Need your help fellows - fighting elevator on final

[skunkwurk]

I finally got the Cap in the air, it's been a while, I know. I'm slow when preparing a plane and have been traveling a lot for work. I have a couple questions, I am hoping the knowledge on the site could help me solve a nagging issue. I hope you don't mind and I'm looking for any and all feedback, so please help if you can. I've flown the plane about 5 flights, pretty awesome plane! The issue which is currently nagging me is final approach, I'm having a hard time getting the plane down due to some porposing (ballooning) issue. I thought it could be a few different things, I thought it could be CG, elevator throw or engine thrust angle. Am I missing anything? Below is what I've done so far.

1. I've looked at CG first, I've checked the CG in the air a few times and it's about neutral, it needs a slight touch of down elevator to keep inverted level or 45 up-line flight. It may need additional tuning as I become more used to the model, but it feels comfortable in the air inverted. I had to add a little weight, i may move a battery if I stick with this balance point. It flies fine, but did not solve the issue.
2. Thought it might be too much elevator on final. I added 10% more negative expo (futaba) to the elevator. I feel I have good control of the elevator throw, flies comfortably and I don't feel I'm fighting the elevator in the air.
   
3. Now I'm considering engine thrust (more up thrust). I think it could be engine thrust as I feel it tries to climb (balloon) a bit on final when I cut the throttle, I need to push on the elevator stick a bit to get it down, when I let off the elevator it porposses. sucks...
4. Are spoilerons an option? It would feel like a workaround instead of a fix.

I don't feel comfortable having to push the elevator to land, I always set my planes up so I can hold slight UP elevator to control a decent with a final flare. This model is a balancing act to get it down and a fight on the elevator, push - pull - push - pull - ugh.... I have not made any thrust changes yet, I figured I should stop at this point and consult with you before I begin to make additional adjustments.

Also, I'm using a 3 blade prop, could it be possible that the prop may require me to have to do anything different on final? I hear 3-blade props produce a braking effect, especially at idle, and additional right-thrust than a 2-blade.

Thank you in advance.

I added a couple pics for you, hope you enjoy them...

Regards,

sc

 

[aarestor]

Fly the plane WOT straight and level in front of you, then chop the throttle and see what it does. If it balloons, put up thrust in it.

 

[skunkwurk]

Fly the plane WOT straight and level in front of you, then chop the throttle and see what it does. If it balloons, put up thrust in it.

Hello aarestor, thank you for the reply. I'll certainly try that to determine if up thrust is needed, what if it coasts w/out any or much climbing?

 

[HRRC Flyer]

Hello @skunkwurk,

Nice looking Cap. . . . . . . Whose ARF/Kit is your Cap? How much does it weigh? This information may be helpful in determining a solution to your problem.

 

[skunkwurk]

Hello @skunkwurk,

Nice looking Cap. . . . . . . Whose ARF/Kit is your Cap? How much does it weigh? This information may be helpful in determining a solution to your problem.

Hi David,

Thanks. It's a H9 33%, it's 25lbs. The manual calls out a range of 23-26 lbs.

sc

 

[Terryscustom]

@aarestor is correct and that will tell you if your up / down thrust is off. If you go full throttle with the plane trimmed and cut the throttle and the plane still flies level, then you are tail heavy.

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.
So to check your CG (after you check for possible thrust issue) you need to fly straight and level at about half throttle. Roll inverted and see how the plane acts then. When you do this at a 45 degree up-line depending on the amount of power you have you have angle of attack and the wings are lifting the plane and you will get a false indication of CG. This is especially the case when you are very close to neutral as you mentioned.

I will tell you that I have a Great Planes Cap 580 that was brand new and I just built it recently. I much prefer that plane just a tad more on the nose heavy side than neutral like I fly my other planes. It does just about everything better not being neutral and draws perfect scale lines.

 

[skunkwurk]

@aarestor is correct and that will tell you if your up / down thrust is off. If you go full throttle with the plane trimmed and cut the throttle and the plane still flies level, then you are tail heavy.

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.
So to check your CG (after you check for possible thrust issue) you need to fly straight and level at about half throttle. Roll inverted and see how the plane acts then. When you do this at a 45 degree up-line depending on the amount of power you have you have angle of attack and the wings are lifting the plane and you will get a false indication of CG. This is especially the case when you are very close to neutral as you mentioned.

I will tell you that I have a Great Planes Cap 580 that was brand new and I just built it recently. I much prefer that plane just a tad more on the nose heavy side than neutral like I fly my other planes. It does just about everything better not being neutral and draws perfect scale lines.

Thank you, Terry. I was hoping you'd make a comment, I appreciate it. I probably should have mentioned this in the original post, but I have a bit of a trim anomaly as well. I feel I need to adjust trim a click or two at different throttle levels, it is not drastic, but not 100% consistent elevator trim either. It almost feels like I need a mid-sub-trim value for level flight at 1/2 throttle. Does that make sense? I did not write all of this down and there is just so much you are trying to feel out on the first few flights, and my spotter was not really taking notes, so I'm going on memory and as I think this through, more keeps coming back to me. Would the inconsistent trim be due to CG or thrust angle or potentially both?

sc

 

[Terryscustom]

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.

 

[Naughty Monkey]

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.

 

[Judge]

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