Flight Model problem: excessive glide ratio

[HWasp]

SOLVED!

 

Hi!

 

I have flown simple test flights with the new Mirage FM, that consisted of idle thrust descents from 10000 ft at different speeds.

Results:

At 300 kias glide ratio was 13,2 (AoA 5)

At 250 kias glide ratio was 14,9 (AoA 7)

At 200 kias glide ratio was 10,7 (AoA 10)

 

All descents on idle thrust holding steady indicated speeds.

 

Using the equation for total drag for delta wing in the attached pdf with an Aspect Ratio of 3,02 the total drag coefficient:

Cd = 0,0373 at AoA 5 (300 kias)

Cd = 0,0739 at AoA 7 (250 kias)

 

With these values and a weight of 10300kg (clean ac 80% fuel) and a wing area of 41m2 i have calculated the following Lift to Drag values

 

L/D at AoA 5 (300 kias) = 5,44

L/D at AoA 7 (250 kias) = 3,95

 

I don't expect my calculations to be accurate, but the difference here seems excessive to me (13,2 to 5,44 and 14,9 to 3,95)

 

The graph in the pdf shows roughly similiar Cd values for a generic delta wing design.

 

I have also flown the same test with the Belsimtek F-5 and the ED Su-27 for reference and have found the following glide ratios:

F-5:

at 300 kias 6,32

at 250 kias 7,91

 

Su-27:

at 300 kias 6,42

at 250 kias 7,13

 

I am aware that these are different airframes but the layout of the mirage does not justify such a great difference (13 to 6 and 15 to 8 )

 

I also did a short test to check if high idle thrust was the problem:

I turned the engine off at 1000 ft AGL 300 kias and measured the time to decelerate to 250 kias.

This took 33 seconds which I find quite hard to believe.

I restarted the engine with some delay and was able to stay in the air after more than 1 minute without engine thrust.

 

I fly the Boeing 737 for a living, which is no Mirage but still is a high performance jet with high wing loading. For me this aspect of the FM seems unrealistic.

The new Harrier doesn't have this problem (maybe because the constant flaps 5).

 

I am afraid that some drag values in this speed region are incorrect as this has been a problem with the old FM as well just not to this extent.

 

I would love to see some charts from RAZBAM that show L/D and drag coefficients if available.

 

I hope that my post will help to improve this great bird!

(or make it a flying brick :))

Expaero04(1).pdf

Edited December 15, 2017 by HWasp
problem solved

[EvilKipper]

You may be correct, though I want to point out that the mirage actually has very low wing loading for a high performance jet. At MTOW it's wing loading is 85 lb/ft2, vs the original 737's 117 lb/ft2, but you were not testing at MTOW, in fact there is no valid loadout in DCS that reaches MTOW as far as I know. At loaded, which is basically the CAP loadout it's only 68 lb/ft2, but you weren't testing at that loadout either, at the weight you tested the wing loading is only 51 lb/ft2. That's WWII prop fighter territory (109G-6 is 40 lb/ft2). Also you're simple delta equation probably didn't consider the leading edge slats. I wish we actually had the data, because I hate to speculate, but since we are speculating, I'll put on the other side of the scale that it's a really huge wing (actually just under HALF the area of the original 737), and aerodynamically it's a very clean shape. Maybe it actually is a pretty good glider. Maybe the numbers are plausible.

[Azrayen]

Nice tests :)

Issue now listed here: https://forums.eagle.ru/showthread.php?t=197059

= taken into account :)

[archangel1973uk]

Also note - you need to find out what the Mirage pilot's rule of thumb was for practicing this - whenever I'm gliding a jet IRL; there's an established estimated simulation for a dead engine. Idling produces a significant amount of thrust with forward airspeed and airflow through the engine and seriously elongates the glide as a result.

 

Normally it involves some combination of airbrake and/or flap with some given RPM

[HWasp]

Hi!

