Flight model question: is drag realistic as planes keep their speed/energy for very long time?

[jetsimace]

When reducing throttle to idle, planes in IL2 tend to keep their speed/energy for quite a long time.
I would assume that speed/energy would bleed much faster due to existing drag.
(Specifically I am referring to P-51B and P-51D, currently I can't tell regarding other planes...)
Any ideas?

[TheWarsimmer]

Cut your rpm. On planes like a 109 you can also turn on the manual override and set a pitch that acts like an air brake.

[ITAF_Airone1989]

I did some test previously and seems that all the German plane have the same glide rate: 10km any 1000m of altitude... And It's the same for the stuka, the h111 and the 109.

But if I remember correctly some plane (Arado) have even a bigger value (15km)

[Talon_]

3 hours ago, TheWarsimmer said:

Cut your rpm. On planes like a 109 you can also turn on the manual override and set a pitch that acts like an air brake.

 

I think high RPM low throttle causes more drag due to increased vacuum pressure within the cylinders, does it not?

[TheWarsimmer]

12 minutes ago, Talon_ said:

 

I think high RPM low throttle causes more drag due to increased vacuum pressure within the cylinders, does it not?

Anytime I want to cut my speed in a hurry that's the method I use. Works much faster. You can test it by approaching a friendly in formation. Cut the mani only and it takes quite a while to slow down and you'll overshoot. Cut them both and you'll slow down much faster.

[DD_Arthur]

3 hours ago, jetsimace said:

When reducing throttle to idle, planes in IL2 tend to keep their speed/energy for quite a long time.
I would assume that speed/energy would bleed much faster due to existing drag.
(Specifically I am referring to P-51B and P-51D, currently I can't tell regarding other planes...)
Any ideas?

It happens in all planes. In my opinion, throttle response and engine torque effect is the area where DCS is well ahead of GBS.

[Avimimus]

In Flying Circus there are some aircraft which are so draggy that maximum control surface deflections (even without turning) are enough to cause them to stall and fall out of the air... not sure about the P-51 though!

[Art-J]

1 hour ago, TheWarsimmer said:

Anytime I want to cut my speed in a hurry that's the method I use. Works much faster. You can test it by approaching a friendly in formation. Cut the mani only and it takes quite a while to slow down and you'll overshoot. Cut them both and you'll slow down much faster.

I didn't try it, but if it really works like you describe, something's really wrong with flight model. The faster the prop windmills, the higher the drag it causes in real life. So high rpm low MAP should slow the plane at a faster rate than low rpm and low MAP.

[354thFG_Rails]

4 hours ago, jetsimace said:

When reducing throttle to idle, planes in IL2 tend to keep their speed/energy for quite a long time.
I would assume that speed/energy would bleed much faster due to existing drag.
(Specifically I am referring to P-51B and P-51D, currently I can't tell regarding other planes...)
Any ideas?

51 is a slick bird. It won’t lose speed as fast as say a 190 or spit. If you’re simply trying to slow down just pop some flaps. The 51 has split flaps so very draggy and not a lot of lift. Also they have high settings when deploying them. There’s a placard on the left side near the flap handle that gives speeds for different settings. 

1 hour ago, Talon_ said:

 

I think high RPM low throttle causes more drag due to increased vacuum pressure within the cylinders, does it not?

Not so much the cylinders because at idle there’s less vacuum pressure. Has more to do with the prop. It’s a much finer pitch setting to maintain rpm so it’s very draggy. If you were to reduce the prop pitch this makes it more corse and aligned more with the relative wind producing less drag. 

[BigGinger]

2 hours ago, 86th_Rails said:

The 51 has split flaps so very draggy and not a lot of lift.

 

This is airplane nerd minutiae, but the 51 has "plain" flaps.  Split flaps can be seen on almost all the British stuff.  Off the top of my head, Hurricane/Spitfire/Typhoon/Tempest all have split flaps.

[354thFG_Rails]

27 minutes ago, BigGinger said:

 

This is airplane nerd minutiae, but the 51 has "plain" flaps.  Split flaps can be seen on almost all the British stuff.  Off the top of my head, Hurricane/Spitfire/Typhoon/Tempest all have split flaps.

