FF kit being stubborn...

[pumptato-cat]

Hey, I'm using an FF '23 kit. (would prefer to not post flight times..)
I get decent flights, but I know they aren't as good as they could be. The plane tends to have either a very fast/slow ascent, decent cruise(short, usually less than 40sec) and then stalls/dives(both happen very inconsistently). Each time it stalls, it dives down right after and loses 4-5 ft of altitude(maybe more) and then wobbles for half a circle or so. There is no AC on, and I do not walk around while it's flying. Also, the flight circle shifts by a TON. It used to be always in one direction, but while recording tournament runs today, it moved in the opposite direction instead of the usual shift. This shift is anywhere from 5-20ft, and usually ends in the plane hitting something or moving almost all the way across the room. It's not consistent, either.

Any ideas on why this may be happening? I've tried adjusting CG(which hasn't seemed to help at all--made the results worse somehow?), angle of incidence, etc. I'm too afraid to adjust the horizontal stab, as that broke the plane last time, but might change that. I'm not sure why the descent is so bad though-if we improved that we could add 30 sec-but it just won't get better no matter what we do. And why would the flight circle shift by that much? Nothing on the plane seems loose or moves around(As far as I can tell) and there are no air currents.

Thanks

[coachchuckaahs]

Is there any way you can post a video? If you need to, post one privately and PM me (and coach Brian) a link.

It certainly sounds like it is marginally stable. This would mean move your CG forward in SMALL increments, and then adjust your incidence of your wing (increase) correspondingly. If the FF kit is similar to prior years, you will move the wing BACK to get the CG forward relative to the wing.

After a CG adjustment, repeat your low-power trim flights until the attitude of the plane is correct (slight nose up, not stalling, not mushing). Generally, we add incidence (or decalage) until we pick up a slight stall, and then we back off until it stops.

Coach Chuck

[bjt4888]

Cat,

Post all of your trim measurements too.

Brian T

[jander14indoor]

Also, look for something broken! If you have a plane that has been flying well, then suddenly behaves inconsistently, that is often the cause. May be a partial crack that lets something move that isn't obvious, but enough to cause inconsistent results.

Jeff Anderson
Livonia, MI

[Maxout]

I haven't built the FF plane yet, but the Div C plane is very short coupled and requires a forward CG. To shift CG, don't slide the wing back because it shortens tail moment and changes the amount of downward on the stab. I had this same problem with the Guru kit (except that it was drastically worse). Also I'm having kinda significant issues with the ongoing policies of certain manufacturers to set the stab at 0-0 and adjust the incidence with the wing because it effectively adds downthrust anytime you add incidence, sometimes making the plane dive even worse. FF extensively tests their airplanes so the effects aren't as bad, but that doesn't change the fact that your flight pattern is exactly what I would expect from a plane with too much downthrust: flat climb, stally glide.

Here's a solution that you should consider if the plane continues to give you problems. This will fix the problems, but you have to actually follow it:
1. Verify that the prop bearing is set exactly parallel to the bottom of the motorstick
2. Cut the back of the stab mount loose and glue a 1/16" thick shim between the back of the stab mount and the back of the fuselage (raises the stab TE 1/16")
3. Zero out the wing incidence to the stock setting on the plans.
4. Slide the wing to the most forward position suggested in the instructions.
5. Shift all ballast to the nose or to whatever position gets the CG 1/4" forward of the location listed on the plans, as measured from the front of the wing (CG is measured from the front of the wing, position relative to the fuselage isn't relevant)
6. If you need additional clay on the nose to get the CG position (measured with the rubber motor on the plane), do so even if it makes the plane overweight.
7. Adjust the wing incidence until the plane glides without stalling, with a rubber motor installed.
8. Begin powered testing, and you should see an improvement in performance. Add side thrust and left rudder as needed to maintain turn throughout flight.

[coachchuckaahs]

Thanks Josh, a far more detailed response based on actual experience flying this year!

