Hi fellas, I have a 80 inch Cap 232 that drops its tail at slow speeds and causes it to balloon up. This makes landing tough. My ? is, could the size of the H stab be increased or the air foil changed to increase the lift of the stab so it won't stall before the wing. OR are my thoughts wrong.

Thanks Kent

Sounds like your CG is too far back and/or you have too much elevator travel. I doubt that stab stall is causing the ballooning. (IMO)

The cg for the plane is recomended at 6 1/8 to 7 inch, I have at 5 inch now. moving the cg from 7 to 5 inch did not change anything. Also tried a bit of upthrust. Also the elevator on low rate only moves a 1/4 inch up.

Thanks Kent

Hmmmm Sounds like you've done your homework! I'd have to see the plane fly and see what it does. Is it possible you have a funky servo pot that's acting up at a particular vibration freq. and servo position?

Well I guess anything is possible, but I am sure the servos are ok. When landing all is well until it starts to slow to a medium landing speed and then the plane starts to drift up. It will stall if I don't touch a little down elevator or throttle up. If I come in at a fast landing speed there is no ballooning but usually it will bounce and then slow down enough to drift up. Rite tough to land. Some people say all aerobatic monoplanes will balloon on landing, BS to me. This is not the first one I had do this but it is the first I could not cure. Any more thoughts would be appreciated.

Thanks Kent

I guess I would start by flying the plane up to a comfortable altitude with minimal fuel and then chop the throttle completely to 'OFF' then let the plane glide down and see what is happening with trim. If properly balanced and the angle of the stab to the angle of the main wing is acceptable the plane should make a smooth, stable glide at your chosen landing/gliding speed. If a handling problem is still observed then we can eliminate the engine thrust from the THIS problem because there is no engine thrust.

If the plane glides/lands without any major flight handling concerns then I would focus on the thrust vector of the engine.

If the engine is OFF and the plane keeps nosing up with increased glide speed look into the center of gravity or the wing angle compared to the stab. I would expect that in the glide, if you add a little nose down then release the stick, the plane should not nose up on its own (This would be similar to the 'dive test' for sailplane CG testing).

My gut feeling is that there is excess wing angle compared to the stab and when the engine is above medium speed the engine is pulling the nose DOWN thus hiding the problem. When the engine is slowed then the adequate downthrust is no longer present and the excess lift produced by the wing takes over.

I would think that if you eliminate the engine thrust completely and have the center of gravity set adequately and you still have a problem then the only thing left to look into is the angle between the wing and the stab/elevator.

I would not create a more powerful stab force to solve the 'symptom' of the problem. That would not be a long term or proper fix in my opinion.

If you are thinking that the downwash of the main wing is stalling (or actually 'blanketing') the stab at a specific speed and angle of attack during landing ... hmmm ... that might be worth looking into.

This might show up in the glide mode noted above by starting at a rather steep glide then very gradually slowing with increased 'up elevator' and watching the plane for a 'change in handling' at a certain 'nose up angle/speed' combination that might indicate a 'downwash' interference beginning.

I recall that Bob Dodgson (of sailplane fame) installed a 'trip strip' on one of his 'modified' planes (a Camano?) with which he was having flight handling problems after introducing the modified location of the stab placement on the fin. He noted an improvement in the handling with the trip strip installed. He later concluded the trip strip changed the airflow over the wing and prevented the stab being affected as it had been previously. Thus he determined that the 'modified' stab location was not the proper location for use with an 'untripped' wing. Maybe you can try this on your plane since it is very simple to install a trip strip, run some tests then remove it later. I believe for his wing the trip strip was .020 inches thick and 1/8 inch wide (multiple layers of some type of tape) and located at 20% of the wing cord and placed across the entire wingspan.

Another area to investigate may be the settings of any flaps (if they exist). Possibly the flaps can be raised to modify the downwash of the wing and observe if there is any change in handling during landing.

On conventional airplanes like the Cap, the stab generates lift downward to stabilize the aircraft. The wing center of pressure is usually behind the Cg which would normally cause the airplane to pitch down except that the stab pushes down to balance the forces. It's not likely the stab is stalling before the wing or the plane would, in fact, pitch nose down. The only way for this not to be true is for the Cg to be way aft. More likely, there is an incidence difference between the wing and stab. Might be worth a look.

Thud_Driver - sorry, you need to read some of the previous posts on the forum where the topic was thrashed to oblivion. For a down force at the tail the CG of the airplane needs to be ahead of the wing CP but that would be a CG of 0 to 25 percent. On an airplane like the Cap which is primarily an aerobatic machine the wing CP which is at 25 percent is ahead of the CG which is probably 30 percent. The CG is ahead of the airplane NP however for stability. If you work out the force vectors and moments that gives you an up force on the stab.

