Problems with x-wind while on ground ?...

I would like to report a strange behaviour I have noticed when performing takeoffs with significant x-wind components.

 

My RL experience comes from the eighties, on the Auster OGMA taildraggers used for glider towing, and also from a Super Cub, but I guess it is sufficient to find strange two aspects about the physics model not only in the Dora but also on the previous versions of the p51d, and yes, I know we cant compare those taildraggers with either a p51d or Dora, but the same basic principles apply :-)

 

1) The aircraft on the ground, particularly during the takeoff run or landing, behave pretty much as if they had a heavy weight on their tails, with the CoG way behind the CoL. Yet, I learned to live with it although when I tried the Dora for the 1st time, I noticed it had inherited that same feel...

 

2) When we set a significant x-wind, with a component nearing 10m/s for instance, during the takeoff run the aircraft will always start to veer significantly downwind, and only after you manage to control it, because the upwind wing also goes down instead of tending to rise, you finally get a bit of weathervaning like you should have felt right from the beginning of the takeoff run. This is true no matter if the x-wind component comes from 9 or 3 o'clock...

 

Since I didn't recall this happening before with the p51d, I decided to test it and found exactly the same results in the latest version of DCS World that was upgraded with the Fw190 D9.

 

Say you are in the rw, with wind from, not necessarily 09:00, you can even test it with, say, wind at 10m/s from 10:00, and start your takeoff run. The aircraft will veer to the right, and the upwind wing will want to go down. This ( the left wing going down ) could be explained by the torque, but if you repeat the same test now with with from 03:00 or even 02:00, at 10m/s, the aircraft will veer to the left, and your right wing will want to go down???

 

I think there's something wrong here. The aircraft always veers downwind ( ? )

 

Ah! I have no takeoff assist or rudder assist.

 

P.S.: Setting wind in the Mission Editor in DCS World is always "different" because it doesn't follow the usual convention of the true magnetic bearing, but rather "where it goes to...". You have three "clock" dials with an arrow inside, parallel to the wind direction you want, and looking at the rw you can then drag it to adjust the x-wind component to you liking.

Ok, I know this effect and it's an effect of the deflected slipstream. As the plane is on the ground with x-wind it means that its sideslip angle is high. THe slipstream axis is not coinside neither with plane axis nor the incoming air but it is placed somewhere between. That's why for wind from starboard the left wing area the slipstream acts for will be greater than on the righ wing. It leads to more lift and more drag.

THis effect is very typical for the powerful prop planes. For example, XP-51 report contains some diagrams for the steady sideslip at power-on and power-off conditions. As the power-off curves are similar to the jets and gliders (the more is sideslip angle the more reverse stick is required) the power-on curves show that stick input is flat or even reversed.

Thx Yo-Yo, anyway, I don't think this "cant" in the slipstream should account for the pronounced "counter-weathercock", at least on the "usual" GA taildraggers... maybe it's different on this powerful prop aircraft?

 

With a x-wind component of 10m/s I would expect the aircraft to promptly want to weathercock ( thus turning upwind, and not downwind ) ?

The average slipstream velocity at TO power is about 30 m/s. I think it has sense, though it reduces as the speed increases. That's what I can say. THe slipstream is truly calculated.

If somebody performs a test for a not so powerful plane in coordinated sideslip it would be easily to explain this difference.

Fixed GoPro over the stick, a ruler to calibrate before or after the test- it's a good replacement of a flight recorder, I think. :)