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Biplanes - quick question

Old 04-24-2016, 02:05 AM
  #1  
stanz358
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Default Biplanes - quick question

Is it just me or do biplanes have a tendency to pitch up dramatically and try to roll left on takeoff? I can understand pitching up since the wing area is doubled but why does it always start rolling left. Ailerons are trimmed correctly, I'm not yanking back on the elevator and once it in the air, flies sweet. Any suggestions?

Thanks
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Old 04-24-2016, 02:35 AM
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Ron
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check wings to make sure you don't have " wash in" stbd wing wash in will cause left roll, especially if you have washout in the port wing.
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Old 04-24-2016, 03:34 AM
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If you are holding up elevator while it's on its takeoff roll, the plane might be going into the air at stall or just slightly above stall. If it stalls it usually rolls left.
Sufficient power accelerates the plane to a speed over stall speed and all is right with the world again.

Paul
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Old 04-24-2016, 04:09 AM
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The turning issue is common with taildraggers when you don't manage throttle and rudder properly.

The pitch issue is likely to be CG and/or wing vs tailplane incidence or thrustline.

Yes, holding up elevator trying to make the tailwheel more effective can aggravate the problems.
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Old 04-24-2016, 05:05 PM
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As a dedicated and long serving Bipe flyer ... OP has described the typical yank of deck too early take-off.

As we start our take-off - we usually have full UP elevator ... which MUST be reduced as we gather speed ... by time lift is starting to act on the wing - we MUST be neutral on elevator ... tail will rise ... model will trundle a bit more and start to lift of deck. It would be normal for a left bias as she climbs due to torque.

BUT if the elevator is a bit late in coming of ... the wing will lift her and she's basically near stall and torque causes the roll left ... that also causes a left swing ... and unfortunately often leads to the cartwheel we see ...

I'm just uploading a video of my Pitts S2 flown yesterday ... you can watch her roll and take off ... even though she's clean away - there's the left bias ... that will always be there ... unless your motor / prop turns opposite way.

Nigel
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Old 04-24-2016, 06:07 PM
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I know its a wet fuel model ... but its still a biplane !



Nigel
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Old 04-24-2016, 07:54 PM
  #7  
stanz358
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Thanks all. Before posting a read up a bit on tail daggers and after reading all of your replies I probably am getting her into the air too quickly. Thanks again
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Old 04-24-2016, 08:16 PM
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I have a biplane that I fly quite often and I always yank it off the deck...I try to blame it on having a short runway but I know I do it myself. I've gotten so bad at it that I already am compensating for it to swing left as soon as it is almost in the air. I realize it is my fault and not the plane, and I need to work on letting it fly itself off the ground.

It is a easy thing to do, hopefully you can work it out before you get "trained" like I am lol.
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Old 04-25-2016, 11:48 AM
  #9  
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Originally Posted by P51 fan View Post
I have a biplane that I fly quite often and I always yank it off the deck...I try to blame it on having a short runway but I know I do it myself. I've gotten so bad at it that I already am compensating for it to swing left as soon as it is almost in the air. I realize it is my fault and not the plane, and I need to work on letting it fly itself off the ground.

It is a easy thing to do, hopefully you can work it out before you get "trained" like I am lol.
Yeah, I only have about 15 flights on mine but I started doing the same thing - figured it's normal behavior and just anticipated and compensated for it. That didn't work to well Friday, crashed it pretty good. But was out yesterday and tried keeping it on the runway longer and it helped. Just starting out so I'm definitely trying not to pick up any bad habits. - thanks
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Old 04-27-2016, 09:50 PM
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I would give it more right thrust. I see this all the time. Few people know how to set up thrust angles. They accept what the manufacturer has provided.
Any aircraft can do what you are describing especially high wingers, not just biplanes.
I have biplanes that take off dead straight with no tendencies to turn whatsoever. All I do is open the throttle and they take off themselves. Spend some time on thrust angles.
Push the plane along the ground without the motor running. It should run dead straight. If not, your wheels or tailwheel are not aligned.
It can be fixed and easily.
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Old 04-27-2016, 11:01 PM
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I can make a 90 deg left turn takeoff with my DC-3 by timing applying full throttle
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Old 04-28-2016, 12:22 AM
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So long as it is not a 180 turn.

