View Full Version : Total Drag (man...)

PRB

March 13th, 2013, 16:13

While doing some calculation on total drag, I may have swerved, uncontrolled, into a truth. I found that total drag equals static (CD0) drag + induced (CDi) drag, and that CDi has to do with dynamic pressure and is also influenced by AOA. So I found that as speed increased, induced drag increased, which made sense to me, since dynamic pressure would be increasing. But I also found that below a certain speed, induced drag started going up again! What was happening? AOA was going up, is what was happening! This implied an “optimum” speed, at which induced drag, and therefore total drag, would be at a minimum. Might this speed be related to that which might produce max range? Did I stumble into something important, or is my math out to lunch (again)?

Tom Clayton

March 14th, 2013, 05:13

(Found this thread by accident thanks to a phantom subscription in my control panel...)

I can tell you from first hand experience that there is a sweet spot regarding speed and fuel consumption. On my bike, I get the best gas mileage around 60MPH, but above that, it starts dropping off due to increased drag. Common sense would state that this concept holds true for any powered vehicle. The trick is in finding that "sweet spot" - which generally requires considerable testing. Before computers, all you could do was start with an educated guess and then fly several sorties at varying speeds, then calculate consumtion after each flight. With planes, altitude also plays a huge role. Higher altitude reduces drag and aldo reduces fuel use due to the fact that fuel metering is based on total air volume. But if you get too high you loose thrust and end up going using more fuel trying to go nowhere fast. Come back down and the increased air density increases both drag and fuel use. Of course, both ground-based and in-flight computers make this easier now.

Dev One

March 14th, 2013, 11:30

There are 4 simple ones:

Minimum (stall speed),

Maximum speed, where max thrust = max drag - probably have one at each altitude

maximum range, also altitude related depending on engine output &

maximum endurance which will also be altitude related.

In other words there are a lot of different parameters to be calculated for optimum performance & modern aircraft come with an Operating Manual with loads of graphs.........I've also not mentioned weight.....which of course varies with fuel consumed & stores which might have been dropped...

Happy hunting the optimums...

Keith

PRB

March 22nd, 2013, 16:20

I guess what I was getting was this: Given straight and level flight, will induced drag follow a curve, such that there is an optimum speed above which, and below which, induced drag will increase? Because that's what my calculations suggest. The only way this makes sense to me is that below a certain speed, assuming straight and level flight, AOA will increase, thus presenting more surface area to the wind, which would make induced drag increase. For the FDG2 B-17E, this "minimum drag" speed, at sea level, is around 130 knots. Does this make any sense?

Tom Clayton

March 22nd, 2013, 19:08

That certainly makes sense, but those calculations are above my pay grade! :isadizzy:

fliger747

April 8th, 2013, 07:28

Paul: It is fairly easy to visualize this if one looks at the curves, such as in a book as Aerodynamics For Naval Aviators. For the effect on performance one has to take into account the power or thrust available, which also changes with speed. Jet power available is approx a straight line, increasing with speed, for prop aircraft it starts out higher at low speed than the jet, but increases at a decreasing rate and actually decreasing past a certain point. The Lift over Drag max is at a higher speed than the minimum power required and would be nominally related to max endurance and max range. Most aircraft use a 99% best range solution, which is about 5% faster than the absolute max range speed. It is quite illuminating to study the differences between prop and jet aircraft and the best speeds. Something I always thought amazing was that the max gliding distance of an aircraft is almost totally independent of it's weight! Cheers: t

PRB

April 8th, 2013, 10:53

Thanks Tom. For jet max speed I've been taking static thrust and then figuring the speed at which total drag (lbs)equals static thrust (lbs). It looks like a pretty good "SWAG" (Scientific Wild-A** Guess). What I don't know is how "static thrust" should be adjusted for the effects of altitude. Off to find that book "Aerodynamics for Naval Aviators".... As a navy person, that title sounds a little like "Astrophysics for Dummies"... :icon_lol:

PRB

April 8th, 2013, 12:18

I found a PDF on the Interweb called Aerodynamics For Naval Aviators NAVAIR 00-80T-80, by H.H. Hurt, Jr. Is this it? It's 434 pages, and looks like great stuff!

fliger747

April 9th, 2013, 14:38

Yes, that's the one, written for pilots and not engineers.

T

Dev One

April 12th, 2013, 00:25

Static thrust is what it says, the force exerted by the engine at zero speed, now depending upon the intake design (& to some extent jetpipe design) the thrust can be lower than this value at zero speed, but can also increase as speed increases at any particular altitude (well mass flow really). It can be interesting to add a gauge that reads out thrust - thats if the model has been designed correctly......

Keith

PRB

April 12th, 2013, 05:58

I’ve read how thrust increases with speed. I’m missing something on this one. Isn’t that sort of like a perpetual motion machine? The faster you go the more thrust you have? So the only speed limiting factor for jets should be the melting point of the metal it’s made of. Like I said, there’s something fundamental I’m missing here.

As for my initial drag question, Tom’s reference does confirm that my arithmetic was correct, sort of. There is a minimum drag speed, due to the fact that induced drag goes down with speed, and parasite drag goes up. I just arrived at it in a different way. My formula for “induced drag” seems to incorporate both the “induced” and “parasite” drag discussed in the Naval Aviators book.

fliger747

April 12th, 2013, 07:18

As it turns out the ratio between increasing thrust due to higher true airspeed at altitude and decreasing thrust due to reduced mass flow is about constant. It just works out that at altitude you use a lot less fuel to do the same thing and go faster.

