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JtD

Flaps and drag

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Coming up a short and entertaining post on flaps and drag, only that it's going to be long and probably boring. I did a few tests in game, derived some drag figures from it and compared it to real life. The focus is on the change of drag with flaps deployment. For those who don't want to read the entire thing, just go to the last paragraph. It contains results and discussion.

 

Motivation:

As you know we do see some hard to believe low speed flaps down turning performances in game, and drag is one major factor contributing to turning performance. My personal expectations also differed from what we see in game, in that I expect less drag at low flaps settings and more drag at high flaps settings. So I put it to the test.

 

Test object Fw190A:

The main test object for me was the Fw190A-8. I picked it because for real life reference, I have wind tunnel test data as well as a Focke Wulf table, that contains the major aerodynamic properties. Also, the NACA 230xx series airfoil used on the Fw190 is fairly well documented, also with various flaps. So, what data do we need, what have we got?

 

Fw 190 real life data:

Drag table:

The drag table is freely available, so I'll walk you through that. It contains zero lift drag for two configuration, one is fast level flight (rads closed, for instance) and one is climb (rads open). It's given in drag coefficient times reference area. Since in game I tested rads closed, I went with the fast level flight figure of 0.485.

It also breaks down the sources of drag, important for us is that the wing creates 0.1647 of the 0.485 total parasitic drag, so roughly 34% of the total drag. This figure is quite representative for the average WW2 fighter aircraft, it's typically 30-35%.

Additionally, further down it gives us the wing area effected by the flaps, which is 50.6% of the reference area of 18.3m², so about 9.2m². With the actual wing area being 15.6m², the flaps effect roughly 59% of the actual wing.

It also tells us what effects landing gear and flap settings have. They add

Gear: 0.55 (not of interest)

Flaps 12°: 0.09

Flaps 58°: 1.50

to the total drag of the aircraft (to the 0.485 from above). If we divide the totals by reference area, they come out as:

Flaps up: 0.0265

Flaps 12°: 0.0314

Flaps 58°: 0.1085

 

bla026.thumb.jpg.11a6042b6b3106b89144f5222a8c1927.jpg

 

Chalais Meudon wind tunnel data:

In a similar manner, we can derive the same data from the wind tunnel tests conductes with the Fw190A-3 in the Chalais Meudon wind tunnel. I'm not sure about copyright, so I'm not going to post more than just the figures. It contains full polars (i.e. lift/drag relationship). Interesting for me were the figures at a lift coefficient of about 0.5, and I read them as (just parasitic drag):

Flaps up: 0.0271

Flaps 12°: 0.0351

Flaps 58°: 0.0951

Which is a pretty good match with the drag table.

 

NACA Airfoil data:

Another source that always is of interest are wind tunnel data for airfoils. For this there are many reports around, I picked one from NACA, TR 664, which contains "wind tunnel investigations of a NACA 23012 airfoil with various arrangements of slotted flaps". Actually it does not only contain info on various slotted flaps, it also contains info on a 23012 with a 20% chord split flap, which happens to be very close to the arrangement on the Fw190 (23015 with 20% chords split flap, so a little bit thicker wing, but same flap arrangement). From there I read:

Flaps up: 0.0105

Flaps 12°: 0.025

Flaps 58°: 0.173

bla027.thumb.jpg.9357ad2aa40867ef49643288291c3ae4.jpg

This is just profile drag of the plain airfoil, and if we multiply it by relevant wing area (not Fw reference area) we should get something out of it:

Flaps up: 0.0105 * 15.6 = 0.1638 (Fw drag table says 0.1647 wing drag, so yes, this works)

Flaps 12°: 0.025 * 9.2 + 0.0105 * (15.6 - 9.2) = 0.2972 (Fw drag table says 0.1647+0.09 = 0.2547)

Flaps 58°: 0.173 * 9.2 + 0.0105 * (15.6 - 9.2) = 1.6588 (Fw drag table says 0.1647+1.50 = 1.6647)

Guess that worked out pretty well. So for drag coefficients, we need to add the drag of the rest of the aircraft (taking the drag table here, so 0.485-0.1647 = 0.3203) and divide by reference area:

Flaps up: 0.0265

Flaps 12°: 0.0337

Flaps 58°: 0.1081

As it is, a very good match with the other two data sets. Which is particularly important, as we can use this approach for other aircraft, were we don't have as much data as on the Fw190 and be certain to land somewhere in the proper region.

 

Fw190A-8 in game test:

How do I get some figures from the game to compare to the real life data? Well, fortunately I don't have to invent a method, because back in the day they also did those types of test. For our question, the answer is glide tests with the three flap settings we're interested in. Based on the altitude loss we can then calculate the drag. So I went into the game and dived the Fw190A-8 with power at idle (that's important to state as the prop condition can have a large effect on the drag) from 900m to 500m at 300km/h indicated air speed. I then hacked all that into an spreadsheet, which looks as below.

