Need your help fellows - fighting elevator on final

[Judge]

Find the trim chart by Peter Goldsmith and print it out. It will guide you through all the trimming step by step and in the right order. My guess, you are slightly tail heavy AND have thrust fighting you. Here is the kicker on trimming though, only make one change at a time and then go back and double check that everything is still trimmed right. It is a long procedure to go all the way through, but you end up with a plane that will fly well and you will be very familiar with it and its tendencies.

I have a Trim App on my iPhone that applies Peter's trim chart in an interactive manner. Very cool. Trim App 2.0

 

[Pistolera]

I know I may get flamed here but we'll see. On a fully symmetrical wing like your plane and nearly all aerobatic planes the wings DO NOT generate lift. Angle of attach makes the plane fly along with modern power levels pulling the plane along.

OK Terry....I won't flame you....MUCH . The wing DOES produce lift, although as you mentioned we need to produce some angle of attack to make that happen, whether upright or inverted.

Skunkworks....if you have the CG where you like it and have resolved any thrust issues, then check vertical down lines to see if it is pulling to the canopy (zero throttle). Most aerobatic planes do this to some degree and most people put a mix in to give a small % of down elev at low/no throttle. This mix also helps reduce ballooning on landing as that is when you have low/no throttle and don't wish the nose to rise back up again.

All of my planes have needed 1-2% down elev with zero throttle, even though every thing else was spot on.

BTW - pretty CAP

 

[Kevin-Young]

How does the airplane do on a downline with no power, does it pull to the canopy or belly? Do you have any type of low throttle to elevator mix for a downline? You seem to have symptoms of both cg and/or thrust so I would suggest starting with cg first and making sure that is correct before you get into other mixing needs. Also how far ahead of the cg is the fuel tank, that may explain you need to retrim mid flight?

Also depending on the radio you are using you may be able to change the step rate of the trim from 4 steps down to 2 so you can fine tune your trims some more.

 

[sweetpea]

All good items to discuss and I didn't think that if your fuel tank is too far forward and at the end of a flight you are now tail heavy. Assuming this is part of the issue and it cannot be moved.........

If this is the case...I'd takeoff with 1/2 tank or slightly less fuel and do some quick CG trim passes and land. This will help you simulate what it will be on landing when you do your trimming for CG. Any extra fuel will just make you nose heavy which is easy to fly and land through.

But I agree with rechecking your CG and checking your thrust line by cutting the throttle.

BTW, that is a H9 CAP. Great plane for a 100cc engine. Shame its discontinued.

 

[skunkwurk]

If you can, post a pic of your elevator linkages, I have a theory but want to make sure they used what I think they did on there for hardware.

To answer your questions, yes. You can always make the plane more friendly by adding a bit of weight but you do have some issues that should be fixed as well.

Here you go Terry. I was only able to take shots of the left stab-half, as I can only leave installed the one side in the trailer. Let me know what you see.

Regards,

sc

 

[skunkwurk]

Elevator trim is only correct for a given speed. Change the speed and the trim required for level flight will change. In a full scale plane you can be in straight and level trimmed flight and climb or descend merely by increasing or decreasing the throttle. So needing different trims at different speeds is expected.

Here's a great site with loads of info:

http://www.av8n.com/how/

Specifically:

Angle of attack: http://www.av8n.com/how/htm/aoa.html

Airfoils: http://www.av8n.com/how/htm/airfoils.html

Hi Judge,

Thanks for the reply, this is excellent as it basically confirms the characteristics of the plane. I have installed a lot of motor for the airframe, I could also be experiencing a side effect of loads of torque made available by the 120.

sc

 

[Terryscustom]

OK, there is part of your trouble with hunting for trim on the elevators. The part that threads onto the stud control horn is free to move and the ball link is also free to move. If the threaded clevis moves to the side and then centered again it will throw you off by a small amount It is best to put a threaded nut to lock that into position or even a dab of "Goop" to keep that from turning on the threaded stud. I actually just did that recently on a rudder that was giving me fits and it fixed it immediately.

 

[skunkwurk]

OK Terry....I won't flame you....MUCH . The wing DOES produce lift, although as you mentioned we need to produce some angle of attack to make that happen, whether upright or inverted.

Skunkworks....if you have the CG where you like it and have resolved any thrust issues, then check vertical down lines to see if it is pulling to the canopy (zero throttle). Most aerobatic planes do this to some degree and most people put a mix in to give a small % of down elev at low/no throttle. This mix also helps reduce ballooning on landing as that is when you have low/no throttle and don't wish the nose to rise back up again.

All of my planes have needed 1-2% down elev with zero throttle, even though every thing else was spot on.

BTW - pretty CAP

Thanks, Pistolera. I'll check the down-line (no throttle) and see what she does, I'll admit, I have not tried that. I have a laundry list of items to check the next time I go out.

sc

 

[skunkwurk]

OK, there is part of your trouble with hunting for trim on the elevators. The part that threads onto the stud control horn is free to move and the ball link is also free to move. If the threaded clevis moves to the side and then centered again it will throw you off by a small amount It is best to put a threaded nut to lock that into position or even a dab of "Goop" to keep that from turning on the threaded stud. I actually just did that recently on a rudder that was giving me fits and it fixed it immediately.

Terry,

I can easily add a nut onto the threaded rod/stud and lock down the clevis. If there is any slop there, it would be very minimal and only left/right. I'll look to see if I can find a nylon nut to use, to keep the weight down.

sc

 

[Terryscustom]

Terry,

I can easily add a nut onto the threaded rod/stud and lock down the clevis. If there is any slop there, it would be very minimal and only left/right. I'll look to see if I can find a nylon nut to use, to keep the weight down.

sc

Yup, and as that clevis moves left or right in shortens the distance between the clevis and your servo arm = trim adjustment issue. May or may not be it, but it fixed my rudder trim issue.