I apologize if this post is perhaps in the wrong forum or too elementary but after some searching I just couldn't come up with anything that directly addressed my questions.

I am a relative newbie, flying for about a year and am self-taught. I started with a GWS PicoMoth, progressed to the GWS TM400 for aileron training, and currently an overpowered overweight GWS Corsair (fun). I love EDFs and have both Kamdax and Alfa MiG-15s. Everything is electric and nothing larger than 24 ounces AUW.

Recently, I purchased a Chinese ARF 48" ws Spitfire and installed an Apache 30M-18T brushless motor, driven by a Polyquest 4s 2200mah lipo and propped for 310watts WOT with a 10x5 APC-propellor. The instructions said CG should be 98mm behind the LE. However, the model is a copy of a Ripmax Spitfire (chinese copy of a chinese copy!) which I was told had a recommended CG of 70-75mm. So there was some comfusion about the CG. I started at 75. I noted the following characteristics.

1) After trimming for level flight, inducing a shallow dive then neutralizing elevator (releasing the stick), the plane would nose up and continue up past level.

2) Application of full throttle from level flight caused the plane to pitch up and assume, in a few seconds, a near vertical attitude.

3) Cutting throttle from level flight caused little immediate change in pitch.

4) Trimming for level flight at 'cruising' speed (1/4-1/3 throttle), required a noticeable down elevator setting.

I interpreted these observations to indicate that CG was too far forward, and perhaps that there was not enough downthrust. I measured decalage to be about 2 degrees (wing positive relative to horizontal stab). I measured the thrustline as best I could and found the thrust angle was actually 1-2 degrees up!

In the middle of all this, I asked for some trimming help on one of the threads where this model was being discussed. I received the following advice:

"With plane trimmed put it into roughly a 45 degree dive and cut power. If it pulls up it's tail heavy, if it dives more it's nose heavy. I would say trim the plane at about half throttle or where you normally would fly at.
It's normal for a plane to climb at higher throttle levels, especially with more powerfull motors. The way to counter this is to add right and down thrust as necessary. The more powerfull the motor, the more you add."

However, do not understand the rationale for the test that was proposed and the proposer could not explain it in an manner that clarified the issues for me. From what I've read, the point of putting the plane in the dive is to assess pitch stability i.e. the tendency of the plane to return to level flight from any disturbance in the pitch axis. If you trim for level flight, leave the throttle setting as it is, put the plane in a dive and release the stick (neutralize elevator), if the CG is ahead of the neutral point (center of lift) then the plane will gradually nose up until it regains level flight. My thinking was that cutting throttle in the dive would be counterproductive because it removes the downthrust and the model must compensate for that loss by changing pitch. My thought was that regardless of whether you are in a dive or level flight, too much downthrust and you pitch up when you cut the throttle; too little, and you nose down. So thrustline setting and CG would be interacting in the test proposed.

Am I thinking about this incorrectly? What would be the best series of tests to employ to bring CG, downthrust (and maybe decalage) into the appropriate ranges? I should say that I can launch, fly and land this plane perfectly well with the CG at 75 or at 90mm and also that I've brought the thrust angle to 0 degrees which has clearly improved the pitch up at WOT. I love the way this thing looks in the air and in low fast passes!

Thanks for any help you all can provide.

DP

It's always safest to follow the manufacturers instructions instead of what someone tells you.
There's folks who will say anything at any time, without the foggiest notion as to what they could be doing to the questioner.
I've never seen or done a power-on dive test.. the way you describe it a pitch-up is inevitable, as the power-off gliding speed will be much less than the speed in a powered dive, so the plane has to pitch up due to the excess lift.
Dive testing power off will locate your c.g. correctly. A mild automatic pullup after doing the dive is safest.
Lots of climb with lots of power is a good thing.

Tall Paul;

Just to be absolutely clear about what you mean by CG testing using a power-off dive:

1) Are you trimmed for level flight before inducing the dive? At what speed or throttle setting?
2) Do you induce the dive and then shut off throttle or visa versa?
3) What dive angle?
4) "Mild automatic pullup" means a gradual return to level flight? Signifying adequate pitch stability due to CG ahead of center of lift?


Second question then is thrust angle. How would you determine the proper setting. I'm guessing that 1-2 degrees upthrust, which is what the model came with, is not correct. By the way, I neglected to mention one other important piece of information regarding CG: when the manufacturer of this model (www.haikong.com) was emailed to inquire about the CG, they responded by saying that they had made a mistake. It should be 75mm, not 98 as indicated on the instruction sheet. Doesn't give you much confidence. And since nose heavy is safer, I started with 75mm.

DP

Tall Paul;

I'm sorry to be so long winded! I'm confused about which dive test you were commenting on since I described two. So could you please comment directly on this piece of advice I was given:

"With plane trimmed put it into roughly a 45 degree dive and cut power. If it pulls up it's tail heavy, if it dives more it's nose heavy. I would say trim the plane at about half throttle or where you normally would fly at.
It's normal for a plane to climb at higher throttle levels, especially with more powerfull motors. The way to counter this is to add right and down thrust as necessary. The more powerfull the motor, the more you add."


Thanks - DP

The dive test helps you determine whether to move the balance point forward or aft. To determine how the thrust angle should be changed, trim for level flight and chop the throttle. If the model has a major pitch down, you need some downthrust. Likewise, if you are flying at low throttle and punch it and you get a major pitch up, then you need downthrust. Thrustline changes are used to reduce or eliminate pitch change with throttle change.

Of course, when you change the thrust angle, you'll have to re-trim the elevator a bit. Repeat the dive test and see how it goes. With a few trial flights, you should be able to nail down the thrustline and balance needs of the airplane.

Thanks Bax - but please, just to be sure I understand what you mean, describe the 'dive' test in some detail. Please.

DP

Really sounds like your plane is nose heavy. Nose heavy planes have a tendency to pitch up as the speed increases. This also means that the stabilizer is not at the right angle for that CG placement. I wouldn't mess with either the stabilizer angle or the thrust angle. I would move the CG back a cm at a time until I got the flying qualities I wanted.

WARNING: When you do this the aircraft will become more and more touchy, especially in pitch, if this is not what you want then you need to reduce your elevator throws to compensate. Eventually you may run into a situation at which your aircraft actually pitches nose down when speed is increased. Bottom line, move the CG back a little at a time until you reach the desired qualities, but be ready for these changes.

An example, I have a Four star 40 that I purchased, i flew it for a while, made a repair and flew it again. I didn't check the CG. My plane was and still is tail heavy, it is almost neutrally stable and is very, very touchy. When I over speed it it wants to pitch down. These qualities might sound undesirable, but I have come to expect them and understand how to correct for them, and now I have a Four Star that can do some absolutely astounding maneuvers.

It's all a matter of preference.

Nmking09