P-40 turn rate/Flight model check

[Holtzauge]

As you suggest, this is exactly what you get in Il-2, if you use the DD's robot in-game test stall speeds as per my post 137:

 

     In-game stall speeds are 153-176 kph  (95 - 109mph) IAS  (F+G up) over the range of weights they use, (see DD 123) compared to the two estimates of 90 and 92 given in Farky's post.

 

     I take the minimum stall speed in DD 123  153 kph as being at the minimum weight: 3264 kg, > CLmax of 1.32

     At maximum stall speed - 176 kph I assume "maximum takeoff weight"  4414 kg. > CLmax 1.35

    Somewhere in the middle, "standard weight"  3819kg, speed (153+176)/2 = 165kph > CLmax  1.34

 

My hypothesis for the discrepancies are a) that the RL tests were actually at lower weights than we have assumed, since I am not sure how you can stall test a fully loaded aircraft without warming it up and climbing to altitude and b) that the RL tests (and game tests by players) were pilot eyeballed from the altimeter and not quite at level flight, unlike the in game robot tests which were. 

 

If that is the case the game is fairly accurate in this respect, so perhaps the issues people have with it are down to the engine limitations more than the CLmax.

 

Yup, something seems to be off in the 90 and 85 mph stall numbers: Assuming 3840 Kg or 8458 lb and a stall speed of 90 mph this gives a Clmax of 1.73 which is simply too high and as Venturi already posted, 85 mph gives a Clmax of 2........

 

In addition: The in-game weight/Clmax  figures you posted above makes sense to me: I mean a Clmax in the range of 1.32 to 1.35 would place the P-40 in the same Clmax category as the Fw-190, Spitfire, Yak, Me-109 etc. A Clmax in the range 1.73 to 2 however just does not make sense so we must be missing something.

[Venturi]

Empiric data beats theory every time.

[unreasonable]

No it does not, that is not at all how science actually works.  

 

If a theory is well established and understood, it takes a large amount of contrary data to overturn it. The data will be questioned for consistency, accuracy and reliability extensively, before modifications to the theory are even considered.

 

More often than actually modifying a theory, a new variable will be assumed to save the theory and make the sums balance: as Holtzauge says, we might be missing something.

[Venturi]

Yes, it is how science actually works. There was also a theory of spontaneous generation, until Pasteur gave empiric data via experiment.

The aircraft stalls at 84mph in clean configuration at 8480lbs. That is our experimental data. The theory - which is our flight model in game- must fit the data and it does not. Therefore, modify the theory until it does.

We have original documents in extensive form, so if the burden of proof is that we need to have original period documents from the native operators of the aircraft accounting for all variables, then we no doubt have met that burden of proof.

I know full well how science works having been funded to do original academic research myself.

I have additional professional inquires to double check on this issue as well.

Edited December 30, 2016 by Venturi

[Holtzauge]

Empiric data beats theory every time.

 

No it doesn't: There is a NACA report based on empiric measurements that say's the Spitfire's Clmax is 1.2. So by your reasoning since this is an empirically derived value it is per definition true?

 

OTOH the RAE measured the Spitfire Clmax just as empirically to 1.36. So which empiric data trumps which? Is it the 1.2 or the 1.36 value?

 

Measurements done with uncalibrated tools, poorly performed or adjusted are just as wrong as theory based on the wrong numbers or erroneous assumptions.......

 

 

Yes, it is how science actually works. There was also a theory of spontaneous generation, until Pasteur gave empiric data via experiment.

 

The aircraft stalls at 84mph in clean configuration at 8480lbs. That is our experimental data. The theory - which is our flight model in game- must fit the data and it does not. Therefore, modify the theory until it does.

We have original documents in extensive form, so if the burden of proof is that we need to have original period documents from the native operators of the aircraft accounting for all variables, then we no doubt have met that burden of proof.

 

I know full well how science works having been funded to do original academic research myself.

 

I have additional professional inquires to double check on this issue as well.

 

OK, will be interesting to see if your inquiries will shed some new light on this but I seriously doubt you will find any support for a power off clean wing Clmax of 2 which is what you are suggesting when you combine 84 mph with 8440 lb.