10 minutes ago I finished another biplane and been out taxiing it around and 'hopping' it. It has no tendency to veer off.
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Old 04-28-2016, 01:46 AM
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Mine only does a bit when leaving the ground, I also think it is because I modeled a 50 year old 1/2A design and enlarged it without changing any thrust angles. It doesn't do it on the ground and just a little when leaving the ground, I just wanted to make a point to the op about "fixing" the issue before he just accepts it and trains himself to deal with it.
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Old 04-28-2016, 03:54 AM
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Originally Posted by fhhuber View Post
I can make a 90 deg left turn takeoff with my DC-3 by timing applying full throttle
I used to have a Cessna 310 that I used for multi engine training.
Every once in a while I'd get a new student fresh from a Cherokee or Cessna that thought pushing the throttles in quickly was the sign of a seasoned pilot.
I had to counter with pulling the mixtures off because brakes weren't enough to stop the left turn. Even with the mixtures off the plane would still turn 45 degrees. Glad we had wide runways.
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Old 04-28-2016, 04:04 AM
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There's making that 90 deg turn on purpose with 3:1 power:weight RC model... and there's doing it by accident at appx 0.7:1 power to weight (?? close ??) with your behind in the seat.

If I blow it with the model I have to glue some foam bits back.

Not worth even trying it full scale.
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Old 04-28-2016, 04:16 AM
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Originally Posted by fhhuber View Post
There's making that 90 deg turn on purpose with 3:1 power:weight RC model... and there's doing it by accident at appx 0.7:1 power to weight (?? close ??) with your behind in the seat.

If I blow it with the model I have to glue some foam bits back.

Not worth even trying it full scale.
Close, 520 hp and 4,300 lbs at training weight.
Just part of the training.
Power was still pretty good, I could get up to 190 mph in 1 mile from a standing start.
When the student got better, I'd shut one engine at lift off.
Sounds worse than it was.
Did it for nearly 40 years and not one scratch.
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Old 04-28-2016, 11:32 AM
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Originally Posted by Panther View Post
I would give it more right thrust. I see this all the time. Few people know how to set up thrust angles. They accept what the manufacturer has provided.
Any aircraft can do what you are describing especially high wingers, not just biplanes.
I have biplanes that take off dead straight with no tendencies to turn whatsoever. All I do is open the throttle and they take off themselves. Spend some time on thrust angles.
Push the plane along the ground without the motor running. It should run dead straight. If not, your wheels or tailwheel are not aligned.
It can be fixed and easily.
I agree with Panther on the right thrust and I would add some down thrust too, also on most bi planes, the top wing is set at a neg Incidence, like about 1 to 2 degrees to help stabilize them in flight, get the tail wheel up in the air by adding a little down to gain speed before take off.
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Old 04-28-2016, 03:18 PM
  #18  
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Originally Posted by Panther View Post
I would give it more right thrust. I see this all the time. Few people know how to set up thrust angles. They accept what the manufacturer has provided.
Any aircraft can do what you are describing especially high wingers, not just biplanes.
I have biplanes that take off dead straight with no tendencies to turn whatsoever. All I do is open the throttle and they take off themselves. Spend some time on thrust angles.
Push the plane along the ground without the motor running. It should run dead straight. If not, your wheels or tailwheel are not aligned.
It can be fixed and easily.
I don't disagree with you because manufacturers advise thrust lines based on what they fit to the model during development ... often underpowered.

But what I would say is thrust lines are seriously dependent on what power levels you apply to the model and of course speed on ground and in the air.

My Nieuport 28 WW1 job was a pig on the ground due to a) short nose moment back to wheels, b) Right thrust, c) ground speed ... she would ground loop as soon as look at you !
But in the air she was spot on. IF the thrust lines had been altered to sort ground - she would have had a different character in the air.

One thing that people do not often do is check out the 'yaw' factor of their models. Because many of my models do spend a lot of time nose vertical ! I do check that factor ... and it really is so simple.