I agree that moving the CG with ballast on this short-coupled plane is a better solution!

Coach Chuck

[poonda]

had a similar problem with the div c kit, everything maxout said will probably work great

i ended up sliding the wing a tiny bit more forward (like a centimeter) than on the instructions and moving the cg a bit (maybe 1.5cm) forward with clay
put a shim under the trailing edge of the stab and increased stab tilt as well
i found that the plane actually flew better if the glide had a really slight stall, though generally you should make sure your plane glides fine with a motor like maxout said

[pumptato-cat]

Thank you so much for the advice everyone!!!
Sorry for the late response, my computer's been broken for days. I finally managed to get a school chromebook though!

bjt4888, I will post trim measurements when I get home.

jander14indoor, I think you're right! The piece that attaches the horizontal stab is broken, and has been reglued+reattached multiple times(crash landings), so this might be an issue. I'm thinking of cutting the piece off and resanding the TB, and then redoing the stab angle(it's 5+ degrees because I messed up gluing it and sanding...) I'm not sure how this might affect flight though, because the stab will be slightly lower than it used to be on the plane(there will be a groove in the back that the stab sits into, which means it doesn't sit on top of the TB, it sits lower. Sorry if that doesn't make sense)

coachchuckaahs, thank you so much! I will PM a video after I do some more flying tomorrow(I've PMed Coach Brian already but the videos are outdated...) What do you mean by marginally stable? I may be misunderstanding this, but the plane drops by 4-6ft. In one case, it hit the ceiling while climbing(I changed washin and was not expecting it to climb so high.. huge mistake) and then dived 18 feet and hit the floor...

[Maxout]

Ok this is basically the same thing I was suggesting, so now we have proof that it works. Thank you for sharing your experience and solutions!

had a similar problem with the div c kit, everything maxout said will probably work great

i ended up sliding the wing a tiny bit more forward (like a centimeter) than on the instructions and moving the cg a bit (maybe 1.5cm) forward with clay
put a shim under the trailing edge of the stab and increased stab tilt as well
i found that the plane actually flew better if the glide had a really slight stall, though generally you should make sure your plane glides fine with a motor like maxout said

[coachchuckaahs]

coachchuckaahs, thank you so much! I will PM a video after I do some more flying tomorrow(I've PMed Coach Brian already but the videos are outdated...) What do you mean by marginally stable? I may be misunderstanding this, but the plane drops by 4-6ft. In one case, it hit the ceiling while climbing(I changed washin and was not expecting it to climb so high.. huge mistake) and then dived 18 feet and hit the floor...

Marginally stable means that it will fly, but does not respond well to disturbances, such as air or hitting the ceiling. Yours is a classic case of a plane that is almost unstable. In good air and no touches, it may be more efficient. But in SO you have to expect some disturbances.

Stability is the relationship between the center of gravity and the center of lift. Read up on static stability margin. A stable plane will respond to a disturbance with a correcting moment, while an unstable plane (tail-heavy) will respond by amplifying the disturbance.

The ONLY way to fix this is to move the CG forward relative to the CL. As josh has pointed out, on this year's short-coupled design you don't want to move the wing back (moving CL back), but rather move the CG forward. Do this by adding clay to the nose or moving existing clay toward the nose (or removing weight at the tail). Re-adjust your decalage to re-optimize for the new CG. Each move of the CG forward will improve stability. But what is the cost? You will need a little more decalage to hold the nose up, meaning more lift on the wing, and lift = drag, so it becomes less efficient. So the trick is finding the stability margin (distance from neutral stability) that gives the minimal flight behavior you need, rather than taking a big swing and having a nose-heavy plane that plows through the air.

A marginally stable plane may also do unpredictable course changes due to relatively minor air disturbances.

A small change in CG (3mm at a time) can have a large impact on this year's plane due to the short tail moment.

You can even have different CG setups for quiet air and noisy air (AC on), but in SO since you only get two flights I would err to the more stable side.

Coach Chuck