Why did my plane do what it DID???

[DBono99]

Hi All Let me start off telling you of my building experience..I've been building from both regular ( Sig Kits) and short kits for the past 7 years..I've must have two dozen planes..With that said, here is my question..
Today my test pilot tried to madien my Bingo 20 that was build from a short kit, with the plans I got from Flying Models.. The plane was tested for CG and was a tad nose heavy if anything. OK 1st flight the plane came off the ground about 5 feet and stating propoising after the 3rd porpoise it hit the ground..I haven't a clue as too what made the plane act like that..All my planes usually madien very well,if not, just some small CG adjustment was needed..


Thanks Dan

[NX-687]

Your connection between elevator servo and elevator is that a rigid dowel or a golden rod , or is there any flex there ?

[skyrat]

I had a similar problem, only on landing, with a highly chopped Sig Kadet Senior. When power was reduced & airspeed lowered the plane would start porposing wildly until it pitched straight up & entered a tail slide. It did this twice, the second time was fatal. CG was in range, elevator connection was a little sloppy but acceptable but the thrust line & wing/stabilizer incedeces were not what they should have been. to much down thrust, not enough negative on the stabilizer.

[DBono99]

Hi Guys: Your connection between elevator servo and elevator is that a rigid dowel or a golden rod , or is there any flex there ?
It's a 2/56 rod inside a plastic tube. NO problems there

CG was in range, elevator connection was a little sloppy but acceptable but the thrust line & wing/stabilizer incedeces were not what they should have been. to much down thrust, not enough negative on the on the stabilizer.

CG was good a tad nose heavy if any,elevator connectons were good,no slop,thrust line incedeces SEEM to be fine, will have to check when its repaired.

Thanks Dan

[JetPlaneFlyer]

I appreciate that the CG was in the range shown on the plan but where exactly was this as a percentage of the wing chord? (could be an error on the plan)

Otherwise maybe far too much elevator movement and/or some 'stickiness' in the linkage.

Steve

[NX-687]

Your incidences plays a part too, and motor incidence :)

[DBono99]

Hi NX "Your incidences plays a part too, and motor incidence"
Can you be specific??
I'm in the process of putting the plane back together. Don't want the same problem showing up again.
What and how should I check.

Thanks Dan
PS All the planes I built whether regular kits or short kits I NEVER a problem with incidences..Then again I NEVER checked any of them..

[ghoti]

Move CG balance point forward, toss into tall grass. Bill

[skyrat]

i need a picture & some rough numbers on the plane & i can probably get you pretty close to where things should be. It all depends on configuration, size, weight & power.

[DBono99]

Skyrat: Here is a copy of the plan.49" WS-440 Wing Area-flying weight 49 oz. What else would you need.

Thanks Dan

[skyrat]

I think I see the problem already! It's hard to tell from just a pic of the plans, but it looks like the plane is built 'flat', no down thrust & no negative incedence on the tail. Measure it all on the plans, you want in the area of 3 degrees down thrust, 2 to 3 degrees negative(leading edge DOWN) on the tail & your CG around 1/4 chord(ie, 8 inch chord, CG 2 inchs aft of leading edge) All angels should be taken from the true camber line if the wing.

[JetPlaneFlyer]

I checked the CG position out with a CG calculator and it looks ok... It gives a theoretical 10% static margin, which is quite generous.

However... the symptoms certainly 'sounds' like a CG issue so it would do no harm to adjust the CG forward a little. You can always move it back later.

I'd not even consider adjusting the stab incidence, you can get the same effect by trimming a little 'up' elevator, but I’d be surprised if this is necessary unless the CG is moved way forward.

Are you sure you have not got too much movement on the elevator and you are over-correcting?

[DBono99]

Hi All Many thanks for the fast and informative replies..Here are some numbers concerning the CG..Wing card constant 9 1/4 including the ailerons Cg according to the plans center of main spar which is 2 3/4" come out to close to 30%from the LE. Now 25% would be 2 5/16" from the LE.

Elevator throws set at 3/8" the plans call for 1/2" up N down..So I should alright there..I am going to put in some UP trim in the elevator next time.
What BUGS me the most that I put together many kits, with out any major problems.

Some thing I just thought about..The plans call for 3/16 sheet for the stab n elev. I made them out of 3/16 stick and they came larger then the plans but not by much..Maybe 1/8" bigger then plans..

