RE: Diving on downwind?
 

There must be something like that, a physical problem with the structure or the equipment, or the way the plane is being flown.
The wind can't do it.

RE: Diving on downwind?
 

A changing thrust line will cause all sorts of problems, including pitch issues, and yes, they could be induced by prop loading. However if you are inferring that prop loading changes depending upon upwind or downwind flight direction, no. Prop loading does not change upwind vs downwind.

The only way I can envision something happening based on flight direction is if you aren’t exactly doing downwind (i.e. slight cross wind) and compensating somehow with controls. For example, if the wind is cross just a bit, but you force a perpendicular flight path with respect to ground with rudder or by rolling slightly into the wind. If this was done, you are slipping or turning in the air to hold a straight line with respect to the ground. Say this slip, skid, roll etc. is to the left when going mostly upwind to compensate for the crosswind and to the right when going mostly downwind. If your firewall is flexing, the left vs right side load due to the constant turning could cause the firewall to flex differently, say it flexes up with left loading and right with down loading. This would cause the plane to climb upwindish and dive downwindish. But, this is not due to the wind, it is due to the left turn vs right turn, wind has nothing to do with the actual effect. You should notice the exact same climb/decent with left/right controls in dead calm. But, this effect could possibly make it seem like the wind was at fault because of the constant correction in a slight cross wind. In effect, this just another wind illusion, similar to my first post, only a lot more complicated.

RE: Diving on downwind?
 

Indeed, the wind may have only been a condition that I noted at the time and inferred that was the contributing factor. I am assuming the prop is working harder going into the wind, and pulling less in a downwind situation. After reading all this, I'm not sure if wind speed has any effect on the engine/prop?

RE: Diving on downwind?
 

You are correct in that last bit. The propeller works no differently when the model is flying into the wind versus when it's flying downwind.

RE: Diving on downwind?
 

Are there any very tall trees or obstacles on the upwind side of your flying site?

I agree that a plane doesn't know if is travelling upwind or downwind.........IF the air mass is laminar. But if the mass is turbulent and you are flying in "rollers" caused by obstacles , you will definitely feel the downdraft effect. In real planes, like Boeings, this effect can sometimes be felt after passing a moutain range. Models can feel the effect from trees.

Often the prevailing wind is from the south in the summer and from the north in the winter. ( At least in some parts of the country)

Recently the prevailing wind may have changed. Obstacles that were downwind earlier this year, are now upwind and causing turbulence.

Thats my guess, anyway.

RE: Diving on downwind?
 

There are lots of trees running along side, and near the strip, with rolling terrain and a pond. On the pit area side, we have the owner's large hanger( it's a private grass strip). There are a lot of updrafts and cross currents, and shifting winds off the nearby mountains. It's rare to have a consistent wind direction with the windsock swinging 90 degrees,or more, most days. Nevertheless, the prevailing wind is pretty consistent usually hitting the runway at 45 degrees. Smaller planes usually wind up doing a modified cross-wind t/o. The big models handle the wind quite easily. While I don't have a lot of hours, I've flown a full size sailplane, ultralight, and 4 GA planes-hi and low wing, and never had one want to take a 'nose dive' like this thing.