My memory is awesome...just a little short...

The downthrust you specified seems pretty high, but...a lot of times a flat bottom / high wing trainer (especially if it has a low thrust line) will have a wild pitching tendency unless addressed through the prodigious use of down thrust. In other words...it'll want to climb like crazy "WFO" , etc. If you trim that out, then it will dive like crazy in a power off situation. You can't win like that. In an attempt to "neutralize" the models behavior, downthrust is used.

The Dihedral you specified (3.5 degrees/panel?) is not at all excessive. The effect of dihedral is rather complex, but...

The lift component developed by a wing acts perpendicular to it's surface. In a "V" situation as is presented by a wing with dihedral, the lifting forces are acting upward, and inward.

Let's say now that the plane banks. Now, one wing is level and the other is tilted upward at a rather high angle. Now, the vertical component of lift between the two halves of the wing is quite different! (total lift is the same)

The lower wing is producing a greater "vertical" (less horizontal) component of lift than the high wing (more horizontal and less vertical component) and thus the aircraft attempts to revert back to level flight. Obviously, a desirable trait in a trainer!

Go with the specified values. Make small changes if you feel they are necessary after a few flights.

Good to hear you are getting "Momma" involved. Good thinking!

That way...a trip to the field is a "family night"!

Have fun!

The downthrust is essentially to correct for the offset of thrustline to center of drag.

with the engine below the wing, it tends to want to pull the nose up as you add power The downthrust acts to pull the nose down, and if the angle is right, they cancel out. Since each engine is a bit different, the angle is unlikely to be perfect... but 0 deg downthrust on a commn trainer, you could loop the plane just by manipulating the throttle.

The thrust line effect is used in some of the cheapie throttle/rudder electric planes to give a degree of "elevator" control. On those, appx 1/2 to 2/3 throttle is level, less is a slight dive (which due to tailplane incidence is liited from being too severe) and high throttle pulls the nose up. Thus... you get the ability to loop and do other maneuvers which nrmally require elevator.

The right-thrust is to offset torque and "P-Factor" the torque an effects of prop pitch related to line of flight tend to turn the plane to the left. Again, this effect is relate to throttle, and its actually possibl to have a plane controlled completly by throttle doing right-left turns, and climbs and dives. (they use this abilty in FF models, to get a left turn climb and right turn glide.)

Brain fade on my part. You're talking "incidence" and I'm spouting "dihedral". Duh!

The manufacturers (especially "way back when") did things simply because "that's what worked on the prototype" and not as the result of exhaustive testing at JPL or something.

Fhubers post is pretty much "on". He brings up a couple things I wasn't thinking about...That is, how to get a modicum of actual control over the model when using a single channel r/c unit. THAT was very common years ago. No elevator, no throttle, just a rudder that sometimes moved when you wanted. Some called it "r/c assist"....others called it "radio interference".

Trial and error, with a hint of Black magic...that's where a lot of that stuff came from!

Have fun!

Don't beat yourself up on the dihedral... he asked a little about it, and you covered it quite well.

The incidence is intende to be in relatin to expecte fuselage level flight, and downthrust is a totally separate issue.

A plane's wing incidence is designed to have the plane level at normal cruising speed and not gaining altitude. If the plane is intended to fly fast, it will have a low incidence. If it is intended to fly slowly it could have a fairly high incidence. (I've seen 8 deg) the incidence also gives you an idea of the speed range of the plane. A high incidence will indicate top speed and stall speed are supposed to be close. (some pre WWI planes, top speed was within 5% of stall speed.)

The difference n angle between wing and tailplane (Decalage) affects pitch stability and CG range. If the angle is large and wing + relative to tailplane, the CG range is moved forward. If the andle is -, the CG can go as far back as 60% of MAC of the main wing with a "normal" appearing aircraft. (old FF models, this was common. The tailplanes lifted, where its more common for us to develop a downforce with the tailplane.)

With a far rearward CG, decalage angle and CG range become VERY limited, and just burning off 1 oz of fuel could make a .40 size plane uncontrollable. (FF models rarely burned more than 1/4 oz durring a flight. The 1930's/40's standard fuel tanks wouldn't hold 1/2 oz!)


BTW... I have a relatively modern trainer, the "TameCat" by AlTech. Total downthrust angle is 9 deg. The wing is at 0 deg incidence, the tailplane is at + 3 deg (partialy lifting tail!) CG is at 37% MAC. Great trainer... but a bit heavy. Too bad they quit making it. Its nearly indestructable, and looks a lot like a F-14 Tomcat.

Since somebody was kind enough to catch me on this before, I'll share the following with you: Doc Matthews is a great designer, but the Whiz family came from the mind of Owen Campen. In this case, the good Doc was just the one who adapted it for manufacture in the appropriates sizes, etc.

We've currently got two Whiz .40s in our club and a group more on request. A really outstanding thing about the Whiz is the way it lifts its tail like a real airplane and taxis just as pretty as you please on the mains and rudder... and it doesn't nose over even on our rough grass field. Also a very fine flyer capable of all the stuff that doesn't require a purpose-designed aerobat.