Boeing Stearman Model 75

[aleatorylamp]

Hello Ivan, hello Smilo,
Well, I didn´t think the flight model stuff was so uninteresting, otherwise I wouldn´t have spent so much ink on it in the posts... but of course it depends on a person´s taste for it.
At the beginning it seemed quite straight forward, but quite soon it got more and more complicated because of the strange way the information was presented by the different sources.

After all if Ivan´s explanations on propellers, which I didn´t fully understand at the beginning, at the end, and BECAUSE of the posts in this public forum, most issues were clarified. So, the forum fulfills its purpose!

Some things are not too clear yet, and will probably stay unclear, because of the fact that many sources followed the FAA´s way of bundling everything up into normalized post-war HP standards. This way, the very probable over 200 units of 280 Hp stock Stearmans with a wooden propeller have been buried and forgotten about in the mists of time.

Also, the R-680-5 is most widely quoted as having 215 Hp, but a document stating engine certificates, shows is as equivalent to a -B5 with 240 Hp. I find that quite disconcerting. Maybe the -5 and the -B5 weren´t the same engine and that could be the mistake: There could have been a -5 with 5.5:1 compression, and a -B5 with 6.5:1. The early PT-13 production run of 26 units without letter had the R-680-5 series engine, without letter. So probably the 225 Hp is correct there, and a stock Stearman never had the -B5 = -5 engine with 240 Hp.

But who knows? ...and who cares? Even if I think it´s annoying that it´s so hard to find out for sure what´s correct, I know that complaining about it in a public forum is no use, but we are human, and humans err.

Anyway, we are trying to have fun, aren´t we all?
Cheers,
Aleatorylamp

[smilo]

well done, Stephan, thank you.
the package has been received
and a reply sent.

apparently, the Boeing Stearman Model 75
is still a valid trainer, even as a sim model.

as for the engine performance work;
even though it's not my cup of tea,
i've stated on more than one occasion,
how much i admire the attention to detail
and all the work being done.
it would seem, i need to state it again.

believe it or not,
i have read all the posts in this thread
and i did not find them uninteresting.
what i find annoying is the assumption
that i did.

post #118 was meant as a playful jab
at Stephan and Stephan alone.
an inside joke between him and myself.
it is all to clear, that the point was missed.
no thank you, Kwong, i will not be drawn
into a pointless quibble contest.
i have much more important things
in my life to worry about.
that's all i have to say about it.
other than, if you feel the need
to get in the last word...feel free

[Ivan]

Hello Smilo,

I guess I am the local curmudgeon for now.
There was a lot of frustration to get to this point, but now I am fairly convinced that Aleatorylamp knows enough (or at least as much as I can help with) on this flight model.

With the last screenshot of engine models, I am pretty sure he found the same manual that I did though hopefully he saved a copy. I did not because it was a bit tedious to do so.
With the comment about -B5 engine versus -5 engine, I can see he has gotten to the same point with trying to match things up between military and company designations as I had though he has an incentive to find the real answer and I did not.

At the moment I am trying to understand some of the issues in working on yet another flight model for an older project and trying to do some tuning on two others to get them closer to release, so there is plenty to do which isn't really much fun at times.
Some of these technical papers I am reading using math that I haven't touched since High School and I am getting lost though I am getting a lot further than I did the last time I tried a couple years ago.
My Daughter has the math background but is off in College, so she can't help and would be too busy to help in any case.
My Son hasn't quite gotten to the classes he needs to understand the math yet though he might by next year.

My apologies Smilo. No wish to draw you into any kind of argument.

- Ivan.

[aleatorylamp]

Hello Ivan,
Yes, it took me many, many days to find the manual, and I could only make screenshots of the most interesting pages as it wouldn´t let me download it. I also found a couple of other documents, that seem to both confirm and refute some of the information!

Maybe they did this on purpose, just so some enemy would not get hold of any more exact info on this trainer, because it was so successfull that more than half of the 9000+ units built are still flying today!!

Thanks to your efforts in getting me to understand the mysteries of the propeller tables, I have been enjoying myself tremendously, trying out all sorts of different engines with the new fixed propeller, all with the same Boeing Stearman airframe.

So, with the information at hand, I was wondering what you would think about including the 5% extra power that the fixed prop was allowed to produce at full throttle. Thus, 100% throttle would really give 105% Power.

This would imply that the simmer would have to reduce to about 96% throttle for normal Vmax, and to 92% throttle for Vno. Not having a supercharger, the extra 5% can´t be done as WEP, in any of the 3 available types.

Normal Top Speed would be balanced for 124 and 125 mph for the 220 and 225 Hp engines.
Then, the short-spell absolute maximum for the 220 Hp engine would be 232 Hp, and the 225 Hp one would do 237 Hp. Speeds would be whatever the simulator should come up with at these powers.

Then, cruising speeds could be 95-103 mph and 100-108 mph for the two engines.
Would you think perhaps that possible Vno speeds (maximum continuous) could possibly be 119 and 121 mph?

Good luck with the maths on your two or three pending projects!

