Stall Speed Bug?

[TequilaChaser]

love reading this thread, but for the less brilliant minds of Aces High, I would think that there are quite a few who are wondering in laymens terms, how this data and knowing/understanding this data will help them  "In - Game"

if it helps any at all.............


back to the office ........

[hitech]

TequilaChaser: It is not about helping people in game, rather it is us trying to figure out if a problem in modeling exists.

HiTech

[Benny Moore]

Those pictures are really something, and they explain why Aces High II has the best stall modelling of any simulator.  I've always said as much.  But I didn't know that airflow was modelled so thoroughly.

My one problem is, isn't the definition of stall that the angle of attack is too high for the wing to produce enough lift to maintain level flight?

[dtango]

Widewing:

The reason stall speed is dropping a few mph when you go with higher nose attitude is because the weight of the aircraft that the wings need to counteract is lower because of the vector relationships where L and W aren't 180 degrees to each other (where L=W), rather it's now defined by L=cos(y)W.



The greater the angle (y) in a climb, the less weight the wings have to bear which = lower lift needed.

You can see this in Pyro's screenshots where at 7 degrees AOA the weight of the aircraft is 12904 lbs where the lift is 12063 lbs just before stall - then at 20 deg AOA weight is still 12904 but lift is 10465 lbs before stall.

Tango, XO
412th FS Braunco Mustangs

[hitech]

My one problem is, isn't the definition of stall that the angle of attack is too high for the wing to produce enough lift to maintain level flight?

That definition only works for 1 speed. And is equivaltent to mine for 1g stalls.

Stalls can happen at any speed and my definition works for any speed.

HiTech

[Pyro]

Originally posted by dtango

You can see this in Pyro's screenshots where at 7 degrees AOA the weight of the aircraft is 12904 lbs where the lift is 12063 lbs just before stall - then at 20 deg AOA weight is still 12904 but lift is 10465 lbs before stall.
 

Not to nitpick but that's a potentially confusing statement.  You mean 7 and 20 degrees nose up attitude, not AOA.  They both are at the same AOA at the onset of the stall.

[dtango]

Regarding the reason the power-off tests flown by Widewing, Mace, and myself appear to have lower stall speeds than power-on conditions, analyzing it a bit more I believe that Widwing and Mace are close to the mark regarding the whole dv/dt affect and the relationship to the stall.



The image above is a typical record of a dynamic stall maneuver, power-off.  Notice the 1-g stall speed (Vs1g) being higher than the FAA stall (Vs).  The reason this is occurs is because the actual normal force acting on the plane is less than 1-g.  Essentially the plane is dropping some which creates this condition.  One thing to clarify - it's not actually the rate of deceleration that results in this but because the aircraft is experiencing less than 1-g load due to aircraft dropping.

Reviewing Widewing's films and reviewing my own films I've noticed that the accelerometer is actually below 1-g in the power-off cases when the departure of controlled flight occurs which tells me that we are in this dynamic situation.

Tango, XO
412th FS Braunco Mustangs

[dtango]

Originally posted by Pyro
Not to nitpick but that's a potentially confusing statement.  You mean 7 and 20 degrees nose up attitude, not AOA.  They both are at the same AOA at the onset of the stall.

Ah yes, my bad.  Tried correcting it but I've gone beyond the 120 minutes edit limit on the bbs .

Tango, XO
412th FS Braunco Mustangs

[Brooke]

I've completed my study of F4U-1 stall speed in Aces High vs. the stall speeds listed in the F4U-1 pilot's manual for clean, power off; clean, power on; dirty, power off; and dirty, power on.  I find excellent agreement between the manual and Aces High.

I did some math to figure out a technique that would be repeatable, that would allow steady-state measurements (which are much easier to take), and that would allow reasonably easy avoidance of changing g loads on the aircraft.

My results and analysis are posted here:

http://www.electraforge.com/brooke/flightsims/aces_high/stallSpeedMath/stallSpeedMath.html

If anyone finds errors with my analysis or data, please let me know.  I've checked it a couple of times, but that doesn't mean there aren't errors, and it would be better if the analysis and data stands the test of being looked at by others.

I'll be interested to see if anyone repeats my tests or applies it to aircraft other than the F4U-1.

[dtango]

Brooke:

Outstanding write-up .  Thanks for taking the time to write it up.  Great idea by using the rate of climb/descent to figure out what stall speed is!

Just to make sure I remember this myself, so the key equations are (in conditions for steady state velocities):

(1) cos(theta) = sqrt [ (1-Rate_of_Climb) / Vairspeed]
(2) Vs1g = sqrt(1/cos(theta)) * Vairspeed

Tango, XO
412th FS Braunco Mustangs

[Widewing]

Originally posted by Brooke
I've completed my study of F4U-1 stall speed in Aces High vs. the stall speeds listed in the F4U-1 pilot's manual for clean, power off; clean, power on; dirty, power off; and dirty, power on.  I find excellent agreement between the manual and Aces High.

I did some math to figure out a technique that would be repeatable, that would allow steady-state measurements (which are much easier to take), and that would allow reasonably easy avoidance of changing g loads on the aircraft.

