen in real life..
in FSX: with props set at 100% pitch, you obtain maximum rpm, and at 0% pitch you obtain your minimum rpm.
in a real plane ( correct me if i'm wrong ) when the prop is set at 100% pitch, you have your highest amount of pitch on the blades, which creates greater friction on the engine and reduces the rpm, and when you have o% pitch you have the minimum amunt of pitch on the blade and minimum amount of friction on the engine, increasing the rpm. ( the blade is also flatter in relation to the air flowing through it ). Yet, afsd reports that when you have the prop levers at 0% your pitch angle on the blade is at maximum pitch which slows the engine down and when your prop pitch is set to 100% the blades are at minimum pitch, and the engine speeds up. However the confusing part is that, with the prop pitch set to 100% ( minimum pitch ) your able to achieve your highest speeds while at 0% pitch you lose speed and slow down.. Thats kinda bassackwards to me..

[SkippyBing]

Pam, one thing I've found with FSX is that sometimes things are treated from a programers point of view, rather than an engineers so although a propellers pitch is referenced to it's plane of rotation in FS it's refence the direction of travel. It's equally valid, just not what you expect to see if you're used to working with aircraft or indeed boats.

Having said that, I think if you read the prop pitch as an angle FSX reports it in the correct sense. I seem to remember when I was working on the Wyvern setting 0 Degrees in that let the engine race up while you sat on the runway doing nothing!

As an aside years ago when I was covering propeller design on my degree course (boats not aircraft but it's the same theory) there was an article about some race aircraft with a fixed pitch prop which I believe was going for some sort of speed record. It had around 60" of pitch (60" of advance per revolution assuming no losses), it was effectively stalled until about 110 kts!

[warchild]

Pam, one thing I've found with FSX is that sometimes things are treated from a programers point of view, rather than an engineers so although a propellers pitch is referenced to it's plane of rotation in FS it's refence the direction of travel. It's equally valid, just not what you expect to see if you're used to working with aircraft or indeed boats.

Having said that, I think if you read the prop pitch as an angle FSX reports it in the correct sense. I seem to remember when I was working on the Wyvern setting 0 Degrees in that let the engine race up while you sat on the runway doing nothing!



ok.. I think i got this straight then. Since the propeller is moving forward "X" number of inches per revolution, we can calculate an angle at any given point on the prop at the beginning of one rotation to that same point on the prop at the end of that rotation. That becomes the plane of movement, and the angle of the pitch of the prop in relation to that plane on a constant speed prop is maintained at 0 degrees in relation to that plane, where it can do its most efficient work, and any reduction in pitch in relation to that plane reduces the props efficiency and ability to move the craft forward, but since it slows the craft down, the angle of the plane of motion changes and the prop therefore remains at zero degrees pitch in relation to that plane of motion, while maintaining a degree of pitch in relation to the shaft it is rotating upon. Does that sound right??

[PRB]

Ok, in the image provided, which prop is set at “fine” pitch? If these were fixed pitch props, which would I choose if I was more interested in take off performance at the expense of speed at altitude? And for the bonus, if propellers developed 100% of their thrust from the “Bernoulli” effect, would you feel any wind when standing behind the plane?

[warchild]

Ok, in the image provided, which prop is set at “fine” pitch? If these were fixed pitch props, which would I choose if I was more interested in take off performance at the expense of speed at altitude? And for the bonus, if propellers developed 100% of their thrust from the “Bernoulli” effect, would you feel any wind when standing behind the plane?

"B" is fine pitch. choose "A" for take off performance.. the rest i dont know except that you will always feel some amount of wind irregardless..

[Brett_Henderson]

Concerning constant-speed-prop (henceforth known as CSP) performance and function:

We first need to divorce ourselves of the idea that the pilot is controlling, or setting a blade-pitch. The pilot selects an RPM, and then the CSP itself will continuously adjust the blade-pitch in order to maintain that RPM (constant-speed).

For example: A takeoff-roll will begin with maximum manifold-pressure, and maximum RPM (NOT finest pitch). As the airplane rolls down the runway and gains airspeed, the CSP will begin "coarsening" the blade-pitch, in order to maintain the selected RPM. As the climb initiates, the blade-pitch will change accordingly, maintaining the selected RPM. As we level off and gain airspeed, the blade-pitch will "coarsen" even further; maintaining a constant RPM... eventually becoming a relatively "coarse pitch". And if you enter a descent, the blades will likely reach max "coarseness". The blade-pitch has gone from its finest, to its coarsest, without the pilot ever touching the prop control.

*********************************************
I'm going to paste part of a disscussion regarding CSP theory:
***********************************************


Let's try a big-picture view of what a constant-speed prop tries to do (on most light singles):

It's not an engine RPM governor directly.. it's a hydraulic/mechanical system using engine oil-pressure to modulate the prop blade pitch. As you advance the throttle, and the engine RPMs try to increase; oil-pressure increases proportionally. This pressure forces the blade-pitch into a higher angle-of-attack, which in turn keeps the RPMs from increasing. The increased power becomes increased thrust via the steeper blade AoA, while the RPMs remain constant.