P.S.:
Incidentally, experimenting with my "pet 280Hp stock Stearman", I got the following results, which may explain the strange 135 mph top speed quoted by some sources:
100%: 280 Hp, 135.1 mph, 2283 RPM Vmax. (close to 2300 RPM)
_95%: 260 Hp, 131.7 mph, 2227 RPM Vno. (close to 2200 RPM)
Curiously enough, giving an extra 5% to this fixed prop engine, we get the 300 Hp the engine has when equipped with a CV propeller.
Another curious thing is that after the prototype stage, the Stearman-75 engine mount was strengthened to cater for engines of at least 300 Hp. Whyever would they have done this?
As it turns out, the engine-mount can cope with upto 600 horses!

Cheers,
Aleatorylamp

[aleatorylamp]

Hello Ivan,
Testing for WEP supercharger effects can be very tedious because it seems to take considerable time before it can be used after start-up. Supposedly it is because the engine has to warm up before it can be used in the first place.

To try out the 105% power option allowed by the engine factory-certification for fixed-pitch prop equipped R-680 radial engines, I experimented with a weak supercharger effect, that would hopefully ruin the engine if this short-time extra power is abused (even though this engine did not come with a supercharger).

So, regulating maximum engine performance to 225 Hp, and activating the supercharger option, with WEP Type as Supercharger, and entering 0.55 in the Boost Gain parameter, I got the 237 hp out of the 225 Hp engine/propeller setup.

I wonder if this would be good to use for simmers?

We had something similar for the Baltimore engines, but those really did have a supercharger.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

What does the Operating Manual say about this?
You did see the performance graphs in the manual of course?
What EXACTLY are you trying to make happen in this case?
How is the 105% power gotten? What prevents its constant use? (I mean in the real aeroplane.)
I am asking about both the Actual Engine and your Virtual Engine.

Are you actually testing the power settings with a Fixed Pitch Propeller?
Use a Constant Speed Propeller for Engine Tuning!

I decided to get screenshots of all the pages of the Lycoming Operating Manual last night.
That was a little tedious but not too difficult.
Next, I just need to process them all which should be really easy except that some pages are skewed in the original.

Last night, Anna Honey got home from her business trip, so the house is not nearly as quiet today.

I have been working on testing the BV 141B and finished doing the first round of testing yesterday afternoon.
I say first round because I figured with all the work on Propellers I have been doing for your projects, I should at least do the same for what I am about to release and the idea is to alter the format of the Propeller Tables without altering the performance.
In doing this, I found a few interesting "Gotcha's" that I probably did not know about when the first version of the AIR file was created, so those need corrected first.

- Ivan.

[aleatorylamp]

Hello Ivan,
The answer to the questions are really quite simple. I actually thought that it was clear from what I´d already posted, but OK: The Stearman engines in question never had a supercharger, so the way the extra 5% was applied, was with the throttle lever´s final stretch of movement, the objective being, to get a little extra power for take-off and climb. If abused for too long it would ruin the engine, I suppose.

What would I want from it in the virtual model? Well, the 11 Hp difference gives a bit more nerve for climbing and 3.5 mph more level flight speed. Who wouldn´t want a little extra speed, even if only 3.5 mph? I can of course forget about it if it is just a nuisance and too small to be worth while.

I just thought the fact that it existed in reality was quite curious.

I had included an information paragraph from the manual, on this extra 5% engine speed, at the bottom of the engine/power table attached to post #117.

Perhaps the scanned small text is difficult to read, so here it is again:

"During normal operation of engines with a fixed pitch propeller, the rated engine speed should not be exceeded at any time. However, the Civil Aeronautics Authority permits a blade angle setting which would allow the engine to turn 105% of its rated speed IF the plane were flown at full throttle in level flight at sea level. This additional 5% above rated engine speed is permitted only with a fixed pitch propeller for the purpose of providing additional power during take-off and climb."

I have now attached a screenshot of the corresponding 225 rated Hp engine performance curve for fixed pitch propellers, which shows the necessary information graphically, and I don´t have any further data on the subject.

I am at present using the single 16-degree graph propeller tables for a fixed pitch propeller, and am adjusting aircraft performance with it. The results are coming out very well. A CV propeller would have quite a different performance and throw everything out of order.

Perhaps the extra 5% engine power is negligible and can be ignored.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

I saw this page of graphs but did not look at it in detail because it was so small.
I have not been reading enough about this power combination to give any useful advice here.
I can think of a couple different approaches, but I don't want to send you off on a worthless pursuit.
A real solution will need a lot more thought and some experimentation (really to see what WON'T work).

- Ivan.

[aleatorylamp]

Hello Ivan,
I really don´t exactly understand what you mean, or the reasons behind your questions, I´m afraid.
It is only a question of deciding how to implement it and whether to do so at all. No complications of any kind!

The extra power is obtained by an extra RPM increase... how else? Both are reflected on the graph.

With the 16-degree (well, 15.95) propeller curves in the 2 propeller tables, the combination prop/motor with the engine torque and friction settings and aircraft Drag settings, are giving the 2 desired top speeds, at 2 power settings. The latter coincide with the graph, both the normal max. Hp and also the extra 5% Hp.

The 220 Hp Continental R-670-4 gives either 220 or 231 Hp and S.L. level speeds are either 124 or 126 mph
The 225 Hp Lycoming R-680-? gives either 225 or 237 Hp and S.L. speeds are 125 or 128.5 mph

The most logical thing to do would be to do it like on the real plane:
Full maximum throttle for take-off and at a given moment for power-climb, and if the pilot forgets to reduce it, he will ruin the engine. Great stuff! End of story!

