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-   -   Thrust Vector Argument, please help! (https://www.rcuniverse.com/forum/aerodynamics-76/11683961-thrust-vector-argument-please-help.html)

kram 10-21-2020 07:46 PM

Thrust Vector Argument, please help!
I'm embarrassed to publicly admit I don't know this, but here goes:

If a single-engine tractor prop plane comes ARF with a +3 degree up incidence (crankshaft vs horizontal stab), What kind of elevator trim will be needed at med-hi throttle???

Specifics: NIB Seagull Yak 54 put together nicely by a friend who asked me to help tune the Saito 125 and do the maiden flight.

CG specified by kit was 3/4" behind the wing tube and acted tail-heavy, but flyable by pilot experienced with intentionally tail-heavy aerobats. I noticed that on landing approach, but during straight flight, main problem is it required TONS of up elevator trim. I eyeballed the crankshaft after landing and it seemed a little "off:" measured 3 degrees UP incidence crankshaft vs horizontal stab/turtle deck. Does that make the wing pull the plane down??? Lots of maiden crashes in the old thread, not all about the above issues.

I dont want to tweak two major things at once, so I recommended moving CG forward, then start putting washers on the firewall if trim still off


speedracerntrixie 10-22-2020 04:26 AM

The only airplanes that I have seen the took any up thrust were bipes and then only around 1/2 degree. 3 degrees is extreme. Tail heavy AND up trim do not make sense at all. I would suggest going back to square one. Level out the fuselage using the canopy frame as a reference. Set the stab to zero, engine to zero but with 3 degrees right thrust and wings at .5 degree positive. Set CG between 28% and 30% MAC. The Yak is a bit short coupled so keep elevator throw to around 15-20 degrees for your first flights.

kram 10-22-2020 06:06 AM


So, do you think UP thrust would pull the plane up or down?

Think about what a 0 wing would do if coupled to a +3 thrust vector pulling level

Right-Left on the engine is 0 and it didnt pull left on TO (or perhaps I just compensated subconsciously), so I think Ill leave it there to avoid changing too many things simultaneously


BMatthews 10-22-2020 12:09 PM

Wait... We don't normally consider the thrust line to the stab. You'd want to compare the thrust line to the wing airfoil's center line. And then compare the wing's center line to the stabilizer center line.

If this were a model set up to be flown by a pilot skilled at aerobatic flying the overall setup would be 0-0-0 for thrust-wing-stab along with a couple of degrees of right thrust to counter the torque to the left. And the pilot would trim the CG so the model was neutrally stable in pitch. And that would be a nice setup for aerobatic flying.

The fact that ARF producers sell to a wider variety of skill levels the model is likely set up with the thrust and wing at nearly neutral. And the stab at some slight negative angle. Or the foam might simply be tweaked or the assembly was done not that great. If the thrust line has some upthrust compared to the wing airfoil then that's very likely not right. Or at least it certainly would be wrong for many other designs.

kram 10-22-2020 12:59 PM

Many thanks.

Ive never SEEN a plane with 3 UP thrust!

Makes me wonder if a glue gun might have slipped at the sweatshop🤨. No obvious visual evidence for that. Im gonna double check the thrust line with some other techniques, but it eyeballs the same way it behaves on the meter

Right thrust is just to make TO easier, correct? If I had to correct for that, it was subconscious, and it was R-L very true in the air, so I dont think Ill add right thrust now

So, what about my Basic Theory question? If you take a nice-flying Yak and added 3 up thrust, how would you expect to change?

speedracerntrixie 10-22-2020 02:58 PM

IMO it would be foolish to even try to answer that question without knowing where everything is set. You need to establish the baseline first and then make adjustments from there. The setup I gave you in my first reply is a very good starting point. The only guys who set up to everything at zero are the hard core 3D guys. Your airplane is not a hardcore 3D airplane. Setting up at zero zero will require up elevator trim to force the wing into positive incidence to support the weight of the airplane. This ends up making the airplane fly slightly tail down. The reason for the positive wing incidence is to eliminate the up elevator trim. This does two things, it allows the fuselage to be level in level flight and since there is no elevator trim when you roll knife edge or pull a vertical up line the trim doesn't give a pull to the canopy. The right thrust will help slightly on take offs but will make true vertical lines require little to no rudder corrections. It also contributes to a more axial roll. If you treat the setup of your airplane as a complete package and realize that everything has an influence on a multitude of other things and learn to adjust things to work with one another you will see a good airplane turn into a great one.

kram 10-22-2020 03:58 PM

Thanks, man

Well try that and get back to you with test flights if the weather gives us a break!

Thrust line should be easy to correct, wing incidence not too involved: we may have almost +0.25 already


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