Of course the inverses is true. As you retard the throttle, the engine RPMs try to decrease, but the proportionally reduced oil-pressure allows the blade-pitch to "relax", which keeps the RPMs from decreasing.

The whole cylce is one-sided, in that an engine under load is always trying to spin faster. A constant-speed prop's governor, is more like a differential, by-pass valve, and the prop-knob is used to select the percentage of oil-pressure available to the prop mechanism. A higher percentage of pressure yields a relatively coarser pitch, and the whole setup is calibrated for the engine/prop/airframe, and its normal operating range.

The spinning prop-blade angle-of-attack itself, is what physically holds the engine (always trying to spin faster) at a selected RPM. It's kinda like pitching for airspeed; where a wing's airpseed equates to a prop's RPM. The constant-speed prop is continuously "pitching" for a set RPM, like a pilot continuously pitches for a target airspeed. The prop doesn't govern engine RPM in a restrictive manner.. it takes the excess power and translates it into thrust via increased blade AoA, instead of increased RPM.

AoA is the key term here... not blade-pitch in degrees. That's where confusion sets in (like relating max-RPM to finest-pitch). The blade-pitch in degrees can go from max to min and back again, regardless of selected RPM, without the pilot ever touching the prop-knob.



Quote


The sensation & sound of pulling the blue knob out for lower rpms.......is like shifting to a higher gear in cruise for highway driving.




True, in that that is what you sense.. and that's what it feels like.. but that's not what happens. A constant-speed prop is always in the highest gear possible; given an airspeed and power-setting. That is what is "holding" it at a set RPM. Doesn't matter if it's a takeoff roll, or a climb, or a cruise, or a descent.. there is no "higher gear" to select.

In an automobile, the "pilot" controls engine RPM with the throttle. You add/subract "thrust" by increasing/decreasing engine RPMs, using a preset "gear". In an airplane, the pilot increases/decreases thrust via manifold-pressure, as RPMs remain constant. As an automobile's speed changes (or throttle position changes), the RPMs change too, regardless of the selected gear. As an airplane's speed changes (or throttle position changes), the RPMs remain constant.

Ponder an automobile at 55mph on a level road. Pretend it has an airplane-like throttle, set to a fixed position maintaining that 55mph. If you select a higher gear; the RPMs will indeed decrease, but the the automobile will accelerate to a higher speed, as the RPMs increase again. In order to maintain the same 55mph in the more efficient, higher gear, you must also decrease the power, (controlling speed/RPM by throttle setting).

On the other hand; when you reduce airplane RPMs for cruise, and do not change the throttle setting; the airplane will not accelerate, and will likely DEcellerate a tad, because the new "highest gear", is holding the engine at a less powerful RPM. This theory applies to the go-around scenario too.. where the prop-knob is not pushed full-forward. It's not that a finest-pitch can't be achieved because of the prop-knob setting; it's that the constantly-changing, highest-gear holds the engine to a less powerful RPM. The finest-pitch can still be theoretically achieved at lower RPMs.. ie.. a steep, slow, full-power climb... theoretical, because that gets into "over-powering" the prop (MP too high for a set RPM).


Quote


But, there are a few howevers, that are somewhat like a gear shift. For instance, if you shove the blue knob in, with power pulled back (no load on the engine)..........before it's time too, the engine and prop will react as if you downshifted a land vehicle while going too fast. You'll be thrown forward against the seat belt, and the engine will be spinning excessive rpms.




This scenario does fit in a way.. because it turns the tables on the prop. A constant-speed prop is designed to manage RPMs when the engine is the primarmy, prop spinning force. For a constant-speed prop, in a power-off descent; it's like trying to push something with a rope. But this is outside of its normal operating range.. kinda like either failure scenario (engine or prop itself). It depends on the specific setup, and their defaults. Singles, twins, and aerobatic setups all vary. Some twins are setup to default to coarsest (feathered), regardless of the failure.. some require the feathering to be done while the prop is still spinning.. some singles will default to finest pitch, regardless of the failures, and many singles don't even allow for feathering... All of this is outside the scope of this discussion, and beyond my expertise. I get involved in these discussions to help minimize the, pitch/rpm/gear, mis-conceptions.

[Brett_Henderson]

"B" is fine pitch. choose "A" for take off performance.. the rest i dont know except that you will always feel some amount of wind irregardless..

You'd want "B" for takeoff, and "A" for cruise.

The finer-pitch allows for higher, more powerful RPMs. The "bite" taken by a prop is a function of blade-pitch AND RPM...

Per my previous post.. fixed or constant-speed, the physical limiter on engine RPM, is the prop's AoA. So, if you have to rely on a fixed-pitch for takeoff, you want the engine at the more powerful RPM.