[Venturi]

No it doesn't: There is a NACA report based on empiric measurements that say's the Spitfire's Clmax is 1.2. So by your reasoning since this is an empirically derived value it is per definition true? If an aircraft stalls at a given speed, at a given weight - then what? It is magic? Theory does not trump reality - which according to the data and math, shows the aircraft performs a certain way. For chissakes, the damn pilot handbook says it stalls at 90mph ASI!

 

OK, will be interesting to see if your inquiries will shed some new light on this but I seriously doubt you will find any support for a power off clean wing Clmax of 2 which is what you are suggesting when you combine 84 mph with 8440 lb. I am not creating this, it is from the trials done by the British at their famous Boscombe Down trial center in 1942. So now we are picking and choosing which period data to believe? You're saying this data is suspect? By what criteria, because it doesn't fit with your preconceptions?

Edited December 30, 2016 by Venturi

[ACG_KaiLae]

I would say gentlemen that something in our understanding is inaccurate or an assumption we have is off. We need more data. I really, really want the original Curtiss flight tests, assuming they haven't vanished down a memory hole.

 

As to answer an earlier question, 90 mph does not reference the weight, or altitude. Again more data needed.

 

I have a question. My understanding is that the P-40 and the P-36 are basically the same plane, with a different engine. Specifically, the wings are the same. While not definitive, is there data on the P-36 we could use to compare against that might be useful? It might be useful to compare the two and see if that could shed some light on the parameters we are using that are generating some of these numbers.

 

As to the FM of the 109, that's a different topic for another time.

 

My hypothesis of what's going on here is that we are probably looking at a combination of multiple different things. First the clmax of he in game plane is probably too low, which would explain the differences in stall speed. I doubt it is a major difference like that which has been discussed in the thread, but there is currently no way to be certain. Second the engine limits in game are probably too conservative. Third that the 109s overperform in turning to some amount (probably not major but still). Add these together and you get what we have.

 

Of course we would need data to support those, so we're back to need more data.

 

I am curious however; I was searching the internet and found a forum thread where someone named Venturi was having a discussion with a RL Allison engine mechanic about the V-1710-39. Would that be you Venturi?

[Venturi]

Here is another PEC done by the USAAC in Sept 1941. It grossly lines up with the British test, I did a mockup on the data points to demonstrate and overlaid it upon the British PEC done in 1942 at Boscombe down [attached].

 

http://www.wwiiaircraftperformance.org/P-40/P-40E_40-384_PHQ-M-19-1300-A.pdf

 

 

P-40E_40-384_PHQ-M-19-1300-A.pdf

Edited December 31, 2016 by Venturi

[Venturi]

Kai Lae, the report specifically states stall speed was 90mph ASI with 8480lbs.

 

 

[ACG_KaiLae]

On my iPad and don't have the docs in front of me, thanks.

[unreasonable]

Yes, it is how science actually works. There was also a theory of spontaneous generation, until Pasteur gave empiric data via experiment.

 

The aircraft stalls at 84mph in clean configuration at 8480lbs. That is our experimental data. The theory - which is our flight model in game- must fit the data and it does not. Therefore, modify the theory until it does.

We have original documents in extensive form, so if the burden of proof is that we need to have original period documents from the native operators of the aircraft accounting for all variables, then we no doubt have met that burden of proof.

 

I know full well how science works having been funded to do original academic research myself.

 

I have additional professional inquires to double check on this issue as well.

 

No it really is not - the theory must fit all of the experimental data, not just your one point.  If your point does not fit, you most certainly do not throw out a theory that fits all the other points.

 

All of the experimental data includes all of the data that has been used not just in the P-40 but in every other aircraft. 

 

Even when all of the data contradicts a theory, it is still not necessarily discarded if the addition of another variable brings the data into alignment with theory.

 

As per examples, the most famous currently is dark matter and dark energy: both postulated to make existing theory fit observations. Biology is full of this too: Neo-Darwinists seem to be able to "explain" almost anything in nature as the result of natural selection. I am not saying the thought behind either of these examples is correct, btw. 

 

As a scientist you must know perfectly well that the vast majority of data points from academic studies that do not fit established theory are never even published, never mind ever leading to a theory being refuted.

 

So either:

 

1)The equations do not apply to the P-40

 

2) The data is incorrect or incomplete, or

 

3) The data is good, the equations apply.

