Speaking of staying at Holiday Inn...P-47M Best Glide Speed Testing and Results

[Stoney]

The recent discussions of real-life performance versus in-game performance got me fired up to do some performance testing in-game, purely for working through some performance calculations.  I chose the P-47M, mostly because its new, its a Jug, and it is something I'll probably fly a lot in the future.  The first bit of data I collected was glide speed tests.

METHOD:

Airfield:  I loaded the TA map offline, and used one of the 5,000 ft high airfields and airspawned each sortie.  No wind was used.

Aircraft:  The Jug-M was loaded with 50% fuel and the 8-gun, 267 rpg ammo load, giving our shapely lady a nice trim 14,078 lb weight for all tests.  Fuel burn (though not used) was set at 0.001.  

Test Regime:  From the tower, I would airspawn SE, quickly raise the gear and recover from the resulting dive, leveling the aircraft out between 6,000 and 5,500 feet, and at around 200 mph IAS.  Using the .speed command, I would enter the desired speed for that test, and after altitude and airspeed stabilized 5 mph IAS above the desired speed, hit [alt-x] and let the autopilot do the flying for reproducable tests.  I would start my stopwatch as the aircraft decended through 5,000 MSL and stop the time as the aircraft arrived at 4,000 MSL.  I tested each speed with both the prop windmilling (quickly turned the engine on and then immediately off) and with the prop stopped (I never turned the engine on).  This was done both to test the two different conditions to illuminate the effect of prop drag, and also to see if the speeds were different.  In game, if you run out of fuel, your prop will windmill.  If your engine is damaged from lack of oil pressure or gunfire, the prop will stop.  With the prop in the windmill condition, I would manually adjust RPM to minimum, which for the speeds involved stayed between 800-1000 rpm depending on speed.

RESULTS:

The results were interesting.  I had previously assumed that best glide speed was close to best climb speed (default alt-x speed).  Best glide speed in both test conditions was much higher than default alt-x speed.  Second, it appears that the glide tests demonstrate that prop drag is modelled.  I thought I had remembered someone saying it wasn't.

Charts illustrating the tests:

This one shows the Vbg with the prop windmilling



This one shows the Vbg with the prop stopped



This one shows the effects of what I assume is prop drag




The only question I have is whether or not you could use IAS instead of TAS.  One or the other is going to remove altitude as a variable, I just couldn't figure out which one.  Either way, I'll have some more stuff from this to present later.

[Stoney]

I forgot to add that best glide speed is acheived at the highest speed that gives the lowest sink rate.  For the test, the timing measured how long it took to lose 1,000 feet of altitude in the glide.  The speed at which velocity X elapsed time was maximized indicates the best glide speed on the charts.

[Chalenge]

You do know that decreasing RPM on a frozen engine will also decrease the drag effect?

[Stoney]

You do know that decreasing RPM on a frozen engine will also decrease the drag effect?

We don't have the means to "feather" the prop on a stopped engine.  We just have the stopped engine in-game.  Can you feather a prop on a stopped engine in real life?  Most props are controlled by oil pressure, right?

Regardless, that was simply a secondary observation from the testing.

[FLS]

I forgot to add that best glide speed is achieved at the highest speed that gives the lowest sink rate.  For the test, the timing measured how long it took to lose 1,000 feet of altitude in the glide.  The speed at which velocity X elapsed time was maximized indicates the best glide speed on the charts.

I'm not clear on what you're saying here. The lowest sink rate is your greatest time aloft but not your best glide for distance. Sorry if I'm dense but I assume you're saying highest speed that gives the lowest sink rate to mean the best L/D AOA.

You're looking for the best glide AOA and we don't have a gauge for that. I assume that's why your using speed even though you know that speed changes with weight, which is why you don't want to burn any fuel for the test. I wonder if you can use the .target command as a relative AOA indicator instead of just to nose to the horizon. Then you could easily find the best glide speed at different weights.

[Chalenge]

We don't have the means to "feather" the prop on a stopped engine.  We just have the stopped engine in-game.  Can you feather a prop on a stopped engine in real life?  Most props are controlled by oil pressure, right?

Regardless, that was simply a secondary observation from the testing.

Stoney I am not going to argue about how things are in TRW or how you think things work I am telling you how things do work by observation. If you have a stopped engine and you hit the minus key (I have it on a lever) the 'prop-feather effect' will decrease drag.

[Stoney]

Stoney I am not going to argue about how things are in TRW or how you think things work I am telling you how things do work by observation. If you have a stopped engine and you hit the minus key (I have it on a lever) the 'prop-feather effect' will decrease drag.

Sorry, I thought you had seen this in the original post,

With the prop in the windmill condition, I would manually adjust RPM to minimum, which for the speeds involved stayed between 800-1000 rpm depending on speed.

and were talking about real life.  I did, during this test, reduce the RPM to a minimum.

[Stoney]

I'm not clear on what you're saying here. The lowest sink rate is your greatest time aloft but not your best glide for distance. Sorry if I'm dense but I assume you're saying highest speed that gives the lowest sink rate to mean the best L/D AOA.