[PRB]

Good stuff, thanks Brett. Until now, I would have answered my question the way Pam did. I started to suspect I had that backwards...

[tracyq144]

Good discussion, I have never really been able to get my head around this topic. One of the reasons I fly the A2A J-3 so much in FSX, I suppose; just push the throttle forward and go!

[warchild]

Ok, soo, stupid question of the century.. If the prop pitch levers are selecting an rpm, then why are they named prop pitch levers?? Are they left overs from the days of variable pitched props??

[bstolle]

Normally they are called prop CONTROL levers and the labels besides the levers almost always state 'high RPM' and 'low RPM'.
The british loved to use the term fine and coarse pitch. E.g. the Vickers Viscount has the prop controls labelled that way. The Rolls Royce Dart engine also had 'fuel trim' levers....

[warchild]

On the other hand; when you reduce airplane RPMs for cruise, and do not change the throttle setting; the airplane will not accelerate,


This is not what we see in fsx, and is part of the reason behind my above query. I can change manifold vacuum via the throttle all day, and not see so much as a one mile per hour change in speed, however, if i change the rpms, using the prop pitch lever the change in speed is instantaneous.. Indeed, on takeoff, the manifold pressure ( vacuum ) is restricted until you begin yur take off roll by increasing the rpm's.

[Brett_Henderson]

Ok, soo, stupid question of the century.. If the prop pitch levers are selecting an rpm, then why are they named prop pitch levers?? Are they left overs from the days of variable pitched props??

For a CSP, it's just called, "Prop control" .. but yeah the old terminology sneaks in there.


There's a little, mixed terminology here, too. There are (for the sake of this discussion) three types of variable-pitch propellers.

1) Ground adjustable 2) Controlable pitch (in-flight) 3) Constant-speed

1 & 2 are where a the blade-pitch is mechaincally adjusted, or set. The pilot (or ground crew), literally set a blade-pitch in degrees (or percentage).

[Brett_Henderson]

This is not what we see in fsx, and is part of the reason behind my above query. I can change manifold vacuum via the throttle all day, and not see so much as a one mile per hour change in speed, however, if i change the rpms, using the prop pitch lever the change in speed is instantaneous.. Indeed, on takeoff, the manifold pressure ( vacuum ) is restricted until you begin yur take off roll by increasing the rpm's.


I'm not sure what to say..very strange.. FSX represents a CSP quite realistically.

Load a default Baron.. try takeoffs with full throttle and try it with 20-inches manifold-pressure..

Or.. level out and set the AP to hold altitude, and then experiment with changes in MP and RPM.

[warchild]

ok, on the takeoff roll in the baron, it seems that the rpm and manifold pressire have to be increased at the sme time and the rpm is controlled by the throttle. But in mid flight, i reashed 120 knots and changed the manifold pressure with the throttle, and maintained the same speed, whereas when i changed the rpm of the engine with the prop controls, the speed immediately slowed down.. in the P-61, this behaviour goes to an even greater extreme in tht you can taxi around on the ground, steering and accelerating with nothing but the prop controls.. You cant take off with minimum MP, but you can move around..

PS, i use a duel throttle body from saitek for throttle pitch and MP. perhaps that has a little to do with it..

PPS. Sorry i'm a pain, but it's imperative that i know this. I gota write the manual for that P-61 and i need it to be right spot on..

[Brett_Henderson]

Gosh.. no pain at all.. I love this stuff :jump:

Ok, I'm attaching two images.. both taken after establishing a level cruise..

The first shows a full-throttle cruise (26" is all you can get at 3000msl), with the prop-control set to 2700RPM (all the way forward)

The second shows the same airplane with only the throttle changed (MP reduced to 20")


Note that as expected, RPMs remain at 2700, but the airspeed is MUCH less.

Takeoffs are quite as cut-n-dry, because the airspeed starts out at zero.. but if you use a long runway.. and try retarding JUST the throttle well into the takeoff roll, you'll see that the RPMs remain ~2700, as MP goes down (and the aircraft accelerates more slowly).

Of all the things where MSFS falls short of realistic.. CSP function is not one of them.


I'm not sure what's going on in your setup.. maybe the control assignments are corrupt, or out of callibration. What you describe is too far from what it should be, for me to really nail it down.


EDIT: the images are in reverse order.. the one on the right is full-throttle (26")

[Brett_Henderson]

PS, i use a duel throttle body from saitek for throttle pitch and MP. perhaps that has a little to do with it..



MP and throttle are one in the same.. the other control would be prop RPMs

[SkippyBing]

Pam, I've just tried the Baron and it seems to work correctly for me, increase the throttle an MP goes up decrease it it goes down. Similarly advance the prop control and the RPM increases reduce it it goes down.
It's worth bearing in mind the CSP will only function properly over a given range i.e. where its max and min pitch will keep the RPM constant as you vary the MP, outside that reducing the throttle will reduce RPM and increasing it increase it.