You’re right. Either way, good for drag, not a ton of lift when deployed. 

[Guest deleted@83466]

4 hours ago, Art-J said:

I didn't try it, but if it really works like you describe, something's really wrong with flight model. The faster the prop windmills, the higher the drag it causes in real life. So high rpm low MAP should slow the plane at a faster rate than low rpm and low MAP.

 

Yes, if it works the way the other guy says, that would be bad.

[[CPT]Crunch]

The faster an engine windmills the greater chances of your planetary gearbox exploding, they were designed and stressed to carry a load one way only.  To design one with reverse strength would cost far more weight in metal than it would ever be worth.  Aviation is all about minimum weight built into the design.

[Guest deleted@83466]

29 minutes ago, [CPT]Crunch said:

The faster an engine windmills the greater chances of your planetary gearbox exploding, they were designed and stressed to carry a load one way only.  To design one with reverse strength would cost far more weight in metal than it would ever be worth.  Aviation is all about minimum weight built into the design.

 

I don’t know much about engines, but I think in the old days, like the 40’s and 50’s, props used to be flown by the “squares rule” for this reason.  Like if you are at 18” of MP, then you don’t want more than 1800 rpm.  But squares rule is actually bogus, because it’s based on arbitrary units (doesn’t work for Ata or boost).  I think on landing approach, it probably didn’t work anyway, because you’re going to be at a high rpm and low manifold pressure.

[Charon]

2 hours ago, [CPT]Crunch said:

The faster an engine windmills the greater chances of your planetary gearbox exploding, they were designed and stressed to carry a load one way only.  To design one with reverse strength would cost far more weight in metal than it would ever be worth.  Aviation is all about minimum weight built into the design.

 

I've heard of it causing problems with the master rod bearing, but never the gearbox. And the anecdotes I've heard suggest it's a TBO problem, not an immediate catastrophic failure problem, as the other sim models it.

 

Quote

Remember, the problem has only been “proven” on airplanes like the DC-6, descending at 300 knots (TAS) and better, for long periods of time, with the MP way back.There are even reports of old-time operators who practiced emergency descents for real: full RPM, throttles closed, and stuff the nose down to high speeds, even redline speeds. Several old-timers have told me they used to do that, with no known harmful effect, and good records of few master rod bearing failures.

Props Driving Aircraft Engines

 

If you can find examples of accident reports during this sort of operation, I'd love to read them.

[Ribbon]

10 hours ago, DD_Arthur said:

It happens in all planes. In my opinion, throttle response and engine torque effect is the area where DCS is well ahead of GBS.

DCS got it quite realistic, torque changes affect lvl flight quite a lot as it is in RL....

I don't expect il2 goes detailed FM as DCS, rather to stick to it's strenght (combat scenery, focus and gameplay variety) which lately really needs refreshment.

[Charon]

11 hours ago, TheWarsimmer said:

Anytime I want to cut my speed in a hurry that's the method I use. Works much faster. You can test it by approaching a friendly in formation. Cut the mani only and it takes quite a while to slow down and you'll overshoot. Cut them both and you'll slow down much faster.

I wasn't able to reproduce this.

 

Test procedure was a P-51B on Stalingrad (Summer) at 2km. Autolevel, no trim adjustments, three trials in one flight. For each trial I ran it up to a hair over 300mph at combat power, set RPM, then cut throttle to idle and waited for the plane to slow below 200mph.

 

I then timed this with a stopwatch in the replays: I started the timer the moment I saw black pixels between the needle and the 300mph marker, and stopped it the moment I stopped seeing black between the needle and the 200mph mark.

 

At 3000rpm, this took 32.5s

At 2500rpm, this took 37.2s

At 1950rpm, this took 41.5s

 

I don't know if the numbers are correct, but the trend at least seems to be in the right direction.

 

If you can find a different trend in some other plane, I'd like to see it. P.S. One possible confounding factor is radiators, which I should have set to 'manual' before this test.