If for some reason the stab enountered a change in downwash at a very narrow range of angle of attacks such that the up lift on the tail was reduced then it could indeed cause a slight pitch up.

If I remember correctly the lift on the tail is proportional to (1 - de/da) where de/da is the change in downwash with reapect to angle of attack. Since there is a variation in downwash with angle of attack and a variation in horizontal tail position with respect to the airplane pitch angle it is entirely possible that this is the problem. The Cap does have the horizontal tail somewhat high compared to most aerobatic machines.

As such, there isn't much you can do about it. Just be prepared to feed in a little forward stick at the appropriate time.

A question popped into my head while reading this thread... Do aerobat models normally have any washout built into the wing?

It's not a good idea to do this since the stall charactaristics would be different upright to inverted.

If there is a stall/spin sensitivity that the pilot can't live with the proper solution for this type of plane is to use outer placed turbulator strips, root stall strips, reshape the leading edge to promote an earlier root stall, or build a new model with a tip airfoil that slightly delays the stall. For example moving the max thickness point further forward often helps unless its a forward high pont already. The leading edge shape can often play a big part too. A sharper nose wanting to stall earlier if all else is equal.

Thanks for the info fellas. Keith the plane does the same when deadstick but a little smoother, less frequency. When at full throttle the plane does pull down if trimmed at full throttle. I have it trimmed at about half speed now, so it climbs under full power at this trim. The speed at which I trim it does not affect the landing characteristics.
I just set my radio up for spoilers , raising both ali. together. Have not had a chance to fly this yet.
Ben , are you saying that the Caps design for aerobatics is not conducive to graceful landings. If this is true then what kind of landing trim or mode do I need to help change the landing characteristics.

Thanks Kent

Thanks Bruce.

Kent, No, I don't know enough about the aerodynamics of Caps to say that, certainly I have found all of my aerobatic airplanes make beautiful landings (pilot get some credit too, brag). Also all of my airplanes have the horizontal and wing pretty well aligned so I wouldn't see the effect.

According to legend and while designing the F-4 and while analyzing the wind tunnel data the engineers supposedly found a severe nose up hook in the pitching moment curve at high angles of attack. They decided it was an artifact due to wind tunnel testing.

Later on in flight test it was found to be real and would insure a departure at certain flight conditions.

If we had some wind tunnel data would we see the same with the Cap? I wouldn't expect it to be big (it wouldn't have to be much) but would expect it to be a function of where the horizontal tail is vertically located relative to the wing. I'll stick on a quick, bad, exaggerated, sketch of what might be happening.

Keep in mind this is just a theory, without wind tunnel data it is a SWAG, Scientific-Wild-A__-Guess. But other than the horizontal tail location the planform, areas, etc. of the Cap are fairly normal.

If I remember correctly the T tail setup used on a lot of commercial airplanes for drag reduction, etc., also has a pitchup at high angle of attack.

The CAP design is a notorious tip-staller.
What you are describing is an incipient tip-stall due to insufficient airspeed.
I've found it wise to land my CAPs somewhat faster than the equivalent Extra, say.
A tad too much aileron in the "balloon" situation and over it will go.

Here's a few thoughts of mine.

First, take a look at where the elevator is trimmed for level flight. This can tell you in general if something is wrong.

The fact that you can't trim for full throttle and the plane wants to climb indicates you have something in the incidence-thrust line area incorrect. I would go over the plane with an incidence meter and compare wing, stab and engine incidences.

A lot of Extra-CAP planes need expo on elevator.

Reflexing the ailerons up an eighth (1/8) inch helps on the landing of the Sig CAP. It's not quite as large, 73", I think, but it's in the ball park so you might want to try this.

Another thing I have used is landing spoilers. You set flaperon wing type, then mix throttle and flaps, having the flaps go up 1/2-1 inch at idle. I set the offset so they retract at half throttle so if I go around, they aren't still up. I arm them in the landing pattern.

Finally, adding a 6-8inch long stall strip at the wing root can cure a lot of tip stalling. Use something like 1/4 -3/8 triangle. You can tape it to the leading edge for a test. This has worked well on .40-.46 sized planes.

Well fellas tip stalling is not the current problem, when the plane balloons it is still move fast enough not to tip stall. I'll go over the whole thing with a incidence meter and recheck to see if something is amiss. I was hoping for an easy answer, I'll try the spoiler-ailiron approach and see what happens. Can any one tell me how to check the wing to stab proportions to see if this is correct?