[Brett_Henderson]

It's worth bearing in mind the CSP will only function properly over a given range i.e. where its max and min pitch will keep the RPM constant as you vary the MP, outside that reducing the throttle will reduce RPM and increasing it increase it.

Excellent point.. and is yet another way in which the MSFS CSP is very realistic.

[warchild]

Pam, I've just tried the Baron and it seems to work correctly for me, increase the throttle an MP goes up decrease it it goes down. Similarly advance the prop control and the RPM increases reduce it it goes down.
It's worth bearing in mind the CSP will only function properly over a given range i.e. where its max and min pitch will keep the RPM constant as you vary the MP, outside that reducing the throttle will reduce RPM and increasing it increase it.

now that makes perfect sense to me as in the P-61, i have to bring the mp up to 20 inches with the rpm at 1500 before i get any response from the csp ( rpm wise )

[Brett_Henderson]

Per my attached images, maybe this will help clear it up:

In both scenarios, the prop is spinnig at 2700RPM.

If you were to start at the lower MP and then go to the higher MP (advance the throttle).. the CSP will begin coarsening the blade-pitch, as needed, to "hold" the RPMs at 2700... and that translates into more thrust WITHOUT the RPMs changing.

You'd hear the engine get louder, because it's generating more HP.. but not hear it "speed up" (or see the tach increase), because that extra power is being comsumed by the higher prop-blade AoA, resulting in more thrust (higher airspeed).

[PRB]

Pam, something must be funky with your setup, like Brett said. What you're descibing should not be how these engines work. Mine works as expected - decrease MP, RPM stays the same, plane slows down, etc.

[fliger747]

Throttle means MP, torque applied to the shaft and thrust. You could leave the engine theoretically at max RPM the whole time and the system would work OK. The reason we don't do this IRL? High RPM causes high engine stress and wear and reduces the specific fuel consumption. Not so important in FS.....

Usual problem with FS modeled constant speed systems, insufficent range of prop blade angle change and the prop is pitch locking against a stop, at which point it becomes a fixed pitch. Another area? Minimum Goverened RPM set too low, or way to high. Another area with geared engines? Boloxing up the gear reduction ratio.

T

[warchild]

Pam, something must be funky with your setup, like Brett said. What you're descibing should not be how these engines work. Mine works as expected - decrease MP, RPM stays the same, plane slows down, etc.

Not sure what it could be. Its a standard setup.. reducing MP will leave the rpms at 2700 with 100% on the cps, and the plane will eventually ( over a few miles ) slow down frm top speed. but decreasing rpms while leaving the MP alone will slow it down a whole lot faster.. ninety percent of the time, on landing, i find myslf reducing the manifold pressure to below 30 inches, and using the cps to control the speed i'm flying at. I'm basically flying the props..
the throttles were set up several times, always the same way. plug them in and set the axis for each pair to thrust, pitch and mixture.. pretty standard..

[fliger747]

FS does constant speed props and engines quite well except for multi stage superchargers. The one thing it does not seem to model well is a MP phenomenon where MP will increase if the prop RPM is reduced first. Proper operation of prop/engine controls require the MP to be reduced first and then RPM to avoid things like blowing cylinders off the case.... For power increase, RPM up first the MP. Same reason.

Neither wings nor propellers create lift by upper section low pressure, high pressure on the bottom side is equally important, especially at higher AOA. Props create much of their thrust by creating a reaward mass flow of air, much like turbojets or turbofans, but tend to be more efficent at low speeds than the later two.

The prop tables as well as the calculations for engine HP using displacement, compression ratio and RPM do darn well when run through the various curves and tables in the .air file.

T

[fliger747]

The proper way to fly any large radial piston engine with a constant speed prop, is to use max RPM for takeoff and usually METO MP. As soon as practical reduce the MP to a climb value (say about 45") and the RPM to a climb value of (say 2550 RPM). Airspeed is varied to allow for proper cooling. Cruise, reduce MP slowly to cruise table value, say 34" or so and RPM as necessary for required speed or range. For descent, MP is usually reduced (slowly) to achieve the desired descent rate with RPM left at cruise value. On approach RPM is increased, sometimes to max, sometimes to the cruise value. MP adjusted as necessary to maintain approach speed. For Naval aircraft the Prop speed may be at the climb value as rapid throttle advancement for waveoff can cause an overspeed, which for the R2800 is quite hard on the engine.

For taxi prop levers are left full forward. Do not think of these as 100 % or 0%. Fiddling around with the Prop levers was a great way to ensure very short engine life.

T

[warchild]

Throttle means MP, torque applied to the shaft and thrust. You could leave the engine theoretically at max RPM the whole time and the system would work OK. The reason we don't do this IRL? High RPM causes high engine stress and wear and reduces the specific fuel consumption. Not so important in FS.....

Usual problem with FS modeled constant speed systems, insufficent range of prop blade angle change and the prop is pitch locking against a stop, at which point it becomes a fixed pitch. Another area? Minimum Goverened RPM set too low, or way to high. Another area with geared engines? Boloxing up the gear reduction ratio.