Thrust can be read directly in AFSD....

T

MaddogK

June 26th, 2015, 13:15

Paul: It is fairly easy to visualize this if one looks at the curves, such as in a book as Aerodynamics For Naval Aviators. For the effect on performance one has to take into account the power or thrust available, which also changes with speed. Jet power available is approx a straight line, increasing with speed, for prop aircraft it starts out higher at low speed than the jet, but increases at a decreasing rate and actually decreasing past a certain point. The Lift over Drag max is at a higher speed than the minimum power required and would be nominally related to max endurance and max range. Most aircraft use a 99% best range solution, which is about 5% faster than the absolute max range speed. It is quite illuminating to study the differences between prop and jet aircraft and the best speeds. Something I always thought amazing was that the max gliding distance of an aircraft is almost totally independent of it's weight! Cheers: t

Sorry for the necro, but I was fishing for some info when I found this thread (which applies to my problem).

The plane I'm working on is pretty slippery, just enough that at idle the prop outputs just enough power to balance the increase of CDi when the AoA goes up to maintain altitude, so I end up with a plane in equilibrium @ idle and very hard to bleed speed to get into landing configuration since I have no flaps, no spoilers, and locked gear.

I'm thinking I need to tweek the lift numbers a hair to get the AoA higher at low speed

-OR-

Somehow use the cowl flaps to induce more drag.

Is there a cowl flap section in the CFG file I can add/edit, or do I need to 'fake it' and use a standard flap section and no accompanying keyed flap in the model ?

...I actually would like my oil cooler to add the drag and affect the oil temperature when deployed, but I don't see how this can be done without creating a gauge (which can be removed eliminating the effect).

Auto- deploying spoiler @ approach speed (deploying my oil cooler instead but without oil temp effects) ??

Ideas ?

Dev One

June 27th, 2015, 23:04

Not sure FS adds any drag with oil cooler or cowl flaps, but you can have an auto spoiler; I tried one that deployed at about less than 1/3 rd throttle, but it sounds as if your model is too slippery for that. I have found that modifying the slope & left/right position of the normal flight range part of the Cl vs Alpha & the slope of the same alpha range of the Cd vs alpha graphs not only affect the amount of trim needed over the speed range but also the drag. With a prop model I've also heavily modified the prop efficiency curves so that they avalanche just above the J value achieved at max speed in level flight, this also can apply drag when throttle is closed, but care is needed.

HTH

Keith

Two examples of use of the prop efficiency curves; Dave Molyneaux's DH 83 Fox Moth over at Flightsim & my Miles M3B Falcon Six over in the cbfsim library. To see the curves though you need to open the air file with Aircraft Airfile Manager, AirEd only shows it in tabular form.

On my WIP dragonfly I'm playing with the Cl & Cd curves, but Dave Molyneaux's Avro Avian uses a similar modification.

Can't find offhand the mathematical definition of drag (Cd), but my failing memory thinks it something like Cl squared in the equation.

K

MaddogK

June 30th, 2015, 07:23

Not sure FS adds any drag with oil cooler or cowl flaps, but you can have an auto spoiler; I tried one that deployed at about less than 1/3 rd throttle, but it sounds as if your model is too slippery for that. I have found that modifying the slope & left/right position of the normal flight range part of the Cl vs Alpha & the slope of the same alpha range of the Cd vs alpha graphs not only affect the amount of trim needed over the speed range but also the drag. With a prop model I've also heavily modified the prop efficiency curves so that they avalanche just above the J value achieved at max speed in level flight, this also can apply drag when throttle is closed, but care is needed.

HTH

Keith

Two examples of use of the prop efficiency curves; Dave Molyneaux's DH 83 Fox Moth over at Flightsim & my Miles M3B Falcon Six over in the cbfsim library. To see the curves though you need to open the air file with Aircraft Airfile Manager, AirEd only shows it in tabular form.

On my WIP dragonfly I'm playing with the Cl & Cd curves, but Dave Molyneaux's Avro Avian uses a similar modification.

Can't find offhand the mathematical definition of drag (Cd), but my failing memory thinks it something like Cl squared in the equation.

K

I had read somewhere on FSD that cowl flaps do affect cooling in fs9, but only CHT. Since what I'd like to affect is oil temp the stock FS cowl flaps won't work without help, but I cannot find any explanation how I could do it without creating a guage. Needless to say, I figured out the drag problem and feel like a total idjit for not seeing the problem earlier- while poking at the CFG file years ago I changed the oswald factor and mistakenly dropped the decimal point. Now that I'm back into the .6 range the wing lift/drag @ AoA is back to something more closely matching the FS stock A/C numbers tho my mid throttle range went right down the toilet.

:biggrin-new:

I'm able to land tho.

Dev One

June 30th, 2015, 08:01

Try playing with the oil temp values in block 543 of the air file, not sure though if its tied in with the cowl flaps though, maybe just engine power, airspeed & altitude.

HTH

Keith

Just looked at the FS9 SDK, there is a parameter available for oil temp, so maybe you will have to create a gauge using A.N. other command (Smoke toggle for example) to open /close the oil cooler flap. But how you would change the rate of cooling....dunno!

K

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