BiPlane Inverted Flight problem

[hoppy]

I built a 120% Infinity BiPlane which flies great, very neutral in all axises. Going from level flight to inverted is neutral.

Now I've built a 60% version which has inverted flight problems.
When going from straight level flight to inverted, the plane noses down sharply. Very little down elevator is needed to correct the nose over.
Powered 45 deg dives with controls then put in neutral causes plane to
pull out of the dive.

Elevator and wings have 0 incidence to one another.
CG has been varied from 20% to 33% of combined chord with no noticable change in flight characteristics.
SLight amount of up elevator required for level flight at all CG locations.

Adding reflex to the wing by raising both ailerons did not change problem.

I'm at a loss of where to go from here. My next step would be to continue moving CG rearward till it won't fly I guess.:(

Any ideas?

Plane pictures - http://www.rcgroups.com/forums/showthread.php?t=606425&pp=100

[JetPlaneFlyer]

You have the right idea... keep moving the CG back.

Steve

[Rodney]

Also, try putting the upper wing incidence at about -1.5 degrees relative to the lower wing. I'm quite sure you will like the change in performance.

[vintage1]

CG back.


Carefully.

[hoppy]

OK, cg back 1/2" (24" wingspan) from recommended. Just a little nose down reaction on inverted now. That's good.

Bad - Plane flies in a slight nose up attitude and will right turn with only ailerons but not left turn without rudder input. Motor thrust angles are good.

Dang thing seems like trying to fly a brick. The 37" version is almost perfect which is what drove me to build this one. Power is 100W for 13oz plane weight.

[MrFlash]

Hi Hoppy ,

I think you need a little downthrust on the motor and check all the incedences (and control surfaces inc ailerons) are set to zero

Because the 60% version will probably have a higher wingloading than the 120% version you will have a problem prob when its in K/E flight transitioning to inverted (the relative side area of the fus wont be enough to keep the airframe flying unless you gun the throttle). You'll find the nose will want to tuck under so when youve progress to inverted, the plane will be pointing nose down and initially requiring down ele to keep the nose up.

Just my 2c but might be worth a shot ;)

[MrFlash]

ooops looks like we posted at the same time (ish).

Whats the AUW weight ??. Reducing the wingspan to 24" puts the wingloading through the roof. To get it to fly slowly and neutral I reckon you'll need to keep the weight to around 6oz or less (with battery)

[hoppy]

All up weight is 13oz

Top wing has +1.5deg incidence.
Stab and bottom wing are 0.

Wing loading about 16oz/sqft

[JetPlaneFlyer]

Hoppy,

The right/left turn thing sounds like a seperate issue to the inverted nose drop.
Are both ailerons moving equally up and down?.. If one aileron is moving more than the other, or if one or both are moving further down than they are up then that could explain the problem.

If it's flying 'nose-up' then this probably means that it's flying too slowly (It cant really mean much else).. Are you sure that you are really getting 100W and is the prop doing it's job?.. Does it 'look' slow?

Steve

[hoppy]

We did it again.....:)

Yes, I agree with the too slow....that might be much of the problem. I'll have to recheck the watts as the pack I used today seemed puny. Anything less than 2/3 throttle put it into harrier flight.

It does however land nicely, pretty slow and level.

[MCarlton]

Are the ailerons on all four wings?

It almost sounds like the top wing is stalled and you're flying on the lower wing. Perhaps?

I have also read that using an opposite incidence to what you have may help. Rig the lower wing with 1 degree positive and the upper wing with 1 degree negative. I don't know the theory, I just read it somewhere and it seems to make sense.

[MrFlash]

I think its just too heavy ..

At that size you ideally want a wingloading of around 3 - 5 oz /sq.ft (ie AUW weight of around 6-7oz with battery) .

13oz AUW will need tons of throttle just to keep it airborne. The minute you enter K/E it will want to screw into the deck ill bet.

[hoppy]

Ailerons on all 4 wings.

I've only tried KE once but it didn't appear to be too bad.....some elevator mix might make it OK.

Thanks to all for responding..... your suggestions have me checking things I assumed were OK.

The pack I used today is a goner - only putting out 50W after 50% used. 1600mah 15C pack. A newer one is around 120W. That will take care of the slow part of the problem.

After reading the Infinity thread again, I see where I can put the cg another 1/2' aft. That is assuming that wingspan/cg = wingspan2/cg2 to solve for a cg based on a shorter wingspan. Can I do that?

That new cg number also agrees with V1's cg calculator so I guess It's right. Just guessing, another 1/2" should do it.....hope hope.

[vintage1]

What pitch speed do you have? Maybe its under pitched and can't get any decent speed? Then it will have to fly nose up to get the lift.

100W into that should be 60mph or so flat out.

[hoppy]

APC SF 9x4.7 replaced by GWS 9x6HD today.

With the + top wing incedence removed, the cg back some more and the prop/battery changed, things are better. Tough to do much more trimming today as the wind was at 14mph and gusty.

Found a small roll axis weight imbalance and the wings were 1/4" off at the wing tips to tail measurement.

Never thought about it much but I guess the trim of a small plane has to be much tighter than a larger one. A 1/4" variation on a 24" span is much larger percentage wise than a 1/4" variance on a 48" span plane.

I'm not getting 60mph but it's much better than that underpowered test yesterday with the worn out pack.

[Rodney]

Get the incidence on the top wing down a bit, about -1.5 degrees relative to the lower wing. Also get about 5 degrees downthrust relative to the lower wing. CG is important, use for fine tuning after getting incidences set.

[hoppy]

Thanks Rodney,
The top wing has some neg incidence now, probably about -1 deg.

The power-off test gave the desired reaction today with the installed downthrust but I need to check it again when the winds subside to see if it is enough.

Winds here in FL are predicted to be ~14mph tomorrow...hope they miss the prediction.:)