I have a Sig Kadet Senior ARF with a OS65LA that got it's maiden flight this weekend. It flew great, the only problem is with the amount of down trim required, at half throttle it needs half of the available down trim adjustment for level flight. Can I correct this with some down thrust? If so how much and should I put it between the engine mount and the firewall or between the engine and the engine mount?

this plane will tend to climb under power and sink at idle. So yes, some down thrust will help. There ia also nothing wrong with leaving the engine thrust line alone and just fly the plane as-is. You can change the trim while flying as you need to. If you want to add down thrust, you can do it in either place. Whatever is the easiest way to add a washer. Behind the top two screws of the engine mount are probably easiest.

Actually, on this type of plane "down thrust", pointing the prop down in reference to the actual thrust line, will cause the plane to climb even more. It's all about the "angle of attack" (AOA). By pointing the prop down, your increasing the AOA between the prop and the wing the greater the AOA of the wing the greater the climbing. Example: engine on plane is at 0 degrees and the wing has a 2 degree incidence- thus you have a 2 degree AOA for the wing. Now if you add 2 degrees of down thrust plus the 2 degree wing incidence you end up with a 4 degree AOA for the wing- which will cause the plane to climb even more. Think of it like this: the prop is going to pull straight- no matter if you put in down thrust- and if its set at 0 degrees the only AOA will generated by wing incidence. Acyually putting in a little "up" thrust might help reduce the climbing. But, you have a high wing flat bottom airfoil with a relatively large engine up front and I doubt that you'll ever be able to trim out the climbing tendencies associated with the throttle.
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It sounds like you're balanced nose heavy.
Moving the c.g. back a bit will reduce the amount of down-trim at full power.
Note the Kadet is designed to have some down-trim for level flight.
This can also be reduced by shimming the rear of the wing -up- about a 1/4".

Listen to DMcQuinn and use the downthrust. The original Seniors were intended to use a full 5 degrees downthrust and if omitted even slight power changes resulted in substantial pitch trim changes with every little power change. The seniors are wonderful airplanes and a pleasure to fly when set up properly. Pitch trim changes with power are always a function of how far away the thrustline is, either above or below the aerodynamic center of the aircraft. The further it is below (as in a deep bellied cabin type such as the Senior) the more pronounced the pitch trim changes with power change. A seaplane with an overhead pylon mounted engine will respond exactly the opposite and pitch down with application of power. An airplane that pitchs up with power is not neccessarily indicative of a nose heavy airplane. Use the downthrust.

Look at the Cadet Senior in my avitar it only has two degrees of down thrust and has no pitch change with power but that is only because the engines thrustlines are raised up a full four inchs from the stock single engine Cadet in wing nacelles and the thrust lines are very close to the aerodynamic center.

John

Whew, thought I'd been doing it wrong all these years.
Easiest way is to place one ~ 1/16" thk. washer under each of the rear engine mount holes. I've tried the spacers between the top of mount and firewall, but it usually takes a thicker washer since some of the spacer gets smashed into the wood and can lead to a loose mount.

My Seniorita- is pretty old now and is powered wih a KB Sportster 20 -(perfect power setup)
these models are simply free flight designs with radios to mess up their real flying character.
The plane is THE perfect basic trainer -for those who really want to see what makes a low speed airframe fly.
If you balance it per the kit -and it is reasonably within weight guidelines -
the model will trim for level flight ONLY when elevator trim and power are matched.
add power - it climbs -screw with the thrust line all you want.
Reduce power a little it sinks - reduce power to ZERO and it dives--then picks up speed and levels out - then dives etc.
I routinely use mine to fly to a low altitude -then trim for a large even turn and --shut off the TX - The PCM holds all controls and the model plays free flite - till I get tired of watching
Beautiful design
Then I go fly my lil lectric foamies -which are at other end of the spectrum of controlled flite .

Huh? Could you clarify? I thought the wings would fly straight, not the prop. Down thrust (4 degrees) works great on my LT-40 and the copies I have designed of it.

Thanks for the help, I know that big flat bottom airfoil wants to float and climb with power. I think I'll fly it a few more times as is, then play with the thrust to try to get it flying level at half throttle with the trim centered. It will give me a challenge....Thanks again for the input.

Yes, downthrust will help immensly. I prefer to trim a plane out at half throttle. Now power up from level flight; if it climbs, increase downthrust, if it dives, decrease the down thrust. Once trimed out with the proper thrust setting, just the speed will changed with increased throttle, makes it much more pleasant to fly.

Wings fly because they have an energy source. With prop planes the engine prop combo are the energy source, on gliders or a dead stick it's gravity. And they follow the power source. Trainer types of planes usually have some engine down thrust designed in, along with dihedral and positive wing incidence to help them climb. And thats what they do when you and throttle- they climb. It's all about AOA's. Remove or reduce any or all of the down thrust, wing incidence, dihedral and the horizonal stab incidence and you can reduce the climbing tendancies associated with power changes. Take a look at pattern type of planes- they go where you point them, and why? Because they usually have everything set at 0 degrees and control inputs (elevator, rudder, ailerons) change the AOA.
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