Calculations of thrust line

I talked to Paul Walker and he mentioned two degrees of outside thrust would be good. Figured out that if I calculate angles of my system with this (using the propeller):

http://www.saltire.com/applets/triangles/tri3s.htm

And if I subtract from BCA the CAB I can get the numbers for my plane. Do you think this is right or do you see any issues with measuring this way (experimentally adjusting the outside thrust and measuring how much it is when it is right).

Jukka

Jukka,

The out-thrust measured from the fuselage centerline is not the total out-thrust the model may end up having...

The center of the propeller, in line with the crankshaft, at the back face of the prop, is forward of the model's CG. The CG flies on a radius from the center of the model's motion. The propeller has ALREADY a degree or more "out-thrust angle" from that geometry.

(The fin and rudder, also, have some offset, usually about double the propeller angle due to the lengths involved...)

PW's suggestion of about 2° out-thrust speaks to the structural offset from the fuselage centerline... If the model is truly tangent to the flight radius at the CG, the automatic out-thrust angle from the curvature of the flight path, AND the structural out-thrust angle to the fuselage centerline, add together...

It is still a small total angle. I like some out-thrust, because if the model gets into a very bad flight attitude, it is always working to pull the model out to the end of the lines... The slower the model goes, the more thrust the engine applies through the propeller. The side vector - outwards - from offset is small, but as the thrust load rises, so does that vector. At times, that may be all we have trying to restore controllability!

I use 2 degrees of engine offset on nearly all of my airplanes. I also use the same 2 degrees of incidence in the tailplane. The simple method I use to determine this angle is to tilt the thrust or incidence line 1/32" for each 1" of travel. For a 3" root chord in my stabilizers I use 3/32" incidence measured at the leading edge. Engine offset can be calculated the same way. It actually works out to be just a hair less than 2 degrees but certainly close enough.

Al

Using your program, if you input 12, 12, & .42, you will get 2 deg. Mount a 12" straight edge (drill hole in middle of a 12" ruller, make sure its straight) on you engine in place of prop, and you would have 2 deg offset when the difference between the tips to leading edge is .42", or 27/64.

Allan Perret

OK all you experts, is there any truth in the rumour that if you introduce side thrust you need down thrust to balance the forces, and/or Al's incidence on the stab? If so, which (or both) is better?
My preference seems to be for stab positive but it probably also depends on thrust vs wing/stab centre lines.
Answers without tertiary level maths appreciated.

Cheers

OK, short answer, or opinion, if you prefer. All control line propeller powered airplanes experience a bit of gyroscopic precession from the very fact that the airplane is tethered and in a constant turn as it travels in a circular flightpath. This is an indisputable physical result of flying a tethered airplane with a gyroscope mounted on the engine. This precession, typically nose up on airplanes flying counter-clockwise, is there all the time while the engine is running and must be compensated for. You can either attempt to trim it out using incidence or down thrust, or you will carry a bit of down in the handle, whether you are aware of it or not, to make the airplane seem to fly neutrally correct.

The effect is slight, but it is always there as long as a prop is turning. I am not comfortable with the thought of using down thrust. Incidence works and will reliably add a bit of compensating nose down pitching moment (nose up inverted) to compensate for precession. Dave and Brett have done some test with variable adjustable stabs and , if I remember correctly, think that about a half degree of incidence not only counters precession but adds a bit of longitudinal stability. I forget the reasoning, but probably has to do with "longitudinal dihedral".

And then there are asymmetries in airplane configuration. Most airplanes have the stab located higher than the wing. When maneuvering, up flaps on outside maneuvers direct wing wake toward the tailplane reducing their effectiveness. Flaps down on inside maneuvers direct the wake of the wing away from the tailplane. This is noticeable in asymmetrically configured airplanes showing a tendency to turn better inside (up). There is no perfect trim for this asymmetry but, practically, we can balance the effect by trimming the elevator down until the turn rate seems balanced. Or, by anticipating the need for trim by building in a bit of incidence to correct not just for precession but also for configuration asymmetry. Again, I think that just 1/2 to 1 degree of incidence is enough for typical airplanes. My semi-scale airplanes have a much greater asymmetry of configuration in that my stabs are usually located much higher relative to the wings. For this, I need more incidence.

Years ago I found that while trimming a series of competition airplanes to turn equally that I was typically winding up with about 3/16" droop of the elevators on most of these airplanes. The 1972 and 1973 NATs winning sea Furys both trimmed out with 3/16" of down elevators. With the Molded Mustangs, I anticipated the need for asymmetry and gyroscopic trim by splitting the 3/16" down elevator trim between the elevators and stab to "fair" (align the stab and elevators) the surfaces. This resulted in 3/32" incidence in the stabs and 3/32" of droop in the elevators which "faired" the assembly with what turns out to be about 2 degrees of incidence. All of my airplanes since have 2 degrees of incidence, "faired" elevators, and track straight in pitch with loss of line tension.

You pays your money and do what works for you. This is how and why I use incidence. These are my beliefs and I won't argue them (any more).

Also, I use 2 degrees of engine offset on all of my airplanes, except those powered by .91s, where line tension is excessive anyway. As far as I can tell engine offset has no noticeable effect on pitch trim in maneuvers. Undoubtedly, there are some negative effects on maneuvers of this configuration, but I have no idea of what or how much. I think this is something which, on balance, probably adds a bit more positive effect to the handling characteristics of our acrobatic models than negative. Whatever. I build it in, trim the airplanes carefully, and learn, with practice, how my individual airplanes "like" to handle in each maneuver.

And this was a "short" answer, Ted style.

Al