I wonder where the CG location as a percent of mac is on each of these Caps especially the original airplane in question.

But the bottom line is that a lot of airplanes with the same planform and tail moments arms that don't have the difference in vertical positioning don't have the problem.

It has to be something that effects pitching moments. Since similar wings don't have a problem like this then you are stuck with the horizontal tail. Keep in mind that the moment change required is very very small.

I doubt it is a stall of the horizontal tail since the flow would seem to be seperating and reattaching awfully easily.

Those of you with Caps and are familiar with the flight path angles and are good at estimating the angles of attack need to do a little arm waving at the airplane and estimate where the wing wake is going.

The thing about a configuration with the wing and tail in relatively the same vertical position is that the tail is almost always in the wing wake except for high alpha maneuvers.

If we can't find the solution with what we have available it would be nice to see the same design built with the stab in a different vertical location and see if there is any improvement in handling during the same flying conditions. Adding a temporary additional horizontal surface higher on the vertical stabilizer to work in unison with the original stab would probably not be sufficient to make a solid conclusion because we would not 'eliminate' the original 'suspected problem' from the experiment.

I am curious to know if there is any noteable difference with the ailerons raised equally on each side. I believe you stated that you are prepared to experiment with that during your next trip to the flying field. I would hate to see this issue go unresolved because then I won't be able to sleep well at night.

I probably have the world's sole surviving GP CAP ARF...
I'd be willing to reinstall the motor (OS 70) and go fly it with tufts around the rear fuselage and have it video'd... or video it myself while someone I trust flies it..

I have a really old GP Cap21 .40 size kit that I flew years ago. It's not flyable at the moment, so I'm thinking back quite a long time. I don't recall it having any odd landing behaviours, though I did always bring it in fairly hot and do "wheel landings", no 3-pointers, as it had a very nast tip stall and could depart with out any warning. In my case, the wingloading was in the "way way to high" range, as I had an OS .61FS in the nose, and the plane glided like a homesick manhole cover. So I'm more than willing to believe that I never got the plane in to an attitude that whould show up the problem.

I'll be rehabing this plane over the winter (assuming I have time) so I'm really curious about this thread. I plan to really lighten the heck out of the structure, since it looks way over built.

Keith I am going to do the ail. experiments this week end I"ll let everone know how it goes.

Kent

I discussed the peculiarities of the CAP configuartion today with Tony Frackowiak.
His observation is the plan-view area of the cheek cowls have to considered when determining the c.g. of that plane.
I agree.
The c.g. will be a bit forward of where it would be on an identical plane sans cheek cowls.
A way to finger this out would be build a "normal" streamlined cowl,... faired from the spinner to firewall, and fly both it and the scale cowl.

YOU KILL ME!!!!!

That is a good one for sure....at our club field it's "Geez, that thing glides like a handful o' car keys!"

Here is a site with a lot of photos of the Cap 21.

http://www.toni-clark.com/english/mo...1/cap_foto.htm

Look at the photos. If the cowl bumps were strakes there could be some effect but the cowls are well rounded bumps with no edge to create vortex stuff or anything. The fuselage of the cap is really deep in the front which makes the area less effective.

I don't have the RC experience of Tony but have spent a lot of time evaluating configurations in analysis, wind tunnels and flight test. We did a lot of quick and dirty evaluations in wind tunnels by using a flat plate rather than a fully sectioned part. For instance leading edge strakes like on the F-15 where the gun is located. They can be replaced with a flat plate of the same planform and you will get almost the same data. But these are lifting surfaces. The rounded bump is not.

Based on that I would pretty well say the cowl is not going to act as a lifting surface and the inputs to the NP are pretty well trivial.

I also can't determine why the CG location (within reason) should have any effect on the balloon effect on landing as noted in the site where weight was added to the nose. He is working with approximately the same weight airplane which will require the same angle of attack on landing. There will be some tail trim lift difference but it is actually pretty small. Elevator response will be a little less sensistive but I can't see why that would prevent ballooning due to an aerodynamic factor.

I

Ben , what you have stated about moving the cg is what I have found by experimentation. I have changed the cg from 7 in. to 4 in. with no change in the balloon. Only change was the elevator sensitivity. The only thing that changes it is a tad more speed, but this is way to much for landing.

Kent

The next "usual suspect" would be the exaggerated bubble canopy.
There was a sport plane around here with somewhat of an Extra look, but slab sided.
The canopy was flat across the fuselage, although looking normal in the side view.
The plane got passed around thru a few fliers, as it was a very poor flier. Difficult to land,until one guy put strakes on the sides of the canopy at the top edge. About 1/4" square and not very long. This made all the difference in the world.
The cowl bulges are quite noticeable on the GP CAP 232 I have, and along with the bubble canopy could be disturbing airflow at high alphas.
So now there's two things to change... the cowl and canopy.
Yesterday while talking to Tony, I was observing his sport plane, the Top Flite Contender has a layout similar to the CAP; low wing, high horizontal. No ballooning problems.. and no fat cowl or exaggerated canopy.

Hey Paul tell some more about the strakes. Do these go on the side of the cowl and canopy longwise, and how big would they be? Also you said high alphas, when landing , this would not be a high alpha?? I'm not exactly sure what this means, I think. Thanks Kent

The strakes were positioned here...
Flat fuselage sides all the way up to the top of the canopy.
The plane won 1st in a Novice pattern contest after the mod.

I'm prepping my GP CAP 232 for flight...
Measured the cowl width... the cheeks are 60% of the fuselage width.. there has to some effect, if only a burble along the fuselage sides.
OS 70, 102 oz. 23 oz/ft^2.