First, this is the energy we lose as we go down at 300km/h, with a plane of 4391kg mass, from 900m to 500m. Relevant is the number on the bottom right.

image.png.6c80f270f328ed5bbf1b875df95ec11b.png

Then we get to the aircraft performance - below the times I got. With these times, I can calculate how much energy I burn per second ('powerloss'). Divided by speed, it gives the drag force acting on my aircraft. I used the mean speed of 310km/h as a simplification, but it's a 1% simplification and my timing is less accurate than that. The total drag includes parasitic drag (no lift drag) and the induced drag (lift related drag). As all our figures about are parasitic drag, we have to calculate induced drag, which is fairly easy if you do it to the accuracy necessary here. Also of interest is the lift coefficient, as we want to know what data point we want to look at in a polar. I used 0.5 above, game test was at 0.52 - close enough. Anyway, if we go back to the induced drag, we have to subtract it from the total drag to arrive at the parasitic drag. Then we only have to divide it by the reference area of 18.3m², and we have the drag coefficients we can compare to the above real life data.

image.png.e3b9c2831ae7423209e6b727537cd0f7.png

 

 

Fw190A-8 comparison

Now we have all we need and can compare game data to real life data, which I summarized in the below table (and a chart, because FM discussions aren't complete without it). Since the airfoil based correction (i.e. the difference between flaps up and the two other flap conditions) can be added to any flaps up condition, not just the FW drag table, I've also added the correction to the clean condition as tested in game. As we can see, the drag in game in clean condition is a bit higher than what real life data suggests, but this may easily be explained with testing differences. So we shouldn't get hung up on this. What we want to look at is the effects of deploying flaps, and it can be seen, that the figures in game are WAY higher than real life data shows. This is true both in the 12° take off flap setting, as in the 58° landing flap setting. Now you can look at the absolute figures and find them off, personally I prefer to look at the relative change. As you can see, the use of 12° of flaps in game increases drag by 75%, where in real life figures were in the range of 25% - so an error factor 3. At landing setting, we're approach 400% drag increase in game, were real life figures were in the range of 300%, so an error of factor 1.3.

image.png.fa9d5d3de59c291f8270225cf69fdda4.pngimage.png.5888fda94290b0d861b8aa8b5781d36a.png

 

Imho, an error of factor 3 is just too large, given that 5% is the target margin for accuracy. In terms of gameplay, the Fw190A-8 does 575km/h clean at sea level, should do 535km/h with flaps at 12° and does 470km/h. So it's near 15% off. This is pretty much exactly as I expected, even the margin works out. With flaps fully down there also appears to be too much drag. In level flight, we'd be at about 335km/h instead of 305km/h, so also a 10% margin. Which is pretty much the opposite of what I expected, so even I learned something from this test.

 

Now this test could be repeated at lower speeds for higher lift coefficients and then compared again, so we have a clearer picture for turning performance, but right now that's all I want to do. Haden't even had a cup of coffee, yet, let alone breakfast.

Edited by JtD
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Boring ?????

 

Your posts are ALLWAYS a chance to learn and food for my thoughts!

 

Thanks for the excellent essay !!!

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Your posts are gold JtD thx for your time, it could not be better explained.

 

A pleasure to read.

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Neat analysis JtD! Very interesting to see that BOX is actually overestimating the drag due to flaps for the Fw-190A8. However, since it seems that turn rate a lower speeds is overestimated (at least for the P-47) this would indicate that thrust is being overestimated. As a happy coincidence, I actually happen to have a figure showing thrust  as a function of dynamic pressure for the BMW801D at 1800 Ps. Alas it is just a figure and I don't know the provenance so its difficult to judge how reliable it is but its the best I've got. IIRC then you did a test with the P-47 earlier at 240 km/h IAS and if I read of the thrust at q=272 Kp/m**2 (240 km/h IAS at SL) I get a thrust of around 1400 Kp. By my estimate that corresponds to a prop efficiency of around 0.69. This is pretty low and I would not be surprised if the prop efficiency used in BOX at that speed is higher.

 

For the Fw-190A8 at 4400 Kg I get a best SL turn time of 21.8 s at 240 Km/h IAS with 60 deg flap in my C++ simulation. Going faster or slower than that increases it, e.g. at 220 Km/h IAS it goes up to around 24 s and at 260 km/h IAS to around 22.5 s.

 

Maybe someone can volunteer and do some tests in-game with 60 deg flap full fuel load and see what SL turn times you get at 240 km/h IAS and then going faster and slower than that to get some more data points?

 

Addendum: The C++ numbers above are for 1,42 ata boost.

Edited by Holtzauge
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Thank you all for your positive feedback. I was going to take on and post about the P-47 next, but found some very implausible things which I need to understand before I interpret.

 

Holtzauge, about thrust - I think it's worth starting a similar topic about low speed thrust. I have a bit of data on this, one bit is in the Fw190 drag table, where you can find the "Schraubenschub" which I take is the thrust provided at standstill. I was already planning on it, with a bit of simplified prop theory included. But all that takes time.

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Just sent a PM with the BMW801D thrust figure attached: I guess you already have the "Widerstansdaten von Flugzeugen" table with aerodynamic data? If not I can PM that as well. If you compare the static thrust in that table for the Fw-190 A8 (1836 Kp) with the figure I just sent you, you can see that the averaged thrust line they have drawn into the diagram hits ca 1836 Kp at q=0 Kp/m**2. This leads me to suspect that the figure I sent you is reliable and that the drawn line is used for T/O calculations and the like. Interestingly, in the figure the "actual" thrust line is much lower initially and then getting higher as the plane begins to move, probably due to the prop being unable to absorb all the power until some forward speed is gained. Anyway, whats interesting is how low the actual forward thrust is, i.e. the prop efficiency, at lower speeds.

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Utterly fantastic. Thank you so much for this.

 

Regardless of whether the devs act on it, please know that you could post 100x more of this kind of detailed analysis and I would enjoy reading it all.

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Just to supplement JtD analysis, here's a neat NACA chart on amongst other things the L/D of different flap designs:

SaRyeGq.jpg

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Could this not be due to a overestimation of the flap lift - and thus a overestimation of the lift induced drag? 
 

Also, it appears that the flaps interact with the boundary layer so as to have a non-linear effect, at least in the initial few tens of degrees.

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really interesting.

 

Will be very interesting know results for more planes.

 

thx for your work

Edited by 666GIAP_Tumu

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