My results and analysis are posted here:

http://www.electraforge.com/brooke/flightsims/aces_high/stallSpeedMath/stallSpeedMath.html

If anyone finds errors with my analysis or data, please let me know.  I've checked it a couple of times, but that doesn't mean there aren't errors, and it would be better if the analysis and data stands the test of being looked at by others.

I'll be interested to see if anyone repeats my tests or applies it to aircraft other than the F4U-1.

A rather complete analysis. Thanks for taking the time.

A couple of points, if I may...

I noticed that you were using 18" @ 2,400 rpm for power-on stalls. I had a conversation with an older friend who did a test pilot stint an Langley in the early '40s. He stated that power-on stalls were generally conducted at Normal Climb power. The purpose being to familiarize pilots with stall behavior at a power setting and configuration that they would commonly experience.

The down side to this is that to induce a stall at the Normal climb power setting (44" @ 2,550 rpm for the F4U), you must assume a nose-up angle of approximately 20 degrees. I used auto-angle. With 25% fuel, zero burn rate, this consistently results in a stall break at 94 mph TAS.

For power-off stall, I set the nose-up angle at about 6 degrees. This resulted in consistent stall breaks at 94 mph TAS. Increasing the angle to 15 degrees resulted in a stall break at 93 mpg TAS, but is a bit tricky to catch unless viewing film at very low frame rate.

As for the F6F-5; the Pilot's Manual provides stall speeds far lower than actual test data recorded in Navy test report NA-83/44177, for a weight of 12,420 lb.

Power-on clean: 96 mph
Power-off clean: 98 mph

Compare this to the Manual (converted to mph from knots, rounded). These speeds are IAS, and they probably reflect stalls performed above 10,000 feet.

Power-on clean: 71 mph
Power-off clean: 74 mph

These numbers very likely represent a different test method and weight  than that used in test NA83/44177. To see how this test is done (you'll need Real Player), watch this F6F training film. You will notice that the aircraft was held in a nose-high attitude during all stall demonstrations and that minimum beginning altitude was 10,000 feet.

Beginning at 10,000 feet, my power-on and power-off stalls were 88 mph IAS and 88 mph IAS. Hmm.... That's still way above the Manual's numbers and those presented in the training film.

My in-game data, using 25% fuel and the 20 and 6 degree nose-up attitudes beginning at 2,000 feet (taken from film) results in 88 mph TAS power-on and 88 mph TAS power off.

As I stated previously, there are significant differences between the manual, training films, Navy test data and Grumman test data. Thus, establishing exact stall figures for the flight model requires selecting data deemed most reliable.

Other planes tested, power-on and power-off, clean, 25% fuel:

P-38L:
98 mph
98 mph

The P-38 Manual gives the following power-off, clean figures
15,000 lb: 94 mph IAS
17,000 lb: 100 mph IAS
19,000 lb: 105 mph IAS

P-51D:
99 mph
100 mph

Fw 190A-5:
102 mph
102 mph

Spitfire Mk.VIII
87 mph
89 mph

I'm satisfied that the most significant contributor to generating variances in stall speeds is the result of increasing or varying climb angle.

My regards,

Widewing

[hitech]

Widewing, are you converting the manual figures to CAS?

[Badboy]

Originally posted by dtango
Just to make sure I remember this myself, so the key equations are (in conditions for steady state velocities):

(1) cos(theta) = sqrt [ (1-Rate_of_Climb) / Vairspeed]
(2) Vs1g = sqrt(1/cos(theta)) * Vairspeed

Tango, XO
412th FS Braunco Mustangs

Yes, but in your first equation you forgot to square the two speed values, and the inner brackets are in the wrong place, it should look like this:

(1) cos(theta) = sqrt [ 1- (Rate_of_Climb^2 / Vairspeed^2)]

But you can actually do it with only one equation and all you need are two items of data, the climb rate, and the stall speed in the climb. If you use the same notation as Brooke that would be:

Climb rate = v_climb
Stall speed in climb = v_SC
Stall speed in level flight = v_stall

From that you can get the stall speed in level flight in one step from this equation:

v_stall = v_SC^1.5 / (v_SC^2 - v_climb^2)^0.25

The results come out exactly the same as before, but it just means you can do it in one step in a spread sheet without using trig.

Badboy

[Badboy]

Hi Brooke

Originally posted by Brooke
If anyone finds errors with my analysis or data, please let me know.  I've checked it a couple of times, but that doesn't mean there aren't errors, and it would be better if the analysis and data stands the test of being looked at by others.

Just found a couple of notational errors:

In this coordinate system, the magnitude of the weight vector opposite the lift vector (its projection onto the aircraft's coordinate system) is W * cos(theta). So, at stall speed in a steady-state climb we have:

W * cos(theta) = 0.5 * rho * v_stall^2 * S * C_L_max

Here you are using v_stall for the stall speed in a steady state climb, but later you use that for the stall speed in level flight. It is clear from later equations you should have typed v_SC instead.

Also:

cos(theta) = sqrt(1 - v_climb^2 / v_sc^2).

Everywhere else you capitalized v_SC.

Just nit picky stuff,  but that does make it difficult to read for anyone not fluent in our favorite brand of gobbledygook

I only looked at it quickly and it looks otherwise ok. I'll take a longer look later.

Badboy

[Widewing]

Originally posted by hitech
Widewing, are you converting the manual figures to CAS?

For which numbers?

My regards,

Widewing