Low-speed F4U

[FlyGhost]

sector!!! haha i clicked on a random article and it was urs...lol   

[SectorNine50]

sector!!! haha i clicked on a random article and it was urs...lol   

LOL!  Way to contribute Fly!

[mtnman]

I've been arguing that the turn radius is inconsequential anyway. Where the F4U modeling goes awry IMHO is in the virtual absence of torque induced roll, creating a measure of roll axis stability that is nothing short of ridiculous. Indeed, if the F4U was so endowed in the real world, accidents would have been all but nonexistent. Its stall behavior makes a Cessna 172 seem wild in comparison.

It may be that Combat Trim is the culprit here... I'll test that next.


My regards,

Widewing

I also felt (at least initially) that the Combat Trim may have been neutering the effects of torque, and mentioned it a few months back when this discussion came up last.  I didn't do any real organized testing, but I found that even when using manual trim I couldn't get the F4U to display any bad behavior from torque effects, even at very slow speed.  I don't think it's the CT at fault any more.

Personally, I suspect the tameness of the torque effects is whats leading to the "uber" flight model stigma.  Especially since the data shown by Dtango and Brooke make the turn radius believable. 

However, I do wonder how many planes have neutered torque effects.  I doubt it's only a factor with the F4U.  I don't really see any planes displaying the torque effects I would expect, but then again I don't have any first-hand experience with them in RL, or the mathematical ability to make a qualified statement.  My feelings on the torque modeling are really no more than my opinion, which has been shown to be wrong at least once (many, many years ago, in a land far away...)

It would be nice to see some data, and a tweaked FM if necessary- I'm all in favor for as realistic of a FM as possible.  I'm not in favor of adjusting the FM based on opinion, but in favor if data supports it...

MtnMan

[Bronk]

  I don't really see any planes displaying the torque effects
MtnMan

Torque effect you say? 109f at very low speed try and roll right.

[Widewing]

Torque effect you say? 109f at very low speed try and roll right.

The same effect is present in almost all fighters, but it's not much an issue. Merely pull off power and it'll roll right. Like the F4U, the 109F is often flown at very low speeds with the flaps out. Therefore it is seen more often in these types.

The real issue as I see it is that most aircraft do not demonstrate the gyroscopic effect to the extent seen in the RL.

Speaking of odd FM things, has anyone ever done roll rate testing at high altitude? I have. Would you be shocked to see the 190D-9 rolling at over 240 degrees per second at 27k and 425 mph (it's around 140 degrees per second @ 225 mph and 500 feet)? How about the P-47 rolling at about 220 degree per second at the same altitude? Very strange, especially when one considers that control effectiveness should degrade with altitude and controls stiffen at higher speeds. I sent HTC an e-mail about this.

My regards,

Widewing

A high altitude

[Bronk]

WW wouldn't air density at that alt factor in a bit?

[Krusty]

Could go the other way with density Bronk... Most planes in extreme dives have to wait til the air is denser to get effective control back in their elevators to get the "bite" they need to pull up.


Maybe the thinner air has less pressure on the ailerons thus reduces roll rate?

(not sure, just thought about this as I read WW's post)

[Widewing]

WW wouldn't air density at that alt factor in a bit?

A mitigating factor is that not all aircraft see a big boost in sustained roll rate at high altitudes. The P-38L is unchanged from sea level. By the way, at 10,000 feet and 400 mph, the 190s roll faster (sustained) than the P-38L... That's absolutely incorrect. Nothing rolled faster at high speeds than the late P-38s with boosted ailerons... Nothing. NACA test data shows that the 190's roll rate degrades rapidly as speeds exceed 250 mph TAS. At 400 mph, its roll rate should be no greater than 70 degrees per second. Actual test data from the P-38L-1-LO shows that the P-38L rolls (sustained) at 95 degrees per second at 400 mph at 10k. Note the chart below... The roll rate degrades with altitude.




My regards,

Widewing

[Bronk]

Thanks to the both of you. I was asking because I was unsure.