Thanks Again Dan
PS The fuselage broke into 2 parts, 1 break by the TE and other break the firewall came out..Both have been expozed back together..I just have a little work on the wing where the troque rods n blocks are..Hope to have the Bingo in the by the week end..

[DBono99]

Otherwise maybe far too much elevator movement and/or some 'stickiness' in the linkage.
The elevator linkage is fine.Throws set 1/8" less the was called ofr on the plans..

Are you sure you have not got too much movement on the elevator and you are over-correcting?

Elevator movement seams fine,my test pilot is an accomplished RC and full scale pilot..So there fore piltui error is ruled OUT!!!

Another thing I just thought of I extended cowl 1/4" to accomdate my electric motor..This have been before withOUT any problems..

Thanks Dan

[JetPlaneFlyer]

You say that the solid tail surfaces were replaced by 'stick'-built'... could it be that the tail is flexing?... I know this would be unlikely at low airspeed but it could explain the symptoms.
An 1/8" increase on the dimensions of the tail wont make any significant difference, adding area to the tail generally increases stability.

Steve

[DBono99]

Hi JetPlaneFlyer..I don't see any flex, all the controls surfaces are tight, no slop or flex. Would you think the wing/stab incidences had any part in the proposing? All the planes I've built I never checked the incidences, To be honest I really don't know how.

Thanks Dan

[JetPlaneFlyer]

Would you think the wing/stab incidences had any part in the proposing? All the planes I've built I never checked the incidences, To be honest I really don't know how.

Thanks Dan

The incidence on the plan looks to be what you would expect on pretty much any sport aerobatic model i.e. everything set to close zero relative to the fuselage datum. The difference between chord line and zero lift AoA of the airfoil (which looks to be not quite symmetrical) should give the required trim.. or if not a couple of clicks of 'up' elevator would do the trick.

It’s such a conventional looking design that the only things I can think of are CG location and slop/stickiness/excess movement in the elevator. If you have ruled out the latter then the only thing I can suggest is to move the CG forward to about 25% chord. A forward CG, within reason, can’t do any harm.

Steve

[Brandano]

Could the plane just be stalling? If it doesn't have a tendency to drop a wing, it would climb up, stall, drop the nose, pick up speed and repeat "ad nauseam". This would happen on a plane with a constant chord, even if the CG is forward enough. Actually, I think a forward CG would exasperate it. How was actually this porpoising, smooth at the bottom and sharp at the top?
[edit]
Actually, even without reaching the stall, some porpoising can happen even on a stable plane just because of the speed variations. Check this article: http://www.aerospaceweb.org/question/dynamics/q0233b.shtml
PIO will probably exasperate things, letting the controls go should have the plane stabilizing on its own.

[Loone]

Based on my experience with the Kadet Senior ARF (2) The elevator is sloppy but does not really affect the flight characteristics of the plane. The original dowels are sloppy, also on number 2 I used carbon rods instead of the wooden dowels as an experiment and supported them all the way down so no bending occurred, and I still got a sloppy elevator. Maybe the Sullivan golden rods might make a difference. But again the slop is not where the flight problem is.

I balance my Kadets by having a friend hold the plane at the end of the wing at the spar and I hold the other side and use the battery to balance it at that point. have used that method for several years with good luck.

Some things I would check are:

the EPA settings on your transmitter or your control speed.

C/G

tail group strength

excessive slop on your elevator.

mhowell.electricdrones.com (http://mhowell.electricdrones.com)

[DBono99]

Hi All Many thanks for fast replies and great information..Here is what I'm going to.
1. Move the Cg to 25% of the MAC which is going to be 2 1/5 instead of 2 3/4.
2. Put some UP trim in the elevator.
3. Reduce the elevator throw from 3/8 to 1/4.The plans call for 1/2.
4. I already have 30% expo. in the elevator, that should enough.

The plane is almost back together..Just have to work on some covering and trim..Wasn't such a bad repair, but I'm getting tired of repairing planes..
A couple of weeks ago I had to repair my Sig Kobra, now it flies great just like before the crash..I hope I have the same luck with the Bingo 20

Thanks Again Dan
PS When I finally get the Bingo in the air, I will report back.

[Loone]

Dan,

If you are running a hitech transmitter be sure to set it to -30% expo

MH

[hoppy]

Same thought I had.:)

[ghoti]

. . . me,too. Be sure to put in the minus sign. Gets real squirrely on plus expo. (Don't ask me how I know.)
Bill

[DBono99]

Hi All I'm using a Futaba whic is - expo same as Hitec..