T


yeah, well, i still cant figure out why if i set the gear ratio to its true 2.2:1 ratio the plane wont fly under 500 mph.. Right now, with it set to 1.44:1 i'm getting the correct top speed, manifold pressure, temps and everything else, but as soon as i change it to 2.0 Mp shoots up to over 50 at idle, and all hell breaks loose.. However, that minimum governed rpm is certainly worth re-investigating..


Addendum. 16:9 as a gear ratio works ( 1.77:1). This matches specifications on th R 2800-AM11 with 2250 hp..

[Brett_Henderson]

When you get into big radials, with gear-reduction, and super/turbo charging; it gets complicated in a hurry... and the MSFS engine/prop modeling falls apart.

You have to start fudging things like, prop MOI, min/max pitch, engine power-scalar, and thrust-scalar.. lotsa air-file work, and ultimately, you'll even have to customize the gauge XML, to make it all realistic from the pilot's perspective.

The core theory of the CSP itself doesn't change.. you just gotta "force" the operational range to fit the airplane.

[Brett_Henderson]

The one thing it does not seem to model well is a MP phenomenon where MP will increase if the prop RPM is reduced first.

Yeah.. that's beyond the MSFS model.. it's easy to visualize though.

A normally-aspirated engine is always trying to "suck" the air into the intake manifold.. it's maximum possible MP, is whatever atmospheric pressure happens to be. At sea-level, on a "standard" day, a wide-open throttle will yield just under 29.92" of MP.

With a super/turbo charged engine, the intake manifold can be at well above atmospheric pressure. The engine "lets" air into it's combustion chamber, rather than having to "suck" it in. With a big ol' radial, with a turbine all spooled up; a reduction in RPM means that the pistons aren't "taking" the air out of the intake manifold as quickly, so the MP goes up for bit.

It would take a very complex, interactive XML gauge (hidden) to accomplish this.

[PRB]

Ok, I'm hijacking my own thread, sort of, which was OT to begin with... If you're supposed to increase RPM before increasing MP, what about during a landing approach, for example, when you're making small adjustments in power? Does every power adjustment require a corresponding RPM adjustment? Even little ones? If so, why do the manuals call for some landing RPM? That implies to me that you leave the RPM set there for landing. But if you have to muck with it every time you want to tweak your landing approach...

[warchild]

Ok, I'm hijacking my own thread, sort of, which was OT to begin with...

heyyy.. glad you did.. I was feeling guilty..

[warchild]

Ok, I'm hijacking my own thread, sort of, which was OT to begin with... If you're supposed to increase RPM before increasing MP, what about during a landing approach, for example, when you're making small adjustments in power? Does every power adjustment require a corresponding RPM adjustment? Even little ones? If so, why do the manuals call for some landing RPM? That implies to me that you leave the RPM set there for landing. But if you have to muck with it every time you want to tweak your landing approach...

OK, i dont know if it's correct or not, but what i do when i'm landing ( since i cant lower the flaps all the way till 170 mph ) is to chop the throttle to idle ( i can hear the engine revving down but the rpms remain the same ) and then adjust the cps till i have 1500 rpm on the prop.. then i can apply full throttle and bring the manifold back to 20 or 30 inches, and use the cps to adjust my approach speed... sounds complex but isnt really..

[PRB]

OK, i dont know if it's correct or not, but what i do when i'm landing ( since i cant lower the flaps all the way till 170 mph ) is to chop the throttle to idle ( i can hear the engine revving down but the rpms remain the same ) and then adjust the cps till i have 1500 rpm on the prop.. then i can apply full throttle and bring the manifold back to 20 or 30 inches, and use the cps to adjust my approach speed... sounds complex but isnt really..

Well, in the P-51, for example, I put the RPM at 2700 (or is it 3000?), like the book says, then yank on the throttle back and forth with reckless abandon to maintain speed and descent profile. In real life I would have blown up the poor Merlin, I suppose. The other cool thing the RPM control does is allow one to use the prop as an air brake, of sorts. The A2A P-51s do this nicely. If I'm coming in "too hot", advancing the RPM a tad will slow the ship down, due to the prop blades turning flatter into the wind.

[Brett_Henderson]

Ok, I'm hijacking my own thread, sort of, which was OT to begin with... If you're supposed to increase RPM before increasing MP, what about during a landing approach, for example, when you're making small adjustments in power? Does every power adjustment require a corresponding RPM adjustment? Even little ones? If so, why do the manuals call for some landing RPM? That implies to me that you leave the RPM set there for landing. But if you have to muck with it every time you want to tweak your landing approach...

The 'P' in G.U.M.P.S. reminds you to set the Prop for landing RPM. This is normally max RPM, and yes, you leave it there in case of a go-around...

The concern about the MP/RPM relationship, is that you want to keep the RPMs "above" the MP.