Or, just put a stopper on the throttle lever and forget about the extra 5 percent, which must have been what the flying schools did, I suppose.

IF we want to use the extra 5%, we could be benevolent and include it in the normal throttle-lever-travel and the simmer wouldn´t be penalized by using it too much.
OR we could put it in as WEP and ruin his/her engine! I really see no complication other than having to decide how or whether to do it.

P.S. The extra momentary available 5% power is only mentioned on engine documents, and nowhere for the Stearman. Being a trainer, I am led to believe for engine protection reasons, it was not generally available in flying schools, except perhaps for specific aircraft where someone would have perhaps done something to an engine to access this little extra performance.
So, if everyone agrees and nobody is against it, mainly for the sake of simplicity, I will refrain from putting it into the Stearman 75 airfiles.

Cheers,
Aleatorylamp

[Ivan]

It is only a question of deciding how to implement it and whether to do so at all. No complications of any kind!

Hello Aleatorylamp,

If that is the case, then it is really just a design decision and you get to make the call, not me.

I just believe that no matter how you do this within the limitations of CFS, there are going to be side effects and you need to decide which side effects are most consistent with the choices an actual pilot would have to make.
With my own projects, I have made those decisions and designed accordingly.
With your project, it is just a little different because I don't know much about the operating limits of the aeroplane and it also happens to be a fixed pitch propeller which has other interesting effects.

One part of the CAA (predecessor to the FAA) note about 105% power has me a little confused because I do not find it to be consistent, but perhaps it really is. I really can't tell without trying to test out a prototype flight model and experimenting with a few parameters.

Now the following is just a bit of undirected rambling but perhaps these comments and questions will give you some idea of where I am going with this:
1. As the Pilot, do you use Full / Wide Open Throttle on the Take-Off Run?
2. This is a Normally Aspirated Engine. You simply CANNOT run too much manifold pressure.
3. What is dangerous and destructive to the engine? Excessive RPM: Sustained operation past 2100 RPM.
4. Is the AIR File RPM Limit really meaningful for a Fixed Pitch Propeller?
5. Can we really break the engine in the simulator? Probably not except for the 5:10 minute use of Supercharger.
6. The CAA authorizes this "105%" as allowed for Take-Off and Climb. What does this mean for a typical time limit?
7. What should the Engine Power versus RPM curve look like?

Hope this gives you some ideas.

- Ivan.

[aleatorylamp]

Hello Ivan,
The points you are making are very interesting, and have made me think a little further.

Looking into the Flight manual of the Navy Stearmans, there seems to be quite a difference in top performance capacity between the Continental and the Lycoming equipped types, as can be seen on the Power/RPM graphs at the end of the manual.

The Lycoming R-680 was later developed into a machine capable of 300 Hp at 2300 RPM, with mechanical improvements on cams, and with the addition of a supercharger, amongst other refinements.

The RPM and Hp graphs shown at the end of the Manual show readings going up to 2300 RPM and above, although some of it is quite difficult to read and to interpret (for me).

Consequently, it is quite debatable whether prolongued operation at 105% of the early versions would have ruined the engine! ...Most probably not.

100% was 2075 RPM on the first "letterless" R-680 Series, and although surpassing the 105% maximum of 2100 RPM was strongly recommended against, mechanically, 2100 RPM for any length of time would probably not have done any harm at all, even for more than 5 minutes!

The comments by the CAA as to its use seem only to have been cautionary measures. At the time when the engine was new, they didn´t know that it could be run faster and with more power.

I have read comments about pilots allegedly having dived at 300 mph without causing structural damage to either engine or airframe...

Consequently, and as there was no supercharger on the engine, it could be a good idea of simply incorporating the extra 5% - i.e. 237 Hp and 2100 RPM - into the normal maximum throttle lever travel, to be used for Take-off and Climb, as would be normal on any other aircraft. Nobody uses full throttle all the time anyway!

I know it´s my call to put in whatever I want into the .air file... but I´m trying to be realistic.
On one hand, just to be safe, I could limit performance to the CAA recommendations, for a flying school, as it were, or I could be more realistic and adventurous, and incorporate the 105% into the normal use of the engine. The more I think about it now, the more the latter seems the best idea.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

First of all, WHICH Engine are you trying to simulate?
It is hard to keep things straight when you keep changing the subject within a single post.
If you want some input from me, you have to keep in mind that I have not done as much reading on the Stearman or the Lycoming R-680 as you have and probably never will. *I* am not building such a project.

Now with that in mind, the Graph you posted from the Operating Guide was for a R-680-B4 as can be seen on the Label at top center.

The engine you are describing would be making 237 HP at 2100 RPM.
The R-680-B4 according to the graph:
at 100% power (2100 RPM) is making 225 HP
at 105% power (2200 RPM) is making 237 HP
That isn't far off but it isn't the same either.

I also believe you are rationalizing to arrive at the conclusion that there are no real operating limits to this engine and that the CAA simply didn't know.
Later engines had more power and capability but they were not simply running the same old engine at a higher RPM.

As for reasons why operating limits are established below where the engine may run without failure, there could be many reasons:
The heat rejection of the cooling system may not be sufficient to run a particular power setting continuously.
There may be a greater likelihood of actual failure during use.
There may be an additional service requirement after WEP use.
Emergency power may reduce the Time Between Overhauls below the manufacturer's guarantee.