Thanks Dan

[DBono99]

Hi All When putting on the wing today, I notice some play/movement by the wing saddle..My (2) 1/4 20 plastic screws were as tight as I could get them. I'm wondering if that could the root of my proposing problem?? I think the movement stems from me making the holes in the former where the wing dowels go a little bit larger..The wing has (2) 3/16" dowels.

Thanks Dan

[JetPlaneFlyer]

I'm wondering if that could the root of my porposing problem??

Very definitely YES! I'd considered that the tail may be flopping about but never thought of the possibility that the wing may have been!

[DBono99]

Hi All Many THANKS for fast replies and GREAT information got GREAT news..The Bingo's 2nd flight went off without a hitch, flies a lot like the 40 glow version. So looking back then, the main problem was probably Cg, although the wing movement didn't help any..I didn't get my hands on sticks today..The sky wasn't soo great..Can't wait for the next time..

Thanks Again Dan
PS I moved the Cg too 25% instead of the original 30%..To do that I had to add about 3-4 ozs of weight to the nose..My test pilot said the Cg was right on!!! I added wing saddle tape to remove the movement in wing..

[Texas Buzzard]

JetPlaneFlier you have had a lot of good advice, maybe mine is just a repeat.

This was posted,"It’s such a conventional looking design that the only things I can think of are CG location and slop/stickiness/excess movement in the elevator. If you have ruled out the latter then the only thing I can suggest is to move the CG forward to about 25% chord. A forward CG, within reason, can’t do any harm.

Steve"

I agree with Steve. I have been building and flying for maybe 40 years and the hunting or porpoising you described is screaming,"My CG is too far back".

With as much experience that you have I doubt the elevator is so sloppily done as to cause hunting. There were two planes I scratchbuilt (both biplanes) that were miraculously saved from wild up and down hunting. Both had to have the CG moved forward. I almost hit a cow with one wild flight. A too forward CG is SAFE to fly. CG too far back is not much fun.

If your stab or wing was not at the correct angle of attack, it would not porpoise, it would just demand elevator correction. It wouldn't hunt.

[macboffin]

Looking at both plans and model looks like zero incidences.Also looks like a very "Square" leading edge on the tail plane, little or no rounding, which can't help airflow.Easiest fix might be to add some positive wing incidence and a little down thrust.

[JetPlaneFlyer]

Looking at both plans and model looks like zero incidences.Also looks like a very "Square" leading edge on the tail plane, little or no rounding, which can't help airflow.Easiest fix might be to add some positive wing incidence and a little down thrust.

The problem is already fixed... read a couple of posts up; a more forward CG and tightening up the sloppy wing attachment did the trick.

Steve

[Thomas B]

I think I see the problem already! It's hard to tell from just a pic of the plans, but it looks like the plane is built 'flat', no down thrust & no negative incedence on the tail. Measure it all on the plans, you want in the area of 3 degrees down thrust, 2 to 3 degrees negative(leading edge DOWN) on the tail & your CG around 1/4 chord(ie, 8 inch chord, CG 2 inchs aft of leading edge) All angels should be taken from the true camber line if the wing.

Negative tail incidence is kind of an old FF trick for gliders. I never use any in a sport R/C model. Better to adjust the wing incidence.

The typical sport aerobatic or scale-ish model seems to do best with a degree or two of positive wing incidence, zero tail incidence and little if any downthrust. Some high wing models need a little downthrust to offset power pitch effects when changing throttle settings due to the thrust line being below the wing.

The little bit of positive wing incidence usually leads to a tiny bit of down trim in normal to higher speed flight, which makes the model look "right" as it zips by, nose slightly down, tail higher. Watch a video of a P-51 flying by at high speed...the nose is down and the tail is up.

This works out well when flying inverted, as the down trim helps in that area.

Nothing worse than an R/C model that needs lots of up trim to fly level...it ends up being very sensitive to flying speeds and seems to be staggering around.

The only model I have ever built that had a pitching up and down problem with correct CG was one that I screwed up the wing incidence on...it ws about 2 degrees negative on the wing and zero elsewhere......it was nearly unflyable.

[DBono99]

Hi Thomas..All the Bingo needed was to move th Cg forward..I went form the original, according to planes 30% to 25%.. The plane is flying really good with the Cg where it is..

Thanks Anyway for you Reply
Dan

[DBono99]

Hi All Well I finally got to fly my Bingo 20 yesterday..Real nice flyer, so then the problem was with the Cg.. Once that was moved forward the plane has NO bad habits..

Thanks Dan