[SectorNine50]

Widewing, did you test those roll speeds off of IAS or ground speed?  Reason I ask is, if your going faster in low density air, wouldn't you essentially have the same control as slower at lower altitudes?

[Krusty]

The chart he posted says adjusted true airspeed. Seeing as it's the same TAS but at different alts, and at higher alts it actually starts rolling WORSE, I don't think it works the way you describe.

[Widewing]

Widewing, did you test those roll speeds off of IAS or ground speed?  Reason I ask is, if your going faster in low density air, wouldn't you essentially have the same control as slower at lower altitudes?

I can simplify this. I tested an F4U at 250 mph IAS at 27k, it rolled at 148 degrees per second. I then tested the same F4U at 250 mph IAS at 500 feet ASL, where it rolled at 91 degrees per second.

Why would this show such a difference of 51 degrees per second?

Some other fighters at 27k:

Bf 109K-4: 130 degrees (should be around 95 degrees)
P-51D: 145 degrees (should be around 95 degrees)
Spit Mk.IX: 143 degrees (should be around 105 degrees)
Fw 190A-8: 247 degrees (should be around 160 degrees)

All greatly exceed any data I've seen.


My regards,

Widewing

[dtango]

Regarding roll rates….

Steady_roll_rate = roll_helix_angle * 2 * V / wing_span

Roll rate is absolutely a function of velocity.  Higher velocity than higher the roll rate.  What you see in AH is just that.  I would expect a higher roll rate the faster you go.  So why doesn't this match the NACA roll rates reported on the 190 and P-47?  A few key things about that report.  The figures are for a) IAS, b) 10k alt, c) fixed 50-lb stick force - this last item being very key in understanding the NACA chart.

The catch is that roll_helix_angle is also a function of other variables including air density and the amount of aileron deflection humanly/mechanically achievable. 

Air density - As illustrated in Widewing's P-38 roll chart, at the same true airspeed because of the reduced density of the air the higher you go the roll rate would be lower because we're not able to generate the same amount of lift at a lower altitude.  Thus if the aileron is deflected the same amount then assuming the exact same true airspeed but different altitudes the plane at a higher altitude rolls slower.  This is what Widewing's P-38 chart shows.

Stick force - Generally the amount of aileron deflection that can be achieved in WW2 planes is constrained by how much an aileron can be deflected.  Unboosted, this depends on how much muscle the pilot has.  The faster you go, the greater the dynamic pressure.  The greater the dynamic pressure the greater the hinge moments on control surfaces to move them.  In other words the amount of force needed to displace an aileron increases with increasing airspeed.  The NACA data referenced uses a constant stick force of 50-lbs.  Here's the chart Widewing is referrring to:



That's why at a certain airspeed the roll rates degrade because at 50-lbs of stick force we're no longer able to deflect the aileron at it's maximum.  I have no idea how Pyro and company model aileron deflection in AH and if they account for degrading aileron deflection decrease to simulate greater hinge moments.  Seems like that would be difficult to do.

Another key constraining factor for aileron deflection is aerolastic bending at high dynamic pressures which distorts the control surfaces.  That's a topic for another time .

Tango, XO
412th FS Braunco Mustangs

[dtango]

Widewing:

You need to do your tests at the same TAS not IAS.  250mph IAS at 27k is a lot faster than 250mph at 500 ft in TAS.

Tango, XO
412th FS Braunco Mustangs

[SectorNine50]

Widewing:

You need to do your tests at the same TAS not IAS.  250mph IAS at 27k is a lot faster than 250mph at 500 ft in TAS.

Tango, XO
412th FS Braunco Mustangs

Not in terms of airflow over the wing.  250 MPH IAS is the same no matter what altitude your at, but to acheive 250 MPH IAS at a higher altitude, you would have to have a faster TAS since the air is lighter up there.

WW, that is odd.  Perhaps the control surface model in AH2 is based upon TAS?  Would that make the roll rate calculate correctly if you pretended the TAS was actually the IAS?