Obviously, this gets confusing with big, turbo-charged engines. It just happens to work out nicely for small GA. You don't want a six-foot diameter prop spinning much above 2700RPM, else the blade-tips go supersonic.. and for all intents and purposes, 27" of MP is gonna be near maximum achievable.

27" / 2700rpm ... 27/27 ... That's the "squared" deal.. keep prop RPM/100 near MP in inches.. most importantly, don't let MP go much above RPM.. why 25/25 is a typical climb setting... 24/24 is a typical cruise setting. Too much MP and the engine is trying to over-power the prop, very stressful on the internal engine parts... No worry about over-powering the prop when it's at Max RPM.. so you don't need to fiddle with it during an approach, no matter how much you change MP.

**********************************

@Pam... the MSFS sounds are unrealistic. In reality, you won't hear it "rev" down, because it's not reving down.. the RPMs are constant.. What you hear in the real world, is it gets quieter as power decreases.. but does not rev down... Engine RPM and prop RPM are the same (or the same ratio, with gear reduction).. You cannot change prop RPM, relaltive to engine RPM.. they're rigidly married.

As for your landing technique ? I won't pretend to know every airplane's specific procedures.. but I'm pretty sure that will get you into trouble, in any airplane. Not only do you risk over-powering the prop (as mentioned)... you need to be at max RPM in case there's a go-around.

Sounds like you might need to plan the approach a little further out. The biggest adjustment to my technique (real world), was when I transitioned form Skyhawks and Warriors; to Bonanzas and Mooneys. While learning high-performace/complex airplanes, you're taught to be at 20" MP, 20 miles out, so you can get into landing configuration and at pattern airspeed. My first attempt at landing a Mooney became a go-around BEFORE I was even on final approach... too hot, too high.. .:icon_eek:

[Bomber_12th]

When landing a high performance prop-driven aircraft, the common practice is to set the prop pitch the same as it is for climb - with the Mustang, it is 2700 RPM, for the Corsair, it is 2550 RPM, for the Allison P-40, it is 2600 RPM, etc. This provides a proper RPM to be at in case of go-around (you don't typically want full RPM, as it will be too much torque on go-around), and at the same time it does provide some good braking, as more prop is turned into the wind. During landing, there shouldn't be any 'jockeying' of the throttle - from my experience, just as in real life, coming off of an overhead break you should throttle back to a setting of about 28-29 inches MP in a Merlin P-51, or 25 inches in an Allison P-40. This starts bleeding the power off very nicely, as you put in the proper RPM setting, where at which time the gear can come down, and various flap settings (as indicated in the manual) are used to progressively slow the aircraft down through base and final (in case of the Mustang, you usually start by puting in 20 degrees of flaps, first, right off the break, and then put down the gear once gear speed is reached). Only smooth throttle changes should be used to make power corrections throughout landing, and properly done, the throttle should remain fairly much in one position, until over the threshold. Whether you're landing a P-51, Corsair, P-40, or anything similar, there is simply no other means to better land the aircraft, and 99% of the time, this is what is done. Otherwise, you have to start much further out, which is quite unsafe (given an engine failure), with flaps being put in earlier, to get the aircraft slowed down enough - and you don't really want to use flaps if you can help it.

Landing a high performance aircraft, an overhead break is always recommended - as is a close-in pattern at a proper altitude - very different to something like landing a Skyhawk or Warrior.

[warchild]

The proper way to fly any large radial piston engine with a constant speed prop, is to use max RPM for takeoff and usually METO MP. As soon as practical reduce the MP to a climb value (say about 45") and the RPM to a climb value of (say 2550 RPM). Airspeed is varied to allow for proper cooling. Cruise, reduce MP slowly to cruise table value, say 34" or so and RPM as necessary for required speed or range. For descent, MP is usually reduced (slowly) to achieve the desired descent rate with RPM left at cruise value. On approach RPM is increased, sometimes to max, sometimes to the cruise value. MP adjusted as necessary to maintain approach speed. For Naval aircraft the Prop speed may be at the climb value as rapid throttle advancement for waveoff can cause an overspeed, which for the R2800 is quite hard on the engine.

For taxi prop levers are left full forward. Do not think of these as 100 % or 0%. Fiddling around with the Prop levers was a great way to ensure very short engine life.

T

Awesome. Thanks Fliger.. I've never had anyone explain those things tp me.. much appreciated..

[warchild]

immmm, dumb question of the day #2: Whats an overhead break??