 

We are not going to throw out the equations just because you have an anomalous data point, but nor is it probable that the P-40 could have a CLmax so wildly out of line with other contemporary designs. 

 

If you believe that the 90 mph numbers and weight as quoted in your source is good - or even that PEC might mean the TAS is even lower - then you have to explain:

 

1) How it is - in terms of the physics of the wing, fuselage etc -  that the P-40 achieved such a high CLmax compared to it's peers

 

2) How is the high CLmax - and therefore high critical AoA - consistent with the other piece of empirical evidence, namely that the AoA at the three point attitude is greater than the critical AoA. (IIRC: trying to find the source for that)

In game this is true, but it would only take a few % increase to make it untrue.

 

The alternative view is that there is some bias or error in the data which accounts for the seeming discrepancy. This seems much more probable IMHO.  

[Venturi]

And where is your experimental data which would refute the boscombe trial data, above?

 

Physics apply to all aircraft aerodynamics, including CLmax. Provide evidence contradicting the Boscombe trials, that is all you need to do.

Edited December 31, 2016 by Venturi

[unreasonable]

The evidence is all the other evidence suggesting that aircraft of similar design had CLmax in the range of 1.3-1.4

[unreasonable]

Here is another PEC done by the USAAC in Sept 1941. It grossly lines up with the British test, I did a mockup on the data points to demonstrate and overlaid it upon the British PEC done in 1942 at Boscombe down [attached].

 

http://www.wwiiaircraftperformance.org/P-40/P-40E_40-384_PHQ-M-19-1300-A.pdf

 

This is exactly the sort of problem I am talking about - you have drawn a straight line through the data points from which you extrapolate a -ve PEC  at about 100mph - hence the claim that 90 IAS might actually be 85 TAS.

 

Alternately instead of simply drawing a line that joins the two ends of the black PEC curve you could have done a least squares line and ended up - I think, although I have not done it - with something at a more vertical angle, thus extrapolating to a lower -ve PEC at stall speed.

 

edit: I just did - running a least squares trend line through those data points (numbers taken from the text version) generates a slope that only crosses the PEC axis at about 65mph. It is +2 at 100mph so about +1.5 to +2.0 in our assumed stall range.

 

Actually the points can more naturally plot a curve at in which the PEC change is constant at lower speeds at about  +ve 2mph. In which case the 90 IAS becomes 92 TAS.

 

Note that the other PEC line over which you have overlaid the points is not a straight line: it plots a curve.

 

All this shows that determining a "right answer" from experimental data is not trivial.

Edited December 31, 2016 by unreasonable

[Venturi]

Again, 90mph ASI stall. I posted it twice. Did you not read the documents?

 

145kph stall speed is still CLmax 1.7...

Edited December 31, 2016 by Venturi

[unreasonable]

I said 90mph stall  "In which case the 90 IAS becomes 92 TAS". Did you not read my post?

 

My point is that your straight line extrapolation is not statistically the best straight line fit from the data points.  At a statistical best fit the PEC at 90mph extrapolates at about +1.5, so TAS at stall = 91.5mph.

 

That was in the edit to my post so you may have missed it. Do it yourself and see - easy enough in Open Office, I expect you have something similar.

 

As to whether it should be a straight line or a curve - it could be either: your Kittihawk graph on which you drew your line is a curve! 

 

Come on, you have done academic research, you cannot go around drawing an arbitrary straight line through the two extremes of a data series and claim that is the best fit. edit: sorry, you did not even do that - you drew a straight line between the two extremes of the underlying Kittihawk curve and claimed that was the best fit for another set of data, which is ridiculous.

 

All of this simply proves that we do not know what the PEC at stall point is - the sources conflict.

 

I agree, however, that a couple of mph either way or tweaking the weight is not going to close the gap between what the data generates in CLmax and the general expectation that the result should be in or close to the 1.3-1.4 range. 

To get 1.35 you would have to knock out 200kg of expended fuel and have a zero TAS of 99mph: ie what the game has now, near enough. 

 

The only other factor I wonder about is the height and temperatures at which the stall-test was done and whether the IAS is in the documents is giving a misleading figure but I do not know enough about the adjustments and effects of height to work that out.