- Ivan.

[aleatorylamp]

Hello, Ivan,
Yes, I understand. The problem is I can only find the graph for the -B4, an engine similar to the one I´m trying to install, which was the R-680-5 (no letter). For the R-680 "no letter" series there is only a graph for the basic one with 215 Hp available, which is not what I want.

I also understand that all the refinements for more powerful engines of the same type developed later, meant they could be run at higher speeds for more time than the earlier types.

Hence my doubts and suppositions of trying to deduce how the engine I want would have worked, but apparently it is a futile exercise, and because of the lack of specific information for the engine, it will probably be best to forget about it and just go for the Navy Version´s newer -B4.

Update: the normal-powered Stearman-75´s I´m interested in are all WWII ones. I would suppose that the -B4 would be OK for that time, as the Navy Stearman Manual was published in 1941. So instead of making an early Stearman 1937 or so, with a R-680-5 (no letter) engine, I could make a 1941 one with the R-680-B4.
No problem.

OK, then. Thanks for your comments!

P.S. I have the feeling that this is getting a bit too complicated, and is creating more bother than benefit. I´m sorry if this has got annoying for you, but that´s why I had suggested a short time ago to forget about it.

I think it is not worth while delving into it much further, for the marginal effect it is going to have on the virtual model´s behaviour anyway.

On the other hand, your comment that what I was trying to achieve could already be quite close, is actually really quite satisfying, given the difficulty in obtaining any better information on the early series of engine I´m trying to put in.

Thanks again!
Cheers,
Aleatorylamp

[Ivan]

The problem is I can only find the graph for the -B4, an engine similar to the one I´m trying to install, which was the R-680-5 (no letter). For the R-680 "no letter" series there is only a graph for the basic one with 215 Hp available, which is not what I want.

Hello Aleatorylamp,

I believe you are really not understanding the designation system here.
Have you found even ONE military designation that has a prefix letter in it?
As an illustration:
The military called the Continental radial engine the R-670.
The Continental company designation for the same engine was W-670.
The Wright R-1820 engine was used in the B-17, F4F, FM-2, Brewster Buffalo, Curtiss-Wright CW-21, etc.
If the military used it, it had a designation looking like R-1820-SomeNumber.
If it wasn't a military contract, it may have been a simple R-1820-Something OR perhaps a GR-1820-Something.
Company designations used a G prefix if it was a reduction geared engine and no prefix if it was direct drive.
The military never had a G prefix on their models.
There should be a company designation for every military version but it isn't likely to be the same.
Thus your R-680-5 corresponds to some Lycoming R-680 series of engine (I am guessing this is a B series engine).

I am not sure which one matches.
I already hinted at this situation back at Post #42 back on August 15 when I first found the manual you are looking at right now.

-------

It also sounds to me like you are giving up on trying to figure out how this engine should work.
Important thing here is NEVER GIVE UP!
Get it wrong perhaps. Put it on hold until you figure things out, but NEVER GIVE UP!

As I see it, (and I may be wrong because I haven't actually worked on it), the problem you have here is actually very simple to solve. I have been trying to hint at the process without just telling you outright because I thought it would be more meaningful if you figured it out on your own.

First of all, tune your Engine power curves using a Constant Speed Propeller. (Sound Familiar?)

Tuning the Engine:
Plot Horsepower versus RPM. (You already have such a graph in the manual, but you want to make your Engine match it.
Here are some numbers without checking the references, so you will need to check them yourself.
Continuous operation appears to be 1900 RPM giving 202 HP at FULL THROTTLE
Set your test Engine's down to 1900 RPM at FULL THROTTLE and see what the output is. Adjust if needed.
Normal Maximum appears to be 2100 RPM giving 225 HP at FULL THROTTLE
Set your test Engine's down to 2100 RPM at FULL THROTTLE and see what the output is. Adjust if needed.
Absolute Maximum appears to be 2200 RPM giving 237 HP at FULL THROTTLE
Set your test Engine's down to 2200 RPM at FULL THROTTLE and see what the output is. Adjust if needed.

I am thinking your question right now is:
Hey! Wait a minute, what SHOULD the RPM limit be???
Answer: It really doesn't matter because we are tuning the Engine's Torque and Friction curves and you will find that when you finally plug in the fixed pitch propeller, the RPM limit is pretty much meaningless anyway though I still think you should set it to something like 2200 RPM in the final version to make things look right.

Now if you want to control the power with an over speeding propeller, you MIGHT want to tune the Torque or Friction way high above 2200 RPM to discourage High RPM operation. Your call on this one, but I would do it.

Tuning the Propeller:
With your neat nifty Wooden Propeller in place. (Set to Fixed Pitch with proper Angle.)
Adjust your AIR file so that the Aeroplane simply will not move. (This is an Engine / Propeller Bench Test, so bolt it down!)
Put in some Concrete Spoilers, or whatever.
Run your Engine at FULL THROTTLE.
Note the RPM.
Compare the RPM to the specifications in the FAA Type Certification document.
(I TOLD YOU that document was useful!)
Adjust Propeller Power Coefficient in Table 512 until you get proper RPM.
Adjust the rest of the Power Coefficient curve (singular) in Table 512 until you get something looking like other Power Coefficient curves of which you have many examples.
Note that the important parts are basically the two ends at J=0 and where Efficiency drops to Zero.
The rest can be a straight line or slight curve: Your choice.