Edited October 29, 2022 by Charon

[357th_KW]

This is exactly how it works in real life as well - I used to fly Q400’s, and anytime you needed to slow down faster (or descend steeper) then you could at cruise idle, you’d bring the props forward to max rpm, and it was like throwing a parachute out.  Much more drag then the speed brakes on any jet I’ve flown.

[Guest deleted@83466]

I only flew the simulated Q400? (Majestic in FSX), and yeah, it did that!

[Charon]

I also conducted tests in the Bf 109G-2. Procedure was as with the P-51, except that I switched water radiators to manual mode and 100% open, to guarantee consistency. Also, I measured 400kph to 300kph, and marked those points when the needle was centered on the dial's mark.

 

I obtained the following:

 

7:30 (most coarse pitch, low RPM): 21.4s

9:00 (intermediate pitch) 18.9s

10:00 (fine pitch) 17.7s

 

Again, this is as I would expect: high RPM/fine pitch produces more drag.

[TheWarsimmer]

I'm not sure what's going on with the results. I'll take a vid later, but my speed drops way faster with low rpm. I thought people knew about the 109 manual pitch trick while landing for a long time.

 

The way I always thought it worked was that the fine pitch created greater drag. Its why you feather a dead engine's prop to as coarse as possible- it will slice through the air and induce less drag. 

 

As far as a 109:

fine pitch = more drag

Coarse = less

 

Made sense to me, but maybe I'm misunderstanding something.

 

 

Edited October 29, 2022 by TheWarsimmer

[Art-J]

^ It makes sense allright, you understand this part well and that's how it indeed works, judging from Charon's results posted above, at least as far as Allied CSP props and 109's prop in manual mode are concerned. The issue is, your posts are somewhat confusing, and self-contradicting. Fine pitch = high windmilling rpm, coarse pitch = low windmilling rpm. Thus, the latter should reduce overall drag, not increase it.

 

Going back to comparison with DCS for a second, prop braking is MUCH more noticeable in "their" Spit vs "our" Spit (that's the very first thing I noticed when Spit IX came to Il-2GB), but I've no idea which sim is closer to reality in this regard.

[1PL-Husar-1Esk]

Yak Panther made long documnet when he proved that the control surfaces are overly effective due to propeller wash. Those would potentially hide P effects and torque on 109 during takeoff off. He stated that control surfaces are  3 to 4 times more effective in some flight conditions compared to real data. This can have more side effect to many flight regimes which look suspicious to us. 

[TheWarsimmer]

8 minutes ago, Art-J said:

^ It makes sense allright, you understand this part well and that's how it indeed works, judging from Charon's results posted above, at least as far as Allied CSP props and 109's prop in manual mode are concerned. The issue is, your posts are somewhat confusing, and self-contradicting. Fine pitch = high windmilling rpm, coarse pitch = low windmilling rpm. Thus, the latter should reduce overall drag, not increase it.

 

Going back to comparison with DCS for a second, prop braking is MUCH more noticeable in "their" Spit vs "our" Spit (that's the very first thing I noticed when Spit IX came to Il-2GB), but I've no idea which sim is closer to reality in this regard.

 

You're right- I got it backwards with regards to how rpm affects pitch. Didn't mean to start such a distracting discussion?. I just tested as well, with similar results to Charon. 

 

Yak 1 69: 400 kph, 80 mani, full rpm, trimmed and level

Cut manifold only: 30 seconds to slow to 300kph

Cut both: 40 seconds to slow to 300kph 

[FeuerFliegen]

19 hours ago, TheWarsimmer said:

19 hours ago, Talon_ said:

 

I think high RPM low throttle causes more drag due to increased vacuum pressure within the cylinders, does it not?

Anytime I want to cut my speed in a hurry that's the method I use. Works much faster. You can test it by approaching a friendly in formation. Cut the mani only and it takes quite a while to slow down and you'll overshoot. Cut them both and you'll slow down much faster.

 

What you're saying here is the opposite of what he said.

 

3 hours ago, TheWarsimmer said:

I'm not sure what's going on with the results. I'll take a vid later, but my speed drops way faster with low rpm. I thought people knew about the 109 manual pitch trick while landing for a long time.