Edited December 31, 2016 by unreasonable

[ACG_KaiLae]

Need more data. The current data points to a conclusion that even if accurate I don't believe that we could get anyone in 1C to listen to because it's so out of line with everything else. Remember if we can't get them to believe it then this discussion is pointless.

[ZachariasX]

Instead of having more data, we should much rather know what exactly they did as an experimental setup to produce given "real world numbers". Incomplete documentation of an experimental setup is a common disease in science. "Mutual understanding of common practise" is just an unerlying cancer discouraging proper documentation and compounding the misery that is often enough present to this day.

 

If you don't actally understand what was done in detail, how numbers were derived, then those results, plausible or not, have to be taken with a grain of salt. Or a ton of salt. They are not "the real world theory has to adhere to". Much in the contrary, established theory can help us to elucidate what the heck they really did. Knowing that, we know what data we we really have at hand. So far it is "calibrated" or "uncalibrated" data, whatever fits current ideas as we can tweak numbers easily to our liking.

 

If someone finds a valid description of the procedure the Brits used at Boscombe Down to produce the data we have at hand, then we can draw conclusions. More data is just food for more pages of a thread.

 

Neither theory or the experimental setup are "correct". They both are artificial tools to understand defined inner workings of a DEFINED SET OF PARAMETERS WITHIN A DEFINED SITAUTION. If this understanding makes it possible for making a reasonably exact prediction of how the outcome of a future experiment will be, then you have a "good" theory that is at least as good as your experiments.

 

People often enough imply an awful lot in their "truth". We better be modest and also try to fully understand the little thing we have at hand.

 

Proper interpretation of an experimental readout is often enough more difficult than preforming the experiment itself.

 

 

Until then, if John Doe comes back with the information that "the P-40 would stall at 90 mph", then we take it as it is: an advise.

 

 

 

[Dakpilot]

One thing I have not really seen is a 'list' of where the P-40 E is flawed in game, actual issues experienced rather than 'it is a bit of a pig'

 

There is no question that the engine limits are very tight and adhere to the manual very strictly, but at emergency power it is faster than a G2 with full fuel and all guns, leaving out the two guns and flying with fuel amount as recommended in various tests and manuals to keep within C of G limits due to the rear fuel tank helps again, in this respect as was done IRL, this will also help the 109  (less fuel) but at these weights this is at least representative of combat performance rather than max weights in 'pure' tests.

 

In short there is nothing wrong with the speeds it can achieve, just how long you can maintain them in game 

 

Should the P-40E outperform the G2 (in speed) for significant times in combat with more relaxed limits? and does that reflect what is known in historic combat reports?  It would be interesting how a de-restricted P40 would handle itself in combat in all aspects, with a much higher useable (permanent) power loading, Even with 10 minute combat power and 5 minute emergency power it would be a very different aircraft, but that is the subject of another thread.

 

As for flight model and turn rate debates I am not a mathematician or qualified aerodynamicist, but a simple pilot, and when I look at the specs of the P-40 compared to its adversaries in general terms, it has similar power to the E7, is bigger, has a more draggy larger airframe and weighs 800kg more. A very big generalisation would assume that it's wing must have a "similar" CLmax to other aircraft of the time, Don Berlin was a good designer but he did not come up with some sort of unicorn on the Hawk 75  wing design, it is based on solid principles and nothing unusual or revolutionary

 

At the end of the day what qualities does the P-40 use to overcome it's 800kg weight disadvantage?  other than tactics

 

The max turn rates given by Dev's are a like for like comparison but at 270 kph/168 mph which are not really combat speeds and all aircraft are not equally in their best performance envelope at this exact 270kph "best turn speed'' It is useful info but not really relevant to expected combat performance

 

Perhaps in addition to the theoretical discussions, two people with comparable and reasonable experience could do some friendly 'mock' dogfighting online P-40 vs 109E and F and give their experience of where the P-40 FM falls short, outside of a competitive online war or duel situation

 

Just a thought 

 

Cheers Dakpilot

[Holtzauge]

I think unreasonable put it very well in this post above: Even if you have empiric data you need to do a sanity check: If someone says someone else has run the 100 m sprint in 7.321 s my first reaction would be that a ballpark figure should be in the order of 9 to 10 s and a figure 2 s faster is off the charts so something must be wrong. It’s the same if you arrive at a reverse engineered clean wing Clmax of 2 for the P-40E: It simply does not comply with the laws of aerodynamics given what we know about the airplanes wing.