[warchild]

Sounds like you might need to plan the approach a little further out. The biggest adjustment to my technique (real world), was when I transitioned form Skyhawks and Warriors; to Bonanzas and Mooneys. While learning high-performace/complex airplanes, you're taught to be at 20" MP, 20 miles out, so you can get into landing configuration and at pattern airspeed. My first attempt at landing a Mooney became a go-around BEFORE I was even on final approach... too hot, too high.. .:icon_eek:

twenty miles?? Sheesh.. we were thinking that ten miles was a lot and to be honest, i'll usually drop flaps to 8 degrees a little abpve 250 knots just a couple miles out, then match the remainder of the flaps with the speed so that i dont have them fully down before 170 mph, then use a steep ( eight to twelve degree ) dive with full flaps onto the runway like one of the ways the manual suggests.. It also says you can glide it in, but I dont knowww.. thats a pretty hefty freight train to try and land without flaps..

the manual also doesnt say anything about rpms or mp while landing. it says to mainin 110 mph over the threshold.. i guess they werent counting on the plane being adapted to flight sim and having someone with very little real world experience flying it..

[PRB]

immmm, dumb question of the day #2: Whats an overhead break??

That's how military planes typically approach an airport to land. They come in low and fast right over the runway, then turn hard 180 degrees to enter the down wind leg, pulling lots of G and reducing power. All those Gs bleed off speed. The goal is to be at gear lowering speed as you come out of the 180 degree turn and stabilize on down wind. Then they finish off the landing in the "normal" way. If there are more than one plane in the formation, they each make the hard turn at some number of seconds of interval, in order to space them out enough...

[SkippyBing]

It also says you can glide it in, but I dont knowww.. thats a pretty hefty freight train to try and land without flaps..

Who says you can't use flaps gliding?

[Brett_Henderson]

twenty miles?? Sheesh.. we were thinking that ten miles was a lot and to be honest, i'll usually drop flaps to 8 degrees a little abpve 250 knots just a couple miles out, then match the remainder of the flaps with the speed so that i dont have them fully down before 170 mph, then use a steep ( eight to twelve degree ) dive with full flaps onto the runway like one of the ways the manual suggests.. It also says you can glide it in, but I dont knowww.. thats a pretty hefty freight train to try and land without flaps..

the manual also doesnt say anything about rpms or mp while landing. it says to mainin 110 mph over the threshold.. i guess they werent counting on the plane being adapted to flight sim and having someone with very little real world experience flying it..


Yeah.. twenty miles is over-kill... that's for initial training.. you tighten things up as you gain experience. One time I was flying over KCMH (Columbus International) waiting for permission/vectors for final.. and the tower guy said, "got a 757 on long final.. if you can turn for the numbers now, you're cleared to land" ... this was at 2500msl and 150knots. I pulled the throttle to idle.. pitched up to bleed off speed.. dumped flaps and gear and dove for the runway..

[warchild]

Who says you can't use flaps gliding?
Welll, in the specific case of the P-61 the operation manual warns against it.. The full wing flaps n that thing create so much drag that you have to use power to stay in the air.. The version we're building here isnt quite that bad, but it isnt far off..

[warchild]

That's how military planes typically approach an airport to land. They come in low and fast right over the runway, then turn hard 180 degrees to enter the down wind leg, pulling lots of G and reducing power. All those Gs bleed off speed. The goal is to be at gear lowering speed as you come out of the 180 degree turn and stabilize on down wind. Then they finish off the landing in the "normal" way. If there are more than one plane in the formation, they each make the hard turn at some number of seconds of interval, in order to space them out enough...

THAT would explain why a lot of P-61 pilots did chandelles as they turned to final.. So maybe i'm not so crazy as i thought i was and maybe this thing simply really does hate to slow down once its moving.. It's smaller than a B-25, weighs 7000 pounds more and has those two monster radials on it.

[fliger747]

Turbine aircraft are flown in a somewhat different manner as they are not prone to cylinder shock cooling from rapid power reduction.

An aircraft such as the P61 is actually pretty slick aerodyanmically. What will initially slow down the plane a lot is drag from the propellers at higher RPM's and low MP. In some of the versions that I have worked with it is not unusual to see -500 HP developed each side!

When dealing with prop pitch issues it is critical to use a utility that shows both actual prop pitch at any setting as well as efficency.

Cheers: T

[warchild]

yup.. i'm finding on the widow that coming back on both throttle and spc levers causes the most efficient slow down.. But you guys have helped a lot here...

[PRB]

Now to confirm the answer to the question “which fixed pitch prop would you choose for high speed flight?”, we find this. See the pic? That is the “Speed Spitfire”, or the “Schneiderised” Spitfire. In 1937 the Supermarine guys decided to modify a Spitfire for the purpose of breaking the world speed record. They never actually made the attempt, but the modified plane is shown here. Among the modifications to this plane were (from the book Spitfire, a Test Pilot's Story, by Jeffrey Quill):

“...The main changes apart from the engine were the fitting of shorter-span wings; a main coolant radiator of substantially less area to satisfy the increased cooling requirement; a long streamlined Perspex windscreen with no clear view front panel; a four-bladed fixed-pitch wooden propeller of greatly increased pitch angle; ...”

And if you look at the photo, the prop is almost feathered!

I don't know why I should be surprised at this, having just noted above that in the A2A P-51D, the air brake effect of max RPM is significant...