When you are finished, Remove the Concrete Spoilers and Unbolt the Aeroplane so that it can actually fly again.

Now here comes the really fun part you were agonizing over:
Keep in mind that I am a 1G pilot with no experience on a Stearman, so beware.
As the new owner of this Stearman, the Pilot should read the manual and limitations (Checklist).
It would say something like Blah Blah, Don't exceed normal maximum 2100 RPM for routine flight.
2200 RPM authorized for Take-Off and Climb only.
Vne 186 MPH..... Blah Blah.

Pilot straps in, says his prayers and starts and warms up the engine.
She taxis to the runway and then what?
Does she use FULL THROTTLE on the Take-Off run?
Yes, because this is a naturally aspirated engine and this is one BIG cotton ball with not a lot of engine power.
(Real aeroplane owners are probably hating me right now,)
Is there any danger of exceeding 2200 RPM?
Probably not because the aeroplane is pretty slow on Take-Off and there is a lot of drag on the propeller (Power Coefficient table).
You will have to confirm whether what I am telling you is true or not. I have NOT built this flight model yet.
Just do not exceed 2200 RPM, but by the time that even gets close, the aeroplane should be off the ground and climbing.
So do we reduce power from FULL THROTTLE in the climb?
Probably not because climb speed is so far below maximum that there should be enough propeller drag to keep RPM below 2200.

Besides, if you look at other manuals, the Climb power setting is typically authorized for 30 minutes or so.

Once settled in level flight after the climb is when FULL THROTTLE may not be appropriate.
Here it is a balance of speed and propeller resistance to see what the actual throttle setting should be to get down to 2100 RPM or 1900 RPM or whatever the Continuous Operation limits are.

You will need to experiment here. I can't predict how things will stack up.

Hope that makes sense.
I wish the Propeller issue I am having with my BV 141B were this easy to resolve.

- Ivan.

[aleatorylamp]

Hello Ivan,
I wasn´t really going to give up yet.
I just thought it was getting too complicated to continue discussing on the thread. Anyway, thank you for your indications. It will take some time to follow, but it already sounds familiar.

For the Continental R-670 series it is all much simpler, including the military designations. These and the types were by far not as complicated as for the Lycoming R-680 series engines.

According to Textron Lycoming, AVCO Corporation, the engines that were produced of the R-680 series were the following: (noteworthy is that the -4 and -5, and the -B5 and -B4 are not the same engine!)

R-680, R-680-2, R-680-4, R-680-5, R-680-6,
R-680-B2, R-680-B4 Series, R-680-B5, R-680-B6, R-680-BA,
R-680-D5, R-680-D6,
R-680-E1, R-680-E2, R-680-E3, R-680-E3A, R-680-E3B

Sticking to my earlier "no letter -5" for a moment, right now, I have the RPM difference for the 225-237 Hp difference much lower than 100 RPM, actually at best only 39 RPM (2163-2202 RPM). This difference would actually more or less fit what I had deduced for a "no letter" -4 or -5, i.e. 2075-2100 RPM.

I started out from the point that the basic R-680 (no number or letter) gave 215 Hp at 2000 RPM, and tested for a 10 rated Hp more powerful engine in the sim. I supposed that normal max. 225 Hp for the -5 engine would be correct at 2075 RPM, and that at 2100 RPM, the full max. 237 Hp would also be correct. Less sharp cam angles, giving lower RPM than the -B4, etc. Then, gas consumption flat out is at a very high 18 Gal/hour... perhaps OK for the older, more inefficient version, which is being squeezed out for more power.

Anyway, I expect that your detailed instruction for a more correct tuning procedure, will lead to the results that are more fitting to the graph of the newer -B4 series - giving a 100 RPM difference between the 225 and 237 Hp - as for this engine we DO have exact details - not so for the "no letter" -5 version.

Thanks again!
Cheers,
Aleatorylamp

[aleatorylamp]

Hello Ivan,
Sorry to be such a pain, but before embarking on your proposed more thorough CV-prop tuning-check (post #139), and to avoid the fears you expressed in Post #133, I just checked some things on the RPM difference between full max. and normal max. power, and also did some preliminary checking for some possible improvements to be obtained by increasing or decreasing the torque/friction balance (still using the fixed pitch propeller).

First: Is the 2200-2100 RPM difference really correct for the 237-225 Hp difference?
100 RPM for only 12 Hp... Hmmm... I´m only getting a difference of 39 RPM here.
I checked, and saw that the Navy Stearman Manual mentions 2100 RPM with 220 Hp.

Doing my testing again, I found I got 58 RPM difference between 220 and 237 Hp:
237 Hp - 2202 RPM (2 RPM slightly too high for Full max. power)
220 Hp - 2144 RPM (44 RPM somewhat too high for Normal max. power)

Then, as you had proposed Vno at with 1900 RPM and 202 Hp, I checked:
202 Hp - 2078 RPM (178 RPM - very high).

Now, to try and increase the RPM difference between the 220 and 237 Hp, I tried out a few things:

First, I tested shifting the ZERO POINT on the graph tables from J=0.86 to J=0.8 (and adjusting the J=0.6 and J=0.7 positions accordingly), but this only reduced the difference by 1 RPM.