 

Looking at the P-40E IAS/CAS/TAS data on page 3 in the report Venturi posted, you can see that in this case the PEC is also given as a negative value but that it in this case means that a value of -5 means that the airplane airspeed indicator is showing 5 mph LESS than the “true” IAS or CAS if you prefer. So in this case a PEC of -5 means that you ADD 5 mph to the 147 water column value to get a CAS of 152 mph (Or in the case of the airplane where the is a calibration error or 3 mph relative the water column value you add 2 mph PEC).

 

So it could very well be so that the stall value of 84 mph in the Boscombe trial could be the result of human error: Someone reads a PEC value of -6 and subtracts that from the airplanes airspeed indicator read value of 90 mph and arrives at 84 mph when it should instead have been added to make 96 mph. I’m not saying that that is necessarily true but it would explain why the Boscombe reverse engineered Clmax is off the charts…..

Edited December 31, 2016 by Holtzauge

[Holtzauge]

<snip>

and when I look at the specs of the P-40 compared to its adversaries in general terms, it has similar power to the E7, is bigger, has a more draggy larger airframe and weighs 800kg more. A very big generalisation would assume that it's wing must have a "similar" CLmax to other aircraft of the time, Don Berlin was a good designer but he did not come up with some sort of unicorn on the Hawk 75  wing design, it is based on solid principles and nothing unusual or revolutionary

 

At the end of the day what qualities does the P-40 use to overcome it's 800kg weight disadvantage?  other than tactics

 

Exactly: While an iconic aircraft of flying tiger fame, the P-40E simply does not have very much going for it in terms of hard figures like wing- and powerloading so unfortunately it will come up short in most comparisons.

Edited December 31, 2016 by Holtzauge

[Venturi]

Hilarious.

[ACG_KaiLae]

Instead of having more data, we should much rather know what exactly they did as an experimental setup to produce given "real world numbers". Incomplete documentation of an experimental setup is a common disease in science. "Mutual understanding of common practise" is just an unerlying cancer discouraging proper documentation and compounding the misery that is often enough present to this day.

 

 

 

Hoping we will turn up documentation that directly lists the clmax of the plane itself. Or, perhaps, the P-36 clmax, which should be comparable?

[unreasonable]

Hoping we will turn up documentation that directly lists the clmax of the plane itself. Or, perhaps, the P-36 clmax, which should be comparable?

 

Or even the wing or airfoil! The plane Clmax cannot be higher than that.

 

Unfortunately the NACA reports I have been able to find do not show tests for P-40, it's wing, or the specific airfoils NACA2215 and 2209 used in the wing.

 

There is a report (get it here:   http://ftp.demec.ufpr.br/disciplinas/TM045/Report460_1935.pdf)which shows CLmax for NACA 2212 which is very close to 2215. Clmax listed as 1.60 - table on page 56.

 

Also looking down the list you see that where there are airfoils in a series listed by progressive thickness, eg  2412,2415,2418 etc (3rd and 4th digits being maximum thickness as % of chord), there is an airfoil with maximum Clmax and the variants with higher or lower thickness show a drop. For the other 2--- series listed, the best of series has CLmax of 1.60 - 1.62 so it would be surprising if 2215 was higher.

 

Given that part of the wing is 2209, and both sections cannot be at the maximum, the average for the two airfoils would probably be a little less, say 1.58

 

Then you usually get a few % loss in the actual model wing compared to the airfoils, say 5%, giving a hypothetical wing of Clmax 1.58 * 0.95 =  1.50

 

edit: btw, Happy New Year everyone - it is already 2017 where I am.

 

If that is the wing Clmax you might get in a wind tunnel test, I do not think it is possible for the plane to have a Clmax higher than that. Given other NACA reports that state that measured plane wind tunnel tests show a drop in Clmax compared to wing only tests of between 0.10 and 0.20 it is likely that the plane Clmax would be in the order of 1.50 - 0.15 = 1.35  .... which is exactly what it is in the game according to the developers' robot tests!