Here's the way my CAP measured out..
I was surprised at the amount of downthrust!
Flew it today.
Wind 15-25 gusty.
Mostly slow stuff, low passes and touch-and-goes. Didn't oberve anything remarkable at all.
There was some ballooning, but only when turning from base to final, and that depending on the altitude... higher up it was less noticeable.. I attribute this to my not pulling the nose up to adjust the glide angle.
I did determine a 35% c.g. is too far aft.
Snapped out of a spin recovery! Too much elevator even on low-rate!
The c.g is going forward...
Here's a dimension photo....
And a few other shots...
http://www.angelfire.com/indie/aeros...CAP-232-12.jpg

Hey fellas chew on this a little. My caps wing is 0 ind, the firewall and engine is 0 and the horizontal is 2 degrees positive. this is an arf so thats the way it was. Is this causing the balloon problem? I hate to have to cut the tail out , but will if I have to

Thanks Kent

That sounds about the same as mine, which is also an ARF, with a lot more downthrust though.
After flying yesterday, I'm thinking "technique" rather than aerodynamics for your situation.

I repeated yesterday's flight, without the distraction of the camera, and still came to the same conclusion.
There's nothing obvious in the flight behavior of my CAP to match the ballooning characteritic.
I did move the c.g forward to 32% to desensitize the elevator, and changed to a 10 oz tank from a 6 oz.
In flight trim is 1 degree nose-up on the elevator, close to what it was yesterday.
Now it snaps out of the top of a loop, instead of the bottom. An improvement!
Next flight will be with reduced low-rate on the elevator.
Overall, the behavior of the plane is quite normal in all speed ranges.

I'll go along with a burble or whatever from the cowls that might be effecting the flow, just not a big NP effect. For years the standard RC pattern ship was low wing, high tail, the Orion, Taurus, Perigee, and a host of others. They all had a relatively high tail. Of course we were so happy to just get the airplane back on the ground that a ballooning effect may have been there, just not noticed.

i flew my Cap 21 today, It is a 50+ inch wingspan, about 5-6 pounds, Saito .56 powered, Cg is where ever it fell out with all the stuff in it. The tank is 16 ounce (I like to fly a lot) All total, not a very scientific approach :-)

I was expecting a handful in flight and some funny things on landing. It turned out to be anti-climatic - thank goodness. I made several power off landings as the motor was trying (and succeeding most of the time) to stop in flight (another problem to be solved). I honestly didn't see any unusual thing happening in landing. After 40 years of doing this you tend not to remember what you input to controls and just watch the airplane, so if a ballooning had started I might have automatically corrected it and not noticed.

It would be a terrible inconvenience for me (what can I say, I am just a nice guy) but if you are still having problems with the Cap ship it to me, postage paid, and I will evaluate it for you for 2 or 5 years and give you a written report :-)

I flew the CAP again today after finding out yesterday that color scheme sucks big time, especially against broken clouds.
It vanished on some approaches while heading directly toward me, when there were clouds behind it.
Added a nice red stripe full span last night, which fixed that problem, but pointed at the same problem when the back end is pointing at me. It just plain vanishes!
Not a happy flier right now.... can't decide if the fix. ... strip and recover with reds, yellows and oranges is worth the expense.

Hi K Hunt,
Would believe your problem is basic. Probably in the design
force arrangement.
The lift of the wing and horizontal tail are not in proper proportion.
Surfaces lift at a square of the speed.
When speed changes so does the lift in that respect.
An aircraft should fly level at full power and stay level with reduced power until lack of sufficent power reduces speed to the
point where lift is no longer sufficent to overcome gravity.
At that point the craft decends.
When the tail-wing lift proportion is correct the craft will remain
level throughout a major amount of power change.
If your craft flys level at full power with elevator trim neutral and
baloons at low power the proportion is off in favor of the wing
In short the tail is not lifting sufficently.
The tail lift needs to be increased. Lift is generated by these factors> area, angle of attack to the line of flight and airfoil.
A simple try would be to add positive incidence to the horizontal
tail.
Note> when tail lift is increased the neutral point moves back.
Thus the balance point must be moved back to compensate.
This sort of thing seems to be a problem for others also.
I have already posted the factors in detail on another posr in this
forum if you would be interested.
Good luck and let me know how you make out, OK?

Hal [email protected]

Hi K Hunt,
Would believe your problem is basic. Probably in the design
force arrangement.
The lift of the wing and horizontal tail are not in proper proportion.
Surfaces lift at a square of the speed.
When speed changes so does the lift in that respect.
An aircraft should fly level at full power and stay level with reduced power until lack of sufficent power reduces speed to the
point where lift is no longer sufficent to overcome gravity.
At that point the craft decends.
When the tail-wing lift proportion is correct the craft will remain
level throughout a major amount of power change.
If your craft flys level at full power with elevator trim neutral and
baloons at low power the proportion is off in favor of the wing
In short the tail is not lifting sufficently.
The tail lift needs to be increased. Lift is generated by these factors> area, angle of attack to the line of flight and airfoil.
A simple try would be to add positive incidence to the horizontal
tail.
Note> when tail lift is increased the neutral point moves back.
Thus the balance point must be moved back to compensate.
This sort of thing seems to be a problem for others also.
I have already posted the factors in detail on another post in this
forum if you would be interested.
Good luck and let me know how you make out, OK?

Hal [email protected]

Thanks Hal I am very interested in the details please tell me the post. also the stab is at 2 degrees now.

Kent

I was interested to note tha search for an aerodynamic situation lead me to a problem with colors.... Here's the fix to the thing going invisible at some attitudes..
Krylon and LusterKote™ over whatever GP used on their ARFs..
Quite visible against a broken sky..

And it's good against an earthy background