 

Thanks for the replies!

 

You may be correct, though I want to point out that the mirage actually has very low wing loading for a high performance jet. At MTOW it's wing loading is 85 lb/ft2, vs the original 737's 117 lb/ft2, but you were not testing at MTOW, in fact there is no valid loadout in DCS that reaches MTOW as far as I know. At loaded, which is basically the CAP loadout it's only 68 lb/ft2, but you weren't testing at that loadout either, at the weight you tested the wing loading is only 51 lb/ft2. That's WWII prop fighter territory (109G-6 is 40 lb/ft2). Also you're simple delta equation probably didn't consider the leading edge slats. I wish we actually had the data, because I hate to speculate, but since we are speculating, I'll put on the other side of the scale that it's a really huge wing (actually just under HALF the area of the original 737), and aerodynamically it's a very clean shape. Maybe it actually is a pretty good glider. Maybe the numbers are plausible.

 

Wing loading and glide ratio are not that closely connected. There are plenty of general aviation aircraft for example that have a low wing loading but have a glide ratio below 6. (PZL Wilga, Zlin-142). Also a Cessna 152 has it somewhere around 10 if I remember correctly.

 

The good old high wing loading 737 glides much better than those and does have a max. glide ratio around 14-16 (737-400, no winglets, no nothing). One of the more difficult things flying any airliner is to get it to descend. They are quite the gliders with clean wings.

 

Now the reason why the mirage cannot have the same glide performance as the 737 is the shape of the wing.

 

A delta wing (and other low aspect ratio wings) generate most of their drag due to the vortices forming around them (except very low AoA , well below 5).

Just google induced drag, vortex drag and delta wing aerodynamics. This is also the cause, why delta fighters bleed energy in turns like crazy.

 

Slats fitted to aircraft are present to reenergise the boundary layer on the upper surface of the wing in order to delay flow separation to higher angle of attack. It basically increases stall AoA, at the cost of additional drag, lowering the glide performance.

 

I have also done a test without engine power, if you read my whole post. That shows that high idle thrust is not the cause.

 

 

As for the speculation part:

 

I think I have provided you the document that I have used as source for the calculations, and also some of the numbers. I also noted that these calculations are not accurate, they are there only to show you that something is way, way off.

Calling this a speculation is not exactly polite.

Edited December 5, 2017 by HWasp

[HWasp]

Also note - you need to find out what the Mirage pilot's rule of thumb was for practicing this - whenever I'm gliding a jet IRL; there's an established estimated simulation for a dead engine. Idling produces a significant amount of thrust with forward airspeed and airflow through the engine and seriously elongates the glide as a result.

 

Normally it involves some combination of airbrake and/or flap with some given RPM

 

True, but the whole reason I have done the glide test and reported the findings is because the Mirage just did not want to slow down in level flight with and without the engine running, suggesting to me that some drag values in that speed region are wrong.

I am quite certain, that an engine failure at 1000ft AGL / 300kts would be a cause for immidiate ejection for most single engine fighters irl.

 

Normally I'm not the guy who is always comparing charts to DCS aircraft to find problems. If it feels ok, then I just fly it happily. I think that many aircraft in this sim have an extremely realistic flight model, certainly the best simulation currently available.

 

I'm very happy, that RAZBAM has taken note of the problem. :)

[myHelljumper]

Normally I'm not the guy who is always comparing charts to DCS aircraft to find problems.

 

I don't see problem doing that and your OP is very well done :thumbup:.

 

I think this is the cause of the too high STR we have currently, at peak STR we are 1 to 2 deg/sec (14~14.5 ingame vs ~12.5 in charts) higher than any chart available (BMS charts and internet charts).

[HWasp]

Thank you!

 

The new FM version has solved this problem as glide is now between 4,2 and 8,2 for the previously measured speed range. I'm quite happy with the results.

 

For me it's case closed.

 

Thank you RAZBAM!