Edited December 31, 2016 by unreasonable

[Venturi]

Of course!

[Venturi]

It is all so easy once we realize that the period documents and the maths don't make sense, right? The evidence is there, let those with eyes see.

[Venturi]

155km/h = √((2*3846kg*9.81m/s²) / (1.225kg/m³*21.92m²*CLmax)) 

  155km/h=√(75459kg*m/s² / 26.852kg/m*CLmax)

  1854m²/s²=(2810m²/s²)/CLmax

  

  gives current in game CLmax of 1.52... assuming 155kph is right and 3846kg is right...

 

Which is better than every other aircraft, so obviously P40E is modeled correctly as the turn fighter it should be.

Edited December 31, 2016 by Venturi

[JtD]

WRT airfoil / wind tunnel tests, there's a comparative test between three wings with an aspect ratio of 6, one M6 profile, one 2215-2209 profile, one ClarkY profile. It's interesting because for one we're getting a typical wing shape tested (P-40 is 5.8 aspect ratio same airfoil) and as a bonus a reference with the ClarkY for which plenty of information is available. The tests were conducted at a Reynolds number of around 3 million, and the maximum lift coefficient given for the 22xx wing here is 1.6 and the ClarkY 1.67. The NACA 22xx series airfoil works with the same physics as all others and predictably performs in the same ballpark.

 

So what we need to look at is how the low stall speed on the real life P-40's was achieved. It is not because of free lunches in physics. Lift coefficients in the region of 2 are unachievable with flaps up and the engine at idle. So clearly, there's a particularity to the tests and/or the aircraft that messes up the results.

 

Technical Note 487

[Venturi]

Well, the PEC for the stated stall speed in the operating manual of the aircraft of 90mph, is estimated from drawing it in on the graph. At that speed the correction is not actually measured on any of the three or four PEC graphs for the P40 that I have found. However for the measured points they have all agreed, as in this one for the P40N, posted earlier [attached].

 

I agree that the wing CLmax is not the whole answer, that is why I have gone on and on about the "aircraft CLmax", anyways CLmax's above 1.7 were achieved by commonly used WW2 fighters - or so my aeroengineer friend says, that the 23015 airfoil as found on the F6F and F4U gave CLmax of ~1.75.

 

I believe what is going on is a combination of things including aerodynamics and non-historical engine performance.

Edited December 31, 2016 by Venturi

[Venturi]

WRT airfoil / wind tunnel tests, there's a comparative test between three wings with an aspect ratio of 6, one M6 profile, one 2215-2209 profile, one ClarkY profile. It's interesting because for one we're getting a typical wing shape tested (P-40 is 5.8 aspect ratio same airfoil) and as a bonus a reference with the ClarkY for which plenty of information is available. The tests were conducted at a Reynolds number of around 3 million, and the maximum lift coefficient given for the 22xx wing here is 1.6 and the ClarkY 1.67. The NACA 22xx series airfoil works with the same physics as all others and predictably performs in the same ballpark.

 

So what we need to look at is how the low stall speed on the real life P-40's was achieved. It is not because of free lunches in physics. Lift coefficients in the region of 2 are unachievable with flaps up and the engine at idle. So clearly, there's a particularity to the tests and/or the aircraft that messes up the results.

 

Technical Note 487

 

 

You mean this one? I note that the measurement of CLmax actually does not show a downwards inflection point, last point was at 1.62 or so. Good find.

19930081415.pdf

Edited December 31, 2016 by Venturi

[JtD]

... anyways CLmax's above 1.7 were achieved by commonly used WW2 fighters - or so my aeroengineer friend says, that the 23015 airfoil as found on the F6F and F4U gave CLmax of ~1.75...

And the aircraft had a clmax in the range of 1.2-1.4 with engine at idle. No WW2 fighter achieved clmax's that high without applying a significant amount of power.

 

And following up that statement, the P-40 manuals typically give two landing speeds - one for engine on, one for engine off. The latter typically being 10-20mph higher than the former. Additionally, instructions call for high engine rpm and a sufficient amount of throttle to maintain it during approach and landing. This way, a significant amount of prop wash is being generated, which would explain clmax's as high as seen. Therefore my theory is that the stalling speeds given in the tests and manuals are not for engine off or even idle, but include the propwash generated by flying the aircraft according to the procedures given in the manual. If that was the case, things would make perfect sense.