Then, I raised the balanced torque/friction settings from .64/15.9 to 0.725/27, but this only reduced the difference by 3 RPM, compared to my initial 58 RPM.

Next, I lowered the balanced torque/friction settings to 0.58/8.15, and this increased the difference, but only by 1 RPM, compared to my initial 58 RPM.

I am assuming that my initial 0.64 torque setting, as well as the increase to 0.72 and the decrease to 0.58 would still lie within the criteria for a reasonable torque setting. Lowering torque further would perhaps not be a reasonable setting, I suppose.

So, my question is whether it will be worth while to conduct the more thorough CV-prop engine tuning test to adjust torque and friction before putting in the fixed pitch propeller, if changing the torque/friction settings will hardly make any difference.

Of course, because I didn´t change the Zero Lift Drag settings, all this preliminary testing may not be accurate, and may not reflect what would happen with the more thorough CV prop engine tuning test, so my line of thought could be totally wrong and useless.

So, the next thing I´ll do is conduct your proposed CV-propeller test!
Cheers,
Aleatorylamp

[aleatorylamp]

Hello Ivan,
Update: OK, I´ve tuned with the CV prop, (not on a bolted down aircraft), for the different settings: 237 Hp at 2200 RPM, 220 Hp at 2100 RPM and 202 Hp at 1900 RPM, all at full throttle, and I have noted down Thrust and Torque readings.
I needed a higher torque graph setting (.612) for full max. power, and the other two are at 0.628 and 0.610, with Friction at 14.7 for all three.

Then I put on the 16-degree fixed-pitch propeller, and on the bolted down aircraft, at full throttle the engine gives 1870 RPM and 232 Hp.
OK!! (FAA dictates: Not over 1950 RPM, not under 1725 RPM).

So, it seems like it´s going along the correct path, this more profound tuning procedure.

More later!
Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Go back and re-tune your Engine with the Constant Speed Propeller.
Your power curve is pretty messed up.

From what I am reading you are getting:
2200 RPM - 237 HP (Good!)
2100 RPM - 220 HP (OK, but just a touch low)
1900 RPM - 202 HP (Good!)
1870 RPM - 232 HP (What the Heck?!?!?)

Why does 30 RPM down give 30 MORE Horsepower?
This kind of abrupt jump is not good. Curves should be pretty smooth and look like the ones in the manual.
Something is wrong here.

Now assuming that your 232 HP is really 202 HP and that the 232 was just a typo, then I suggest you go back to the shop and get a new propeller.
The one you have is pretty worn out!

If someone out there knows better, please correct me on this issue, but as I see it:
The FAA specification allows for a max diameter 98 inch propeller but will allow for repairs taking it down to 96 inches or so.
The FAA requirement specifies the resistance of a propeller as being such to limit it to between 1725 RPM and 1950 RPM with the Engine at FULL THROTTLE and Zero forward motion.
What does this mean?
I see this as a Brand new Propeller will be limit things to 1725 RPM.
As the Propeller is used and nicks get sanded down, the blades get smaller and thinner and re finishing the tips all reduce the Power Coefficient.
When the Power Coefficient is such that the Propeller winds up to 1950 RPM at FULL THROTTLE with Zero forward motion, it is time to replace it.
Your Propeller is used and worn.
Get a New One!

- Ivan.

[aleatorylamp]

Hello Ivan,
I put in 220 Hp for 2100 RPM because that´s what the Navy Stearman Handbook states for the N2S-2, although granted, the FAA DOES state 225 Hp for 2100 RPM, which however leaves very little RPM difference for the simulator.
100 RPM difference of only 12 Hp? A difference of 17 Hp would sound more realistic, I´d wager.

QUOTE:
1870 RPM - 232 HP (What the Heck?!?!?)
UNQUOTE

This is what I got for Full Throttle with the static bolted down plane after I installed my fixed-pitch propeller.(Altough it WAS dragging around the concrete block at 6.9 mph!!)
Mistake: I had transferred the old Torque graph! No wonder...

Well, first I´ll try and make a new propeller to get RPM down to 1725 RPM first, and lower the Hp.
Thanks for explaining what this means (and everything else, of course).

Now I´ll test for the rest of the points on the curve between J=0 and J=0.8 (where the curve drops to ZERO). ...without the concrete block!

OK, then. Let´s see what ELSE happens...

Update: OK, I´ve got the new propeller on, and it´s dragging the concrete slab at 1725 RPM.
After several trials Hp is now at 198 Hp, and the Torque Graph setting is slightly lower than what it was with the CV propeller tests for max. full power. I suppose I´ve finally understood.

Update 2: It really came out as I´d feared in my earlier post #141. I got basically the same performance results as after I tweaked my own fixed-prop tests prior to this one, although slow speed performance is now correct. Below 100 mph I had it far too high before! However, the full max. is now a bit on the low side.
So, it´s much of a muchness... sixpence of one and a half a shilling of the other.

I had to put in a bit of a "step" in the graph in table 512, between J=0.6 and J=0.7. Speeds and RPM for 220 and 237 Hp are so close together, that both J values affect the result, and I tried lowering J=0.7 some more without getting too close to ZERO, and raising J=0.6 a bit, to separate the speeds a bit more. Oh well...

