Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Wmaker on July 27, 2012, 09:25:24 AM
-
Probably over a year ago as I was wondering about the Me410's high wingloading and how its stall speed and hence "manevuvering envelope" would compare to the aircraft already in AH. At the time there was a very nice website (www.worldofkrauss.com) with very nice plots and data on a huge number of airfoils including the airfoils to which the foils of both Mosquito and Me410 are based. Unfortunately, the website is at least currently offline and I can't double check or link the to the data I used. From those, I calculated the theoretical stall speeds at sea level for both using 50% fuel/stardard loadout weights. I arrived to figures that are almost identical which is 119.3mph for both aircraft.
Now I tested both Mosquito and Me410 in game using these weights. I did power-off stalls at 500ft by trimming the plane by hand using the speed where the shaking/buffet starts as the indication that the aircraft has entered into a stall. This is assumption is based on Hitech's comment on the stall buffet in AH. Everything here was done with aircraft in clean configuration as the lift coefficients of 1.10 for the Mosquito and 1.42 for the Me410 where obtained from the data (www.worldofkrauss.com). In my tests I noticed that the Me410 stalls practically exactly where I thought it would , at 120mph. However, Mosquito stalled at 107mph, roughly at 13mph lower speed than I had expected. I calculated using "theoretical altitude" of 0ft. The air density difference of 500ft adds ~2mph to the stall speed.
So HTC's modelling seems to agree with my calculation as far as the Me410 is concerned and Mosquito's Pilot's Notes seem to roughly agree as far as Mosquito is concerned. Weight with 50% fuel for the Mosquito is 18463lbs in game. In Mosquito's pilot's notes a stall speed of 105 knots (120.8mph) at sea level with 18000lbs weight is given. The weight of the Mosquito with 50% fuel in AH is 18463lbs. So based on that, the stall speed should have been slightly higher than what the Pilot's Notes mention but instead, it's clearly lower at 107mph in AH when the 500ft of altitude and 463lbs of weight should increase it, not decrease.
The relevant bit from the Mosquito's Pilot's Notes:
(http://i46.photobucket.com/albums/f147/Wmaker/Mossiestall.jpg)
P.S. I know my explanation could have been a lot clearer but I just wanted to get this out here. It does contain the data though.
-
They might simply be defining the stall at a different point. Compare when the buffeting starts and it's pretty close, 2 mph, if I read you correctly.
-
They might simply be defining the stall at a different point. Compare when the buffeting starts and it's pretty close, 2 mph, if I read you correctly.
I think the 2mph is his adjustment for being at 500ft instead of 0ft.
-
They might simply be defining the stall at a different point. Compare when the buffeting starts and it's pretty close, 2 mph, if I read you correctly.
As Karnak said 2mph was the difference between sea level and 500ft per my calculation (1.25kg vs. 1.20kg diff. in air density). I have to find that quote fromHT but he said that the aircraft in AH as entered stall regime when the buffet starts. Anyway, altitude for altitude, weight to weight, the difference per my test/calcs is ~16.7mph which is quite significant difference when talking about aircraft's stall speed. Using the buffet as baseline makes my testing more conservative but it is based on HT's statement. I have to try to find that quote.
-
The stall regime is a range of speeds where drag increases without increasing lift, it's not a specific speed. That's why I suggested comparing the speed where buffeting starts. Unless you know how the stall was defined for the pilot notes it will be difficult to compare speeds directly.
-
The stall regime is a range of speeds where drag increases without increasing lift, it's not a specific speed. That's why I suggested comparing the speed where buffeting starts. Unless you know how the stall was defined for the pilot notes it will be difficult to compare speeds directly.
Heh, the buffet starts when the aircraft just barely keeps the altitude and variometer drops right after. I sure pilot notes mark a speed where the aircraft stalls ie. can't maintain level flight. At speed mentioned in the Pilot's Notes, Mosquito is in slow level flight in AH.
-
Can you post a link to that 1.1 lift co? That sounds way out of range of typical wwii aircraft.
HiTech
-
What is the normal range for WWII aircraft in terms of lift co?
-
nm
-
Can you post a link to that 1.1 lift co? That sounds way out of range of typical wwii aircraft.
It indeed does sound like it, but when looking at the airfoil itself it comes clear why it is lower than the contempory airfoils. Like I said, unfortunately the website where I got the data from has gone offline. RAF 34 was a very thin airfoil with a very small radius of curvature of the leading edge, which largely contributes for the small lift coefficient. It isn't really surprising that the Pilot's Notes are consistent with that.
In fact, that data from the Pilot's Notes would suggest that it is even lower.
I hope I could post you the shape of the air foil but I can't find a good pic at the moment, it would explain a lot more. It's much thinner, almost like a WWI-type airfoil. It truly differs from the normal NACA airfoils of WWII.
-
RAF 34 section below. Bear in mind the Mosquito officially was RAF 34 (mod), due to the difference in section at the radiator. Will see if I can find a view of same.
(http://i937.photobucket.com/albums/ad212/mhuxt/RAF34.jpg)
CL and CD graph for RAF 34 here - don't know if it's of use.
http://www.hzdr.de/db/Cms?pOid=12531&pNid=1680
-
RAF 34 section below. Bear in mind the Mosquito officially was RAF 34 (mod), due to the difference in section at the radiator. Will see if I can find a view of same.