[Brett_Henderson]

A thing to ponder:

That is in effect, a cruise prop. If they did their math correctly; the point at which the engine cannot spin the prop faster, will be the RPM where the engine's HP peaks.

During the takeoff roll, and as it accelerates, the RPMs will be held well below where peak HP is generated. Now of course, with that much engine, it's moot... but per this discussion, it's worth noting that takeoff/climb performance will be greatly reduced.

This is where the CSP shines. It allows the engine to work in its peak HP RPM, regardless of MP, or airspeed... or keeps the RPMs in a less abusive RPM, for cruising... and so on..

Also.. it's worth mentioning that the prop-controlled, max-RPM for a "conventional" airplane engine is rarely where the engine's HP peaks. If you were to put the average Lycoming or Continental on a dyno; you'd find that the HP peaks well into 3000RPM range. There are two factors in play, when deciding what max-RPM a CSP is set to.

1) Prop blade-tip speed
2) Engine life

So, even though an engine could generate more HP; whatever the CSP max-RPM might be, it's considered the max HP RPM.

Back in the day, a common cheat for getting your High-Performance endorsemnet (airplane over 200HP), was to take something like a Piper Arrow, or Cessna Cardinal ( both rated at 200HP), and then adjust the CSP to allow for a slightly higher, max-RPM.. yielding 200+ HP :wiggle:

[fliger747]

Once upon a time (like 30 years ago) I flew a fleet of Cessna 206's in an Alaskan bush village. One of the planes was a much peppier performer than the others, and had a Robertson STOL kit as well. The Robertson kit was not so popular due to lateral control issues at slow speed, but it was a zippy plane.

In for one of it's 100 hr inspections metal was found in the oil screen. Further investigation found that the tach was off a couple of hundred RPM and she was turning almost 3000 RPM and making (for a while) much more than 300 hp, and the cruise powers were off as well.

Some large engines may actually develop max HP at slightly below the max RPM figure. A case in point is the difference between the R2800 B and C models. The C is really quite a different engine in many ways. One of the places they got a couple of hundred extra HP was a refinement of the oil scavengeing sysem. At High RPM the B was wasting a lot of power just splashing oil around the case. With the new system, a higher gear reduction ratio, new cylinders etc the engine was able to put out more power and operate at a higher effective RPM for Takeoff and WEP.

For the above fixed pitch spitfire the prop would have been designed for a quite narrow effective speed range, one at which the engineers would have hoped the airframe and powerplant could maintain. Takeoff and initial climb performace might have been pretty miserable. Sort of like having a race car with only a top gear.

T

[Brett_Henderson]

Some large engines may actually develop max HP at slightly below the max RPM figure

The question would be... Why would the prop be designed and set up to ever let the engine get above max HP RPM ?

With most GA, there's no guessing.. there is always more HP to be had.. but why would you ever want a big radial reving higher than where the most HP is generated ?


Now..on occasion I've heard that you'll stumble across a small airplane (mostly experimentals).. where the highest airspeed can be reached, at slightly less than max-RPM.. but this would have to do with prop/airframe dynamics.. or a prop not perfectly suited for the engine/airframe.. or even an engine so worn that the power-curve is distorted.

[warchild]

ya gotta realise that the Spit was a military plane, crewed by military people.. If the engine was destroyed, so what?? it would get replaced, ut more importantly, it went ut every day and was put to risk of being completely destroyed alone wih its pilot anyway, so why not go for it?? the CO wasnt usually a mechanic so he wasnt always that hard to convince about why something was a "good" idea.. the crew wanted to find out what would happen, and they found a way to do it. didnt matter what happened to the plane. it was expendable.. Most things in war are like that..

[SkippyBing]

Most things in war are like that..

Errr the Spit in question was flying in '37, two years before war in Europe broke out. They were just trying to break a speed record.
The pitch they've chosen isn't surprising, when the aircraft gets up to speed the prop will be taking the minimum number of turns per unit distance advanced. With a CSP it would look like that once you were going fast enough, it may help to think of it as a screw driving into a plank of wood, the greater the pitch the further into the piece of wood it'll go per turn. But to work well you have to get up to the right speed in the first place otherwise it'll be slipping and working at less than optimum efficiency.

[fliger747]

For a one off racer, easier to carve a fixed pitch prop rather than have Dowty Rotol cough up a special constant speed. Also lighter and even production Spits didn't have Constant Speed props, two pitch selectables if I remember correctly.

HP/RPM, the answer is in THRUST. These engines had geared props so tip speed was not generally an issue. With Augmentation such as water methanol injection, higher MP's could be utilized as well as the higher RPM. Use of a higher RPM to produce nearly the same HP reduced internal cylinder pressures, another important consideration. Also the engine torque curve more closely matched the desired climb settings, overall generally a more important operating area.

T

[SkippyBing]

production Spits didn't have Constant Speed props, two pitch selectables if I remember correctly.

I believe they gained constant speed units sometime in 1940/41 after the two pitch selectables proved less helpful vs ME-109s!