Edited December 31, 2016 by JtD

[Venturi]

And the aircraft had a clmax in the range of 1.2-1.4 with engine at idle. No WW2 fighter achieved clmax's that high without applying a significant amount of power.

 

And following up that statement, the P-40 manuals typically give two landing speeds - one for engine on, one for engine off. The latter typically being 10-20mph higher than the former. Additionally, instructions call for high engine rpm and a sufficient amount of throttle to maintain it during approach and landing. This way, a significant amount of prop wash is being generated, which would explain clmax's as high as seen. Therefore my theory is that the stalling speeds given in the tests and manuals are not for engine off or even idle, but include the propwash generated by flying the aircraft according to the procedures given in the manual. If that was the case, things would make perfect sense.

 

So, testing in game and in the Boscombe trials and in the manual should give exactly the same results, then? Stall at 90mph with PEC one way or another.

Since both would have idling engines with propwash.

Edited December 31, 2016 by Venturi

[Venturi]

Increasing engine power (thrust) to "maintain" the aircraft means... it's not gonna stall.

Doesn't make sense that this would be what was done to find "stall" speed... although a idling engine does.

 

You can see here that the throttle is completely off at stall... hear the power go to idle about 5 sec before stall, and the throttle is pictured being fed back in afterwards.

 

https://youtu.be/rki8tnsLodQ?t=11m40s

Edited December 31, 2016 by Venturi

[ACG_KaiLae]

WRT airfoil / wind tunnel tests, there's a comparative test between three wings with an aspect ratio of 6, one M6 profile, one 2215-2209 profile, one ClarkY profile. It's interesting because for one we're getting a typical wing shape tested (P-40 is 5.8 aspect ratio same airfoil) and as a bonus a reference with the ClarkY for which plenty of information is available. The tests were conducted at a Reynolds number of around 3 million, and the maximum lift coefficient given for the 22xx wing here is 1.6 and the ClarkY 1.67. The NACA 22xx series airfoil works with the same physics as all others and predictably performs in the same ballpark.

 

So what we need to look at is how the low stall speed on the real life P-40's was achieved. It is not because of free lunches in physics. Lift coefficients in the region of 2 are unachievable with flaps up and the engine at idle. So clearly, there's a particularity to the tests and/or the aircraft that messes up the results.

 

Technical Note 487

 

To me currently this is the 10,000$ question. How would you suggest to resolve this? My knowledge is pretty limited here. It would seem we would need more test data that would support or disprove the RAF information? Is there another thing we should do, test, or dig for?

[Venturi]

The other thing to think about, JtD, is that if the CLmax on the P40E was only higher from propwash - why then are the other aircraft such as the Bf109, which was tested in the same manner resulting in CLmax of 1.4, and which I confirmed using the CLmax equation as I used it for the P40E -- why is that test OK, and the P40's not?

[Crump]

So, another Clmax discussion? The last one was (surprisingly) productive in the end.

 

That is because the previous discussion resulted in learning on the behalf of several of the participants.

 

Some things to keep in mind for this discussion:

 

1.  Airspeed measurement was not accurate enough in the 1940's to calculate CLmax from a flight test.  That is exactly why Grumman used acceleration measurements and the normal force coefficient in their inflight measurements.  That data gave good agreement with their measured results for their wing design.  They did not use airspeed and weight estimations in a simply lift formula analysis.  It is simply NOT accurate enough.

 

2.  3% error in level speed (including the low speed realm) is typical tolerance for a 1940's era aircraft so 6% is twice the error.

 

 

 

If you calculate CLmax using a test of a plane (RL or game) then by definition you are calculating a whole aircraft CLmax - if you want wing CLmax that would have to come from a wind tunnel test of the wing only.

 

This is a big rabbit hole and a misconception.

 

This stems from a fundamental misunderstanding and not being formally educated in the science of flight.  The wing IS the CLmax of the design and is the value used in aircraft performance calculation.  It gives you the newtonian physics definition of WHAT the airplane WILL DO. 

 

Think about the fact in your mathematical system, you use the WING area as your reference area.

