flight dynamics problems

[Mathias Pommerien]

Uh no, it's so boring! I hope we don't need to go down that road.

[pilottj]

Even with the twitchy rudder I was able to get it out of Lukla, Nepal, climb to 12500', mostly in the clouds, and was able to make it to Kathmandu fine. The senitivities can make overcontrolling in turbulence easy, good thing there are no passengers on board...they would need the airsick bag in a hurry

LOL I was going to jokingly suggest adding an attractive female passenger...perhaps she is there to laugh at poor Heidi in the J3 as you and your lady passenger pass them and will get to the pristine bush strip long before they do

[Mathias Pommerien]

LOL I was going to jokingly suggest adding an attractive female passenger...perhaps she is there to laugh at poor Heidi in the J3 as you and your lady passenger pass them and will get to the pristine bush strip long before they do

LMBFAO!!!!
I'll put that on the to-do-list.

[sdflyer]

Hello

Sorry for being that obnoxious

Power on:
E.g. if you try to do a ‘flat’ turn with the rudder only at around 120kts she yaws extremely fast (about 90deg/sec) and if not checked with the elevator she will be in a 90deg nose down attitude after a 90deg turn.
If you just try to keep the nose down from dropping with a bit of pull during the flat turn, she will immediately perform a rapid flatspin like maneuver .
Full rudder produces a very high roll rate but the yaw rate is much less, a behaviour that’s normally only associated with fast jets and swept wing planes.
When performing a hammerhead you can start the rudder input at as low as 2-5kts and she will come around within 1-2sec.
If you apply full rudder on ground at e.g. 2-3kts she does a very fast (2sec) 360deg turn almost on the spot .

[/quote]
Please pardon for my ignorance, but what is "yaw rate"? My understanding that airplanes of this era inhibited less stability comparable with modern aircraft. The effectiveness of the rudders greatly depends on the size of vertical stabilizer which in turn define its effectiveness and "sensitivity". In other words, I would expect Me-108 be less statically stable that C-172 Although I have to admit I have never flown Me108 in real life it's hard to tell what to expect. Therefore, I wonder about what is assumption on how "things should be" based on?

Power off:
The roll rate with the rudder is almost as high as with the ailerons.
It’s not possible to do a slip at approach speed e.g. 80kts with full rudder because the aileron authority is too low to counteract the yaw.
On the ground at only 2-3kts with the throttle almost at idle the yaw moment from the rudder is excessive. Turns feel like there is a tailwheel steering.
If you look at the various videos with the 108 taxiing there’s quite a lot of rudder movement with only very little yaw at low speed as there’s almost no airflow over the rudder.

Yaw is airplane movement around vertical axis. Roll is airplane movement around longitudinal axis. In my understanding, aileron produces roll i.e. movements around longitudinal axis; therefore, technically speaking ailerons cannot counteract the yaw (although they can produced adverse yaw) Although I agree that rudder shouldn't be effective at the ground speed 2-3 kts I believe that steering on the ground is done through differential breaking not rudder. During the taxi rudder visible moves because in the most airplanes brakes are controlled through the top part of the rudder pedals.

Stall

In an accelerated stall she does a violent snap roll, almost like the Fw190.
Something that shouldn’t happen with the slatted wing and that didn’t happen with the 109 either.
She’s rather unstable along the longitudinal axis even in a ‘normal’ straight and level stall, again that shouldn’t be the case with the slatted wing.

Well I used to stall T-6 which ended up in a violent snap roll which was mostly attributed to the prop effects. Although I agree that slatted wing should delay separation of the airflow at the wing root I don't think they really can compensate for torque or P-factor

Once stalled there’s no stall break (trimmed for a 70kts glide) and the nose doesn’t drop with full up elevator.
Interestingly even when you release the stick she remains in the stalled nose high descent attitude

The nose would drop given that CG is not aft . Not sure if that the case with Me-108

[Mathias Pommerien]

just to throw in to the discussion, Greg has identified an issue with the conversion of the Cm_de and Cn_dr Datcom/Roscam Data to the M$ airfile, or better, in the silly way M$ sets those values 10 times higher just to put a dumper at another place to make up for the crap.
The result you see in the airfile is an over-sensitive rudder that also produces a bit too much roll, even though the basic tendencies are correct, as sdflyer's comments correctly indicate. also the effects of the slats are slidely under-modelled.
We will post a patch either later tonight or tomorrow that adresses the rudder's over-sensitivity issue along with some other cosmetics to the visuals that have been reported. The roll and slat queries will be adressed in a few days.

[Ebo]

Thanks Mathias for the explanation and for the efforts to get this resolved.

It is much appreciated.

Tausend Dank dafür!