Build up speed ... then pull vertical ... watch which way she falls of ... BEFORE speed decays. Many models require right rudder to counter that fall off ... It will always be a compromise of motor thrust line and rudder offset to arrive at best.

nigel
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Old 04-28-2016, 03:22 PM
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Don't go randomly changing thrust angle...

If you have the OEM motor and prop in a RTF, the thrust line will be fine as is.

If you have the recommended power system installed in a kit, the thrust line shown on the plans will be fine.

It takes a significant change in prop and power to need to change the thrust angle.
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Old 04-28-2016, 05:23 PM
  #20  
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Originally Posted by fhhuber View Post
Don't go randomly changing thrust angle...

If you have the OEM motor and prop in a RTF, the thrust line will be fine as is.

If you have the recommended power system installed in a kit, the thrust line shown on the plans will be fine.

It takes a significant change in prop and power to need to change the thrust angle.
My sentiment as well ... as my post said ... thrust lines are usually set by manufacturer based on what they fly the model with.

Nigel
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Old 04-28-2016, 10:39 PM
  #21  
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Nice to see you back Chellie.
In my latest build I used the manufacturers thrust angles. I actually built 2 planes the same.
On both planes when the motor was fitted it was off center considerably. This can make a difference in flight characteristics.
That is like mounting a vertical fin at an angle and then using rudder to correct it.
I moved both motors over so the propshaft was more central.

You can see the manufacturers holes for blind nuts but I use hex head self tappers. They don't come loose. You can see the motor is much closer to center after the mod.



Not the correct spinner assy. Just using it for set-up Also extra ply added behind the motor mount firewall for better screw depth.



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Old 04-29-2016, 04:48 AM
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Originally Posted by CHELLIE View Post
I agree with Panther on the right thrust and I would add some down thrust too, also on most bi planes, the top wing is set at a neg Incidence, like about 1 to 2 degrees to help stabilize them in flight, get the tail wheel up in the air by adding a little down to gain speed before take off.
Funny you should mention that as I just built 2 identical planes and the wing incidence is quite different between them.

One is Negative and the other is 15mm Positive. I have cut the strut and fitted a spacer on the rear strut and have it pretty close to negative now. I hopped it across my paddock and it behaved nicely so I am about to take it down the road for a workout shortly.
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Old 04-30-2016, 12:15 AM
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Originally Posted by Panther View Post
Funny you should mention that as I just built 2 identical planes and the wing incidence is quite different between them.

One is Negative and the other is 15mm Positive. I have cut the strut and fitted a spacer on the rear strut and have it pretty close to negative now. I hopped it across my paddock and it behaved nicely so I am about to take it down the road for a workout shortly.
I have flown a lot of bi Planes, and having neg Incidence on the top wing sure helps, the motor down thrust helps too to stabilize the plane in flight, the down thrust has to be added a little at a time and then check the way the plane flies, same with the right thrust, its a game of testing the affects of adding different thrust and off set thrust and seeing how the plane flies, same with CG, move it around until you find a sweet spot the MFG CG is just a starting point, and for beginners, its best to be a tad on the nose heavy side, that way the nose will drop before the plane stalls, Take care and have fun, Chellie
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Old 04-30-2016, 12:21 AM
  #24  
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before i got into electrics I had a 40 size Ultimate Biplane, I put a K&B 61 in it and it Hauled Buns I bought a new computer Transmitter for it and and the Radio system failed on me, no more Airtronic radios for me, and that was the end of that plane

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Old 04-30-2016, 12:23 AM
  #25  
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Originally Posted by Panther View Post
Nice to see you back Chellie.
In my latest build I used the manufacturers thrust angles. I actually built 2 planes the same.
On both planes when the motor was fitted it was off center considerably. This can make a difference in flight characteristics.
That is like mounting a vertical fin at an angle and then using rudder to correct it.
I moved both motors over so the propshaft was more central.

You can see the manufacturers holes for blind nuts but I use hex head self tappers. They don't come loose. You can see the motor is much closer to center after the mod.



Not the correct spinner assy. Just using it for set-up Also extra ply added behind the motor mount firewall for better screw depth.



yea, that looks like to much right thrust to me
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