 

The way I always thought it worked was that the fine pitch created greater drag. Its why you feather a dead engine's prop to as coarse as possible- it will slice through the air and induce less drag. 

 

As far as a 109:

fine pitch = more drag

Coarse = less

 

Made sense to me, but maybe I'm misunderstanding something.

 

 

 

You're right, but I think you described it backwards in your previous post.  Low RPM is coarse.  Maybe you're thinking it's low because technochat in the 109 reads 0%, but in reality that means highest RPM / fine pitch; 100% for coarse pitch / lowest RPM / closest to being feathered.

 

 

 

Edit - I didn't see your last post when I wrote this, but I see you understand now.

Edited October 29, 2022 by SCG_FeuerFliegen

[Voidhunger]

There is also a strange "bug" for a long time. If you are landing and your plane is moving, you cut off the throttle and the propeller instantly stop.

But if you wait until your plane stop and then cut off your trhottle it takes time until the proppeler stop.

 

anyway I agree that the planes are gliding for a very long time

Edited October 29, 2022 by Voidhunger

[MAJ_stug41]

I havent tested it, but I think P-factor is understated in this game or overwhelmed by something else, as artj mentions above.

 

I have noticed that with fully feathered pitch, if the engine has stopped windmilling, it may be induced to to so with a bit of yaw (and presumably pitch) of the aircraft which convinces me that p-factor IS modeled. If changing the angle of incidence to the wind on the propellers (aerofoils, of course) induces different values of lift and drag, then p-factor must be a thing, because p-factor is the same thing just while the prop is spinning. I doubt the game is literally calculating the forces on and created by each blade at all times, it must be a generalization. I doubt any game models fluid dynamics to that detail.

Edited October 29, 2022 by stug41

[69th_chuter]

Something else to keep in mind: 

A typical constant speed flyweight type prop governor is only going to have an operational range of, roughly (application dependent), 1600-3200 engine rpm.  below that rpm range the prop is pegged on the low pitch stop and above the range its pegged on the high pitch stop regardless of the control setting.  Electric props in manual mode do not have this limitation as that mode bypasses the governor.

[Charon]

I also tested braking with radiator vs. fine pitch in the 109G2, from 450kph to 300kph.

 

In the first test: I opened the rads and cut power starting at 450kph: 26.6s

Second test: I returned radiators to auto, gave them time to settle, and at 450kph, I cut power and set maximally fine pitch. 21.3s (and hit 3000RPM: not advised, especially at higher speeds!)

Third test: I kept both at auto and simply cut power at 450kph. 31.2s

 

So both do make an appreciable impact on braking. The prop makes the better brake, but abuses the engine.

 

I also ran one test where I slipped the plane aggressively: this took only 13s (although it was harder to keep all the variables controlled, and I wound up gaining about 150m, which of course let me decelerate faster). Still, this matches my intuition that slipping is more effective than either (and is also less fiddly).

[R33GZ]

Slipping in a hard turn with all your bells and whistles out is even more effective... that'll get you back on Terra firma REAL quick

[Soilworker]

I don't want to go off topic too much but I'm curious about the use of low RPM settings on CSPs, I find I never really set it below about 70% for most planes, am I doing something wrong here? Are there common uses for lower settings I should be aware of? I know the rule of thumb that you shouldn't advance the throttle lever past the RPM lever which is a main reason I run higher RPMs but some info/advice from people more knowledgeable than myself is always welcome.

 

Edit: Feathering notwithstanding. 

Edited October 29, 2022 by Soilworker

[RoteDreizehn]

18 hours ago, =VARP=Ribbon said:

DCS got it quite realistic, torque changes affect lvl flight quite a lot as it is in RL....

I don't expect il2 goes detailed FM as DCS, rather to stick to it's strenght (combat scenery, focus and gameplay variety) which lately really needs refreshment.

... I don't expect il2 goes detailed FM ...

sorry, I cant share this opinion. Its very annoying that they say 1c produces hardcore sims and have an intermediate FM in IL2.

 

Edited October 29, 2022 by RoteDreizehn

[MisterSmith]

Per the forum rules; please provide documentation and repeatable testing, in the proper format regarding FM claims. Closing this one up.

 

Smith