[bstolle]

1. what is "yaw rate"?
2. My understanding that airplanes of this era inhibited less stability comparable with modern aircraft.
3. I would expect Me-108 be less statically stable that C-172
4. Although I have to admit I have never flown Me108 in real life it's hard to tell what to expect. Therefore, I wonder about what is assumption on how "things should be" based on?
5. Yaw is airplane movement around vertical axis. Roll is airplane movement around longitudinal axis. In my understanding, aileron produces roll i.e. movements around longitudinal axis; therefore, technically speaking ailerons cannot counteract the yaw
6. During the taxi rudder visible moves because in the most airplanes brakes are controlled through the top part of the rudder pedals.
7. Well I used to stall T-6 which ended up in a violent snap roll which was mostly attributed to the prop effects.
8. Although I agree that slatted wing should delay separation of the airflow at the wing root I don't think they really can compensate for torque or P-factor
9.The nose would drop given that CG is not aft . Not sure if that the case with Me-108

1. the speed at which the plane rotates around the yaw axis, same as roll and pitch rate
2. I don't see any reason for that with the 108. It's a very sound design. Also e.g. the Ar 96 was a very stable and nice trainer.
As my father used to say: (he flew the Ar 96 , Fw190 and Me163) there hasn't bee much improvement since the WWII trainers.
But that's a good point as I noticed that the fuselage doesn't oscillate at all after a strong rudder input which indicates that the yaw stability might be too high.
3. Well, especially concering stability you can't really compare a high wing and a low wing plane of the same class.
4. There are quite a few videos, pilot reports and as all neccessary data are known, it's a 'classic' design so there shouldn't be any surprises.
As Mathias had the support from real 108 operators I assume that they would have told him about non-standard etc... behaviour.
I assume the the flying characterstics of the 108 were equally important to Mathias as the rest of the plane.
5. Ok, we are splitting hairs here. The rudder induces too much roll on this 108 so to be 'technically' correct the aileron is not able to counteract the roll induced by the rudder/yaw.
6. As with all planes without tailwheel steering you can e.g. see the rudder being deflected fully to the right but the plane still turning left (without differential braking of course) due to the missing slipstream
7. Again you can't really compare a heavy T-6 with a higher power/weight ratio and a higher wing loading with the light weight and sleek 108.
8. As mentioned above she's already unstable during a normal stall where torque and p-factor are negligible (the slats hopefully delay the flow separation at the tip and not at the root)
9. I doubt that the 108 would have been certified if the CG would be out of the aft limit when loaded according to the flight manual

[bstolle]

just to throw in to the discussion, Greg has identified an issue with the conversion of the Cm_de and Cn_dr Datcom/Roscam Data to the M$ airfile, or better, in the silly way M$ sets those values 10 times higher just to put a dumper at another place to make up for the crap.

That's one of the main problems in FSX FD development. It's quite often simply not possible to use real world data as FSX loves to either interpret them wrong or not at all.
If it would be that easy there would be a lot of much better flying FSX planes available

[Mathias Pommerien]

just to throw in to the discussion, Greg has identified an issue with the conversion of the Cm_de and Cn_dr Datcom/Roscam Data to the M$ airfile, or better, in the silly way M$ sets those values 10 times higher just to put a dumper at another place to make up for the crap.

That's one of the main problems in FSX FD development. It's quite often simply not possible to use real world data as FSX loves to either interpret them wrong or not at all.
If it would be that easy there would be a lot of much better flying FSX planes available

Yes shure, we know that. The problem is a software-related compiling issue currently, not getting the understanding of aerodynamical basics straight.

[sdflyer]

Hello bstolle

1. Thank you.
2. Sorry again I don't know what is yaw stability means. I guess you are referring to directional(vertical) dynamic stability. It could positive, negative, neutral. For example if after pressing and releasing the rudder after few oscillation an airplane returns to equilibrium (initial state) it means it has positive dynamic vertical stability. I believe Me-108 exhibit positive vertical dynamic stability. On the other hand, how sensitive rudder controls in particular airplane is very subjective and depends on pilot's experience/proficiency. Opinions vary it's really hard to model something based on pilots impressions. However, those impression could be incorporated in addition to aerodynamic data.
3. May I ask you why can't I compare high and low wing airplanes? In my knowledge stability is not achieved by placing wing high or low or even middle For example, between high and low wing airplanes the better lateral stability will exhibited in one which has higher dihedral.
4. While I agree video may provide some interesting information, without the "numbers" it's very hard to determine actual airplane behavior.
6. Most so called tricycle gear airplanes ( those without tailwheel aka taildragger ) are actually have steerable nose wheel. The steering is coupled with the rudder pedals therefore when pilot steers the rudder moves as well. Slipstream does not control the rudder - pilot does.
7. It doesn't matter if airplane heavy or light if it has propeller on front of it it will be affected by toque, hygroscopic precession, slip stream and P-factor. When critical angle of attack is reached one wing could stall earlier than another. The reason for that could be an engine torque, a wind gust, weight and balance and etc.
8. Aft CG doesn't necessarily mean CG beyond aft limit. For example, P-39 by design had aft CG and therefore had particular flight characteristics By the way, a lot of modern airplanes today have aft CG among others MD-80s, CRJ, F-15 and etc. Generally speaking aft CG improves maneuverability and range.