NEW S.L. Testing: Torque Graph: 0.625 / Friction: 14.7 (Zero Lift Drag was better left unchanged).
100%: 235 Hp, 127.5 mph, 2195 RPM - quite close.
_94.5%: 220 Hp, 124.35 mph, 2143 RPM - instead of 2100 RPM
_90%: 202 Hp, 120.7 mph, 2081 RPM - instead of 1900 RPM

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

Be careful about combining specifications from different model engines unless that is really your intent.

You sure seem to run a LOT of quick test cycles!

In my prior experience with switching from a Constant Speed Propeller to a Fixed Pitch Propeller, there hasn't been any significant power change that could not be attributed to general noise from imprecise tests, so I am not sure what is going on with your situation.

It might be worthwhile to plot your Engine Horsepower versus RPM to compare against the ones from the manual.
The numbers might be a bit different, but the shape should be pretty close.

If you don't like having your concrete blocks hauled around too much, just create a garbage Propeller Efficiency Table (511) for testing with efficiency values all at Zero. That should work.
Or if you can't keep the aeroplane from moving, then adjust the initial couple steps of the Power Coefficient Table to be the same so the curve is flat and it does not matter if it is really stationary or not.
Basically you just need the number for Advance Ratio Zero.

There really should be no great "Steps" in Table 512, but do as you feel you need to do.
They sometimes cab be seen in actual Power Coefficient Graphs.
I would be concerned at this point that there are other indications if this step appears to be necessary to get the performance correct.

By the way, if you do not get an exact match, I don't think folks are going to ever call you on it.

- Ivan.

[aleatorylamp]

Hello Ivan
Thanks for your feedback!
It was actually more a matter of what I could make the sim come out with, rather than simply which engine specifications I was going for.
Given that the only Lycoming with reliable, complete information is the 225 Hp R-680-B4 on the Navy N2S-2, which is reported in the Handbook as giving 220 Hp at 2100 RPM, this is the engine I´m trying for.
One thing is what I want to get, and another thing is what the sim gives me!

I managed to even out the curve and get rid of the annoying step, and even improving it a bit by using a lower Torque/Friction balance level, just to make the curve prettier and ever so slightly more fitting!
Top performance is now also more correct.

The results with 0.58 at the end of the Torque graph and 8.15 on the friction graph, combined with the corrected below 100 mph performance for the low "J" factors, gave the following improved results:

Full-max: 2201 RPM, 237 Hp, 127.9 mph, 100% throttle. (105% rated power)
Nor-max: 2145 RPM, 221 Hp, 124.5 mph, _95% throttle. (100% rated power)
Nor-max+: 2165 RPM, 226 Hp, 125.6 mph, _96% throttle (100% the other rated power)
Vno.___: 2080 RPM, 202 Hp, 120.6 mph, _89%
VCr high: 1888 RPM, 154 Hp, 108.5 mph, _75%
VCr slow: 1756 RPM, 126 Hp, 100.3 mph, _65%

Ceiling with 50% fuel: 103 fpm at 18500 ft (Oh !!!)
Initial RoC, 50% fuel: 808 fpm at 1400 ft

It was only possible to get a 221 Hp reading for Normal max. at 95%, because the 94% one went to 218 Hp. The same applies to the 225 Hp reading, which had to be 226 Hp at 96%, because 95% went to 223 Hp.
Of course, if one expects that BOTH of these should be at 2100 RPM, it just can´t be done - not even just one of them, actually!

I´ve prepared a picture of propeller tables 511 and 512. The numbers in the box at the bottom of each table refer to the curve position marked by the red dot. AAM isn´t bad to use, is it?
Cheers,
Aleartorylamp

[Ivan]

Hello Aleatorylamp,

You seemed to want to discuss how to calculate when the efficiency of a propeller drops to Zero.
It would have made sense to copy the two or three posts from the BV 141B thread over here so that THIS message does not seem to be such a non sequitur.

It quotes 98AA66, 98AA64, and 98AA66 propellers for the 220-225 Hp Lycoming radial engine.
The first number is the propeller length in inches.
The second number is the pitch in inches, (theoretical advance, supposedly no-slip).

Looking at the Sensenich Propeller Certificate List, these propeller reference numbers are found preceded by the letter "W". I found a converter to convert pitch inches to degree-angles at 75% prop radius.

W98AA66 standard prop. 66 inches pitch = 15.95 degrees.
W98AA64 climbing prop. 64 inches pitch = 15.49 degrees.
W98AA68 cruising prop. 68 inches pitch = 16.41 degrees.

Remember this post of yours? We will use the numbers from here for examples.
I will be using only the first example for discussion for simplicity.

The name of the W98AA66 propeller means that it is 98 inches in diameter and Advances (<--- Important Word!!!) 66 inches per revolution at zero slip. Yeah, you already knew that!

So what happens if we have the propeller on the front of our neat little aeroplane turning at 1 revolution per second? (slow!)
If the aeroplane is moving less than 66 inches per second, the propeller provides thrust.
If the aeroplane is moving MORE than 66 inches per second, the propeller does not provide thrust and is actually being driven by the airstream.
So in this case, the ADVANCE RATIO at which the aeroplane is moving 66 inches per second is where the EFFICIENCY DROPS TO ZERO.

Note that the angle of pitch is calculated by Trigonometry using the Tangent Function.
Y = 66 inches
X = 98 inches * 0.75 * Pi
Tan (Theta) = Y / X = 0.285829
Theta = ArcTan (0.285829) = 15.95 degrees.