You're looking for the best glide AOA and we don't have a gauge for that. I assume that's why your using speed even though you know that speed changes with weight, which is why you don't want to burn any fuel for the test. I wonder if you can use the .target command as a relative AOA indicator instead of just to nose to the horizon. Then you could easily find the best glide speed at different weights.

You're correct.  I am looking for the best L/D AoA, which I achieved by finding the speed.  I was trying to put it in layman's terms.  I guess I failed.

[Krusty]

This is interesting, being.. what? 20mph higher than default climb speed? What's the jug climb speed, 170?

I seem to recall Pyro or somebody stating once that the best dead-stick glide speed for most planes (i.e. creates the most lift) is often 10mph less than the default climb speed.

Interesting to see that may not be the case.

[Chalenge]

Sorry, I thought you had seen this in the original post,

With the prop in the windmill condition, I would manually adjust RPM to minimum, which for the speeds involved stayed between 800-1000 rpm depending on speed.

and were talking about real life.  I did, during this test, reduce the RPM to a minimum.

Yes but thats the windmill condition and not the stopped condition you are talking about. Real life? Ha!

[Stoney]

This is interesting, being.. what? 20mph higher than default climb speed? What's the jug climb speed, 170?

I seem to recall Pyro or somebody stating once that the best dead-stick glide speed for most planes (i.e. creates the most lift) is often 10mph less than the default climb speed.

Interesting to see that may not be the case.

Just remember that speed is only good at 14,000 lbs or close to it.  Weight changes will affect the speed.  Also, there is a speed around 160 mph TAS that will maximize your glide time--i.e. give you the most time airborne (the absolute minimum sink rate at that weight) but will not allow you to glide as far.  Best glide gives you the most distance for the altitude you have.

[boomerlu]

On TAS vs IAS:

This is a really complicated question... whew! Note, I'm not an aero guy, just a physicist, so I might be a bit off. Ok, to glide, we must balance the equations

Gravity + Vertical Component of Lift = Vertical Component of Drag
Horizontal Component of Lift = Horizontal Component of Drag

Doing so gives zero net force on the aircraft so it can maintain a constant velocity. Notice that lift and drag are both proportional to the density of the air, but gravity is NOT. The horizontal component is thus independent of density.

The vertical component on the other hand does depend on density. Simplifying we get

2 mg / (rho V^2 A) = CD SinTheta - CL CosTheta
CL SinTheta = CD CosTheta
=> 2 mg / (rho V^2 A CL) = TanTheta SinTheta - CosTheta

So, theoretically, we know m, g, rho, V, A, CL and we have to solve for Theta the glide angle.

Umm... looking at this, I have to question Stoney: did you factor in the glide angle as part of your distance calculation? We want to know the maximum HORIZONTAL distance covered after all. Edit: this isn't to cast doubt, but this is an important part of the calculation, and you didn't mention it in your method.

Ok, it looks like there's no easy way to factor out the altitude dependence unless indicated airspeed happens to be inversely proportional to the square root of the air density. If it is, then IAS is the way to go. If it's not, then keep the measurements in TAS so it's easier to calculate distance traveled.

[Stoney]

According to this website,  http://www.allstar.fiu.edu/aero/BA-Background.htm , we can get the glide angle from this equation:



Delta H = 1000 feet

Delta T = 46.53 seconds for prop not windmilling and 41.81 for prop windmilling  195/190 TAS Vbg respectively.

I was going to work through that tomorrow and then do the drag coefficients for both as well.

[boomerlu]

Stoney - I was JUST about to slap myself for making something more complicated than it needed to be... but...

Yes that's the easiest and best way to calculate the glide angle. As you probably know, it follows directly from the trigonometry of the glide situation. Gonna get no argument there. . Of course it is an experimental value rather than the theoretical one I derived.

However, the reason I went through the complicated derivation was to show that there's no simple way to just "factor out" the air density (and by extension the altitude). In fact, I didn't set out to prove this, I was just curious myself.

What's going to happen is that for a given TAS but varying altitude, the optimum glide angle will change.

Oddly enough... if the airplane "flies the same" aerodynamically at equal IAS's, then that means the lift and drag produced must be the same at equal IAS's. This further means that the lift forces at different altitudes must be equal at equal IAS's, which tells me that (with K being the IAS)

K1^2 rho1 = K2^2 rho2 = CONSTANT
=> K1 = sqrt(Constant/rho1)

i.e., the IAS is inversely proportional to the square root of the air density.

With this fact, the factor of rho CANCELS in the the force equations if we use IAS instead of TAS.

Thus, the best glide speed in IAS will hold at any altitude. The horizontal distance traveled per unit of altitude however will not change - you will just travel that distance more quickly.

Wow I just proved my initial conclusion wrong.

[Stoney]

For practical application, this is one of the few things where close counts--i.e. a 5 mph difference in glide speed won't have a huge impact on total distance travelled.  Theoretically, once the angle is established though, we could create a table for best glide speeds at that weight, for any altitude.

Vbg IAS for both conditions was 175 and 180 for the windmilling and stopped condition respectively.