Thanks Kent

Kent,
I have been flying the kit built version of the Lanier CAP 232 (80" wing) for several years, and it is by far the most difficult airplane I've ever had to land( for the same reasons as you indicate). I feel it's an aerodymic interference between the wing and the stab at a certain AOA as Ben has indicated. Others I have talked to have helped the problem by reflexinging the ailerons or mixing a little aileron reflex in for landing. My solution was to limit the elevator travel to 1/2" and put in 60% expo. This lets me slow to approach speed in the "soft" zone of elevator movement, and as the plane slows just above the runway surface, the elevagtor moves into the high rate for the flare. I always land tail wheel first even with only 1/2" of elevator travel. It has taken me a while to perfect the landing and each one is still a challenge.
Good luck!

Kent,
Just saw your question as I was posting the last reply. My incidences are all set at 0 degrees as per the plans. Engine, wing, and stab all 0. I've checked each very carefully with an incidence meter several times as well as the C.G. That's why I'm convenced it's an airflow anomaly.

I've done what pmw did with my CAPs, limit the elevator travel for the landing approach. But only 33% expo.
Lots of travel in flight, for those fantastic snaps!
I am very antsy landing one of those things.

I think it has not been suggested but sealing the gaps on the stab may help here. Not sure of the results on your particular plane but in my Cap, it helped alot.

PMW my cap is the H9 80". I just checked the stab , wing ,and engine all are at big fat zero. Some people thing its cool to see me land tail down, if they only knew it was a controlled crash. I am currently trying to figure out how to do the spoiler-ail thing on my radio now. I sure hope it helps.

My elevator only moves 1/4 to 3/8 inch on low with 45% expo. Still very touchy. I have all the control surfaces sealed top and bottom on the plane.

I'm building a Carden 35% Cap now with a DA 100. I will be thoroughly disgusted if a dream plane has this stupid problem. I got the 80" Cap kind of as a practice Cap 232. To gain more experience with large planes and gas motors and such.

I guess I see what happens.

Kent

Guys,
I have been having this EXACT same problem with my Dave Patrick Edge.
When I took off on the maiden flight, everything was at 0-0-0. I checked the incidence of everything a MILLION times over.
The plane immediately dove to the ground, and took a lot of up elevator trim to fly level. However, it still ballooned on landing like crazy.
Since then, I have put upthrust, and downthrust in the engine and horizontal stab, but NOTHING has cured the problem. I come in with throttle, and I can't chop the throttle enough to land, or the plane will balloon up. I have slammed the ground many times due to this problem, and now have completely flattened out my HEAVY GAUGE aluminum gear. I am so sick of this problem.
Many guys have told me I have to "FLY" the edge to the ground, and I AM flying it to the ground.
BOTTOM LINE: If I come in at a high enough speed to prevent ballooning, I am WAY TOO HOT TO LAND.
I am in KHunts position, and I don't know what to do.
P.S. I have had a couple of my buddies who are pretty much 3D pros fly the plane, and they are also slamming the plane on the runway, due to this problem.
I have not tried SPOILERS yet, which probably will help a lot, but I SHOULDN"T HAVE TO TRY SPOILERS!
I was considering giving the stab a couple degrees of positive incidence, and then giving the engine a couple degrees of UPTHRUST, and seeing if that helps. Reason for this upthrust would be to keep the plane from wanting to nose down when I put some positive incidence in the adjustable horizontal stab.
DOn't know if this will work, but I'm at my ropes end. This DP Edge is close to being sold. That's a shame because it's so pretty, and flies so well otherwise.

Just for the halibut, throw some lead up front and see if it helps....or take some careful measurements and post it here for someone to calculate the tail volume coefficient and CG on this model. Maybe there's a misprint or mis-calculation on the orig. kit plan. Who knows!

Data needed:
Wingspan, root & tip chords, leading edge sweep offset
H/Stab span, root & tip chords, leading edge sweep offset
Distance from wing L.E. to stab L.E.

There's an Excel program download somewhere on this forum that does all the nitty gritty stuff for you if you have the above measurements in hand to plug in.

Good luck

I should note that I am a little more than midly interested in this thread. I have a new, ready to fly, with a shiney Saito .58 4 cycle on the front, Cap 21 made from a old ModelTech kit.

I just got over the disappointment of finding out that old age has robbed me of the ability to hand launch a Multiplex MicroJet fast enough to avoid the ground (went to a bungee launch with great success) and now find that I will have to baby the Cap on landing.

There is little or no justice.

I assume that the RealFlight simulator does not duplicate the noted characteristics.

Here is some information about the ballooning problem on CAPs:

http://www.giantscalerc.com/projects-pirate%20232.htm

Might be worth looking at... seems like it is just a problem with the aircraft.

It doesn't make sense to me that the h.stab would be stalling. I think it may just be landing technique for that particular aircraft.