Note that the angle is the important part here.
If the propeller were twice the diameter and Advanced twice as far per revolution, the angle would be the same.

Advance Ration Formula

J = V / n * D

When the values for the W98AA66 propeller are substituted in this formula
ALONG WITH Propeller rotational speed, the result is the Advance Ratio at which Efficiency drops to Zero.

This is purely from a Geometry viewpoint.
Reality is not quite so simple which is why I keep giving an approximate range of values.
The Propeller is not a single Blade Element at 75% Radius.
The Blade Cross Section is an Airfoil and not completely flat so even at Zero degrees AoA, there is still some lift.

I have made some working assumptions there which probably would not stand close scrutiny, so you will need to make your own assumptions.

Hope this helps.
Let me know if you need some additional clarification.

- Ivan.

[Ivan]

J = V / (n * D)

Left out the Parentheses on the last Post.
The Parentheses are not needed when writing on multiple lines.

- Ivan.

[aleatorylamp]

Hello Ivan,
OK.
Propeller Pitch in inches of Advance per propeller revolution,
translates into a speed for a given RPM, at which the propeller´s
work becomes useless, i.e. zero, at least in theory.

This theoretical speed is higher for any given practical operating
speed with its corresponding propeller RPM.

Theoretical speeds and practical speeds have their corresponding
"J" factors.

How big the difference is between theoretical and real speeds and
"J" factors is, depends on the aircraft.

As it happens, for the Stearman-75, I am getting consistent differences
of 10 mph and 0.05 in the "J" factors. You had calculated a recommended
"J" factor of 0.8 for the Stearman.

This would add another 0.13 to the "J" factor difference, making the
difference 0.18, i.e. 3.6 times larger, which would account for the
working assumptions you were making.

A correct propeller efficiency curve steeply goes down to zero,
as opposed to stock CFS propeller efficiency curves, whose angles
get much shallower as they approach zero. To improve these, it is
necessary to see where the curve reaches zero.

You were discussing improvements on the efficiency curves of the BV-141,
on the BV-141 thread, showing the improved curves of the Ki-61 and the
Condor, as well as illustrating the "before and after" BV-141 curves.

My query about how to go about these calculations met with your suggestion
I should go and look on the Internet. I wouldn´t have asked if I had found any
understandable information on the Iternet. My explanation that followed
as to why looking on the Internet had been useless for me led to your cutting
comment that the subject was off topic on the BV-141 thread.
Off topic? What was off topic?

Why is it on topic only for the Stearman thread? I already had the solution for
it here, the zero at J=0.8 you recommended - Mentioning the Stearman in this
context was only for comparison purposes, just like you mentioned the Ki-61
and the Condor.

Cheers,
Aleatorylamp

[Ivan]

Hello Aleatorylamp,

My query about how to go about these calculations met with your suggestion
I should go and look on the Internet. I wouldn´t have asked if I had found any
understandable information on the Iternet. My explanation that followed
as to why looking on the Internet had been useless for me led to your cutting
comment that the subject was off topic on the BV-141 thread.
Off topic? What was off topic?

We must have very different search methods on the Internet because that is actually the source of most of the information that I myself am using. I don't have any formal education in this field either; I have just spent a fair amount of time looking around for scholarly articles on the subject.
The data for this Stearman project was the same way. The performance information took about 15 minutes to find. The Lycoming R-680 manual that we recently discussed was found in well under 5 minutes (probably closer to 1 minute) after I started looking for the data and I am not even that interested in the aeroplane! This isn't a new phenomenon. It goes back at least as far as the P-3 Orion where there were lots of manuals and useful diagrams online but you just either were not finding them or were not looking.

Regarding just a general definition of Advance Ratio, Wikipedia gives an excellent explanation and is where I looked when I needed a quick reminder of what the formula was. It isn't hard to find!
The rest of the terms give a lot of hits on a search engine and I go typically to the sources that are organised the neatest and closest to the way I wand the data to be presented which tends to be documents from either NASA or University classes on Aeronautical Engineering.

Why is it on topic only for the Stearman thread? I already had the solution for
it here, the zero at J=0.8 you recommended - Mentioning the Stearman in this
context was only for comparison purposes, just like you mentioned the Ki-61
and the Condor.

The reason I thought your posts were off-topic was because you were asking for an explanation of how I arrived at the Zero efficiency but in the context of the Stearman as an example. We have already spent literally weeks discussing the issues of the Stearman's Propeller here.
If you had wanted an explanation of how I arrived at that information for the Stearman, you should have asked about it in the thread about the Stearman.
Otherwise, there is no continuity in EITHER thread.
That is why I chose to post the response here: because it makes logical sense in the context of the last several weeks of discussing propeller efficiency numbers along with the various graphs that have been posted.

You may have had a solution (actually you are quoting out of context for J=0.8), but may not have understood WHY that was the solution which is why you were asking and why that question belonged here.

The examples of the Ki 61 and Condor are good illustrations because both are either released or in a "reasonable form".
The Stearman's Propeller Graphs are not because you are still trying to figure them out (and still misquoting by the way).
The BV 141B Propeller Graphs were also not a good example because they were also still being edited.
They were posted because they were the subject of the discussion.

The latest has not been posted because it isn't necessary for illustrating anything important in that discussion and also because they are still subject to edits as needed if I should encounter something I don't like while testing.

- Ivan.