(http://i937.photobucket.com/albums/ad212/mhuxt/RAF34.jpg)
CL and CD graph for RAF 34 here - don't know if it's of use.
http://www.hzdr.de/db/Cms?pOid=12531&pNid=1680
That is more what I would expect it to be.
HiTech
-
http://img846.imageshack.us/img846/4540/raf342.png (http://img846.imageshack.us/img846/4540/raf342.png)
http://img716.imageshack.us/img716/8851/raf231.png (http://img716.imageshack.us/img716/8851/raf231.png)
http://img444.imageshack.us/img444/9923/raf34.png (http://img444.imageshack.us/img444/9923/raf34.png)
-C+
-
http://img846.imageshack.us/img846/4540/raf342.png (http://img846.imageshack.us/img846/4540/raf342.png)
http://img716.imageshack.us/img716/8851/raf231.png (http://img716.imageshack.us/img716/8851/raf231.png)
http://img444.imageshack.us/img444/9923/raf34.png (http://img444.imageshack.us/img444/9923/raf34.png)
-C+
Thank you Charge! I didn't have the sense to save them. :(
This is the relevant bit:
(http://img846.imageshack.us/img846/4540/raf342.png)
Here's the general shape of the profile:
(http://img444.imageshack.us/img444/9923/raf34.png)
Me410's profile's general shape at the root for comparison:
(http://www.laboratoridenvol.com/info/airfoils/NACA-23018.png)
EDIT/Oh, and thanks Scherf for that doc!/EDIT
-
Doesn't a 1.1 Clmax require a thin symmetrical airfoil? Sherf's link seems to show at least 1.35 Clmax for the same cambered RAF 34 section.
-
Doesn't a 1.1 Clmax require a thin symmetrical airfoil? Sherf's link seems to show at least 1.35 Clmax for the same cambered RAF 34 section.
1.1 Clmax can be a result which can be arrvied to any number of ways. The link Scherf provided indeed shows higher Clmax, on the other hand the data I used shows roughly 1.1 here:
(http://img846.imageshack.us/img846/4540/raf342.png)
I've noticed that the Clmax figures can vary between sources for number of profiles. I think that is partly due to the fact that sometimes they are theoretically given for AoA which the profile is unable to achieve without flow separating.
The stall speed in the Pilot's Notes agrees with the data above. Two separate sources agreeing with each other and the rather big difference between the game and primary source data were the reasons why I started this thread.
-
Mossie uses the 34 mod as Scherf mentioned which I'm guessing is referring to the inner wing section. When you look at the planview the inner wing is pretty impressive and modeling the whole area as an RAF 34 section seems like it might underestimate the lift coefficient. It seems like the 1.35 in Scherf's link might be conservative depending on the radiator's effect on the airflow.
-
Wmaker that chart can not be anything from a real test, looks more like someones attempt at fluid dynamic estimation.
The reason is simple look at that chart at 8 degrees. It shows a decreasing Lift co with increasing AOA, this does not happen pre stall.
2nd
You must look very carefully at pilot notes. If it was giving an IAS (which is the most useful when flying the plane) It does not serve much purpose in analysis with out knowing the CAS.
HiTech
-
Wmaker that chart can not be anything from a real test, looks more like someones attempt at fluid dynamic estimation.
The reason is simple look at that chart at 8 degrees. It shows a decreasing Lift co with increasing AOA, this does not happen pre stall.
Yeah, it definately is from some CFD software because the plot isn't smooth. What I thought it would give rather accurately is the point where the lift starts to decrease.
You must look very carefully at pilot notes. If it was giving an IAS (which is the most useful when flying the plane) It does not serve much purpose in analysis with out knowing the CAS.
Good point. The speed measurement most probably isn't entirely accurate but it is good for the pilot to know what speedo indicates as the aircraft stalls. The AoA between the pitot tube and the incoming airflow probably can increase this error even further (That error is also modelled into game, though).
Some Mosquito stall speeds from a book about the war time history of Boscombe Down (British flight testing establishment):
It does not mention wheather these were IAS or calibrated.
W4050 (Prototype, Feb '41) flown with "modest weights"
105mph, clean
90mph, landing config
(considered high at the time)
DZ594/G, 17650lbs
115mph, clean
100-105mph, landing config.
PR.XVI: DZ540, 22300lbs
132mph, clean
118mph, landing config.
MK.XIV: ML937, 25279lbs (Cookie onboard)
143mph, clean
-
What is the normal range for WWII aircraft in terms of lift co?
For production U.S. fighters the coefficient range was roughly 1.4-2.5 iirc.
-
"The reason is simple look at that chart at 8 degrees. It shows a decreasing Lift co with increasing AOA, this does not happen pre stall."
Assuming RAF34 resembles NACA 63-412 at abt. 8 degrees the profile comes out of "low drag bucket" (which is similar to that of P-51 due to symmetry of the leading edge) where the lift does not increase but there is a somewhat sharp rise in profile drag and I assume that is what is seen in the figure.
-C+