 

Aside from that very obvious fact, fuselage lift and the specifics of the tail design does have an effect on wing CLmax.  Fuselage lift is basically negligible and is not considered unless you want to include the lifting surface wetted area as part of your reference area.

 

It is simply a wash as the lift formula does not care where the lift force comes from in the mathematical mechanics.

 

The tail design and more importantly the CG location in relation to that tail design does make a difference.  In the realm of significant digits however that too is a huge rabbit hole unless you are examing a specific loading condition (CG location).

 

Basically, in a normal tail design where the horizontal stabilizer exhibits a downforce, the CLmax of the wing is decreased at a forward CG limit and increased at the rearward CG limit.  That change in Clmax is the result of tail downforce.  That is why Grumman gave a range of Clmax in their wing design.  The low range is the forward CG limit, the high range is the aft limit, and the middle value is normal CG position.  

 

The bottom line is the wing CLmax is what is used to determine performance and something the design team will get right or they will not end up with a viable aircraft.

[Crump]

 

 

23015 airfoil as found on the F6F and F4U gave CLmax of ~1.75.

 

And the FW-190....

 

That 1.75 for the F6F is a little high as Grumman clearly lists their wing design as having a Clmax range of 1.5 at the low end, 1.58 at normal, and 1.61 at the high end.  Their design specification uses a Clmax of 1.61 and the operating limitations which carry the weight of law, agree with that.

 

The significance of the operating instruction is the fact Grumman would be in a world of hurt if they published stall speeds in their operating instructions that pilots could not practically achieve in the air.  Grumman engineers would be heading to jail as US Navy pilots died. 

[Venturi]

Yes and the cg datum is given in the Boscombe reports too.

In other words, 90mph....

[unreasonable]

 

155km/h = √((2*3846kg*9.81m/s²) / (1.225kg/m³*21.92m²*CLmax)) 

  155km/h=√(75459kg*m/s² / 26.852kg/m*CLmax)

  1854m²/s²=(2810m²/s²)/CLmax

  

  gives current in game CLmax of 1.52... assuming 155kph is right and 3846kg is right...

 

Which is better than every other aircraft, so obviously P40E is modeled correctly as the turn fighter it should be.

 

 

These might be your in-game values but they are not the developers'.  Look at DD123 and Han's comments about robot tests. Surely we have to take their data as definitive.

 

Stall speed in flight configuration is given as 153-176 kph - it is already established (subject to official confirmation) that in-game there is no PEC..

 

There are four weights - the minimum 3264kg, full maximum 4414kg - "standard" is 3819kg. 3846kg appears nowhere: in your calculation you are taking very nearly the minimum of the range of speeds with a weight about 18% higher than their "standard". Of course this gives a high CLmax. 

 

I covered this in post 320:

 

      I take the minimum stall speed in DD 123  153 kph as being at the minimum weight: 3264 kg, > CLmax of 1.32

      At maximum stall speed - 176 kph I assume "maximum takeoff weight"  4414 kg. > CLmax 1.35

      Somewhere in the middle, "standard weight"  3819kg, speed (153+176)/2 = 165kph > CLmax  1.34

[unreasonable]

As for Crump's comments, this has all been covered ad-nauseam in Fw190 threads, so I will just summarize, I hope not misrepresenting anyone.

 

The maximum CLmax for the plane cannot exceed that for the wing, and for the wing cannot exceed that of the airfoil. Everyone agrees.

 

Actual wings have manufacturing imperfections, are not completely smooth and have lift interfering gun-ports etc. Planes have interference between wing and fuselage plus +/- lift effects from non - wing surfaces.

Consequently Clmax is always less for wing than airfoil, and for whole plane than for wing.

 

I think everyone agrees with that, the disagreement is about the size of the differences, especially between wing model and whole plane tests.

 

Crump argues that the wing Clmax represents the goal towards which the designers aspire, and that differences represent problems that are solved, so that the measured plane Clmax should be at most a few % lower than the wing.

 

Others agree that this is the goal, but point out that problems are not in practice always solved. This view is backed up by NACA empirical tests that show the 0.10-0.20 differences from wing - plane I previously mentioned.

 

Whatever your view is here, the developers clearly disagree with Crump and their game choices reflect this, I do not expect the P-40 to be treated any differently.