I'm building a Kadet Senior with some slight modifications. I have one already that has less down thrust than what the kit says it should have. The plane has a 91 OS 4-stroke on it and flies great. On my new plane (kit), it calls for a lot more down thrust and I'm trying to decide whether to go with the suggested down thrust or mimic what I've already got in the other plane I've already flown.

This plane will also have a 91 on it and I intend on flying it the same way, low and slow. I like getting the plane to stay in one place when flying into the wind and I like flying slow and low (close to the ground) I do use full throttle once in a while, when going straight up but mostly, i'm just a couple of clicks off of idle and just puttering around.

What I'm wondering is, if I do install the extra down thrust, could i trim out the plane so that the tail would ride lower (sag) and at that attitude, would the plane still be pulled somewhat straight (due to the down thrust)? Which in turn, would give it more lift on the wing since it's not cutting straight through the air but has more incidence to it, in relation to the direction of travel. It may be obvious that I know enough to be dangerous but I think about things like that.

ANY info is greatly appreciated.

It doesn't work that way. The model needs to "ride" at the angle that provides the wing with the angle of attack to the airflow so that it generates just enough lift to hold the model up. This is why when you're flying fast the model seems to be flying tail high (and is where the phrase "high tailing out of here" came from). And when flying slow the tail is low and nose high because it requires more angle of attack at lower airspeeds to generate the lift needed. You can't alter these angles with more or less downthrust.

Getting back to your model flying well with less downthrust..... The need for downthrust is linked partly to the location of the wing and thus the center of drag of the model. And partly it is related to the CG location and the resulting trim angles between the wing and stabilizer. The more rearward the CG is located the less the trim angle (often called decalage) is between these two and the less tendency there is to nose up with a gain in flying speed. The downthrust is there to help counter the tendency to nose up with a more forward CG and the extra decalage needed to fly level with a forward CG.

Thanks for the info. If I am understanding correctly, I should be checking my old Senior to be sure that angles/locations of the stabilizer and the wing are in fact, built to the same specifications as the one that I'm currently building?Since my old plane has much less down thrust, it may be a possibility that whoever built it years ago, might have altered these areas to allow/compensate for the different down thrust?

If everything else is the same as designed, what exactly would be the benefit of removing so much of the down thrust designed into the plane? What difference would it make in the flying characteristics?

thanks again, just trying to wrap my head around this concept before the CA dries!

It is all a matter of leverage -
The angle the propeller is trying to proceed and where that force is located (and acting from) RELATIVE to the wing

Tcrafty,
Whatever you decide to do with the firewall is not as permanent as it sounds. I have changed the amount of down-thrust and right-thrust using washers between the engine mount and firewall. A little experimentation with your current Sr. will help you visualize what Bruce explained.

If you decide to build the plane with less down-thrust the main risk is it might make the plane want to gain altitude when you advance the throttle after trimming for level flight. The Kadet Sr's design dates back to the '60s and has a very efficient, flat bottomed airfoil (perfect for low and slow!) that can be very sensitive to throttle changes resulting in the pilot constantly having to re-trim the elevator to maintain level flight. It is possible that Sig increased the down-thrust setting at some point as engines became more powerful.

If the thrust angle is off the plane will tend to change pitch with changes in throttle. Too far down, and the plane will lift its nose when you let off and drop its nose when you apply power. Too far up, and the opposite happens. When the thrust angle is just right, the planes pitch sensitivity to speed is minimized. Proper thrust angle changes with changes in CG and wing/tail incidence also, so those need to be set right before tuning the thrust angle.

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That's odd. My great uncle (born around 1886) used that phrase all the time, and wasn't an aviator or a modeler. He was, however, in the horse cavalry (pre WWI). I think it's a reference to how a horse holds its tail when in a full gallop.

Of course, a deer hunter might think it refers to how a whitetail raises its "flag" when spooked and runs off.

I like the airplane version, too, but the one "balls to the wall" is my particular favorite.

thanks for all the input. I'll be spending some time on the building next week again, too many things going on right now. I'll probably reduce the amount of down thrust since there might be a moment when this plane might find itself hovering and I don't think the down thrust would help in that situation and it doesn't sound like it has any benefit for the type of flying that I do. There will be some right thrust though.

thanks again for the info. I'll post how the plane flies once I get it in the air. maybe even a video!

Your idea a few posts back to study the angular setup of the earlier model is a very good one. As I mentioned before the need for downthrust is linked to this angular setup. The CG also being related to the angular setup.

Remember that if the elevator is not on the zero line with the stabilizer then you have to measure the angle of the tail using the leading edge of the stab and the trailing edge of the elevator as your chord line. At that point the deflected elevator makes the overall surface act like a cambered airfoil with this new chord line as the base angle. So it is important to factor in this trim on the controls when measuring these angles.