Guys, i have a new fibreglass fuselage of the Extra 300L which is designed for gas engines of 50 to 80 cc,s . the plan states no right thrust to be added when mounting the engine but when i measured the fuse it has left thrust built in plus the fin also has abit of left and not dead straight ahead. i thought normally right thrust is built in but not left , any help ? thanks.

Since you are in the southern hemisphere, can we blame the Coriolis effect? Otherwise I would suggest a mistake made by the mold maker.

The fuselage ? has left thrust? in the firewall?

The fin not being straight is a clue. Are there any model designers who used fin offset for any model?

Freeair,

Both things seem to be wrong:

The fin will force the fuse to crab to the right when the engine rpms are low.
For all engines rotating counterclockwise, some forces develop at high rpms and take-off that tend to yaw the fuse to the left.
At high rpms, the efects of the fin and the left thrust will be fighting each other.

Can you contact the manufacturer of the fuse and the designer of the model?

The left angle on the fin makes sense to me. A lot of full scale airplanes have the fin angled a few degrees to the left, to line up with the spiraling slipstream from the prop. Normally this slipstream spirals down under the fuse and back up and over the left side of the fuse, striking the fin, causing the tail to be forced to the right, hence the nose to the left. A little left angle on the fin will eliminate this force. Model designers build in right thrust for the same reason, as the angle of the slipstream misses the fin. The left thrust is a mystery to me. Are you sure you didn't measure that left thrust while the fuse was upside down?............sorry!!

Lm

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Actually, you can't build in enough thrust in any direction to get the slipstream to miss the fin.

And the angled fin..... Do you know of any model airplanes that have angled fins? I've not seen one outside of pure scale models and a few free flighters in a very long time in the hobby. But heck, I certainly haven't seen every R/C model made.

What most models have right thrust for:
This is a very often repeated test included in flight tests for trimming model flight. It shows that for most, the thrust is a response to an aerodynamic action called p-effect. P-effect is a result of the prop pulling better on one side than the other. It happens when the airplane pitches up or down. As you see from the trimming advice above, it happens, and changing the thrust cures it.

It's funny, but there is no test for p-effect in any other direction.................... left for example, or down.

Not to mention, the type of aircraft you have is typically set up 0-0-0. You are trying to achieve a neutral aircraft. I believe it is poor quality on the part of the manufacturer. The thrust could be dealt with but the fin alignment would cause me to send it back.

The trimming advice above suggests adding more right thrust , or reducing right thrust. Neither will have any effect at all on P-factor.
Only down thrust will reduce P-factor.

Lm

There are two results. One resists the effect and the other alters it. Change the angle at which the prop encounters the slip and the direction of the result of the p-effect is changed. Change the angle of the thrust and it counters the force. Two different reasons for the effect.

As for down thrust.......... P-effect isn't actully up or down. Our description of it usually is, and therein lies the rub. P-effect actually happens when the plane is yawed. And it'll then cause an up or down force instead of the left right force it causes when the pitch axis is affected. So there isn't just one way (or direction) that reduces p-effect. The direction that reduces it is a direct function of what change of axis induces the effect.