Plane Not Capable of Takeoff - can't taxi straight.

[Merv]

Thanks for the advice!

@quorneng makes a good point, your main gear may be too far forward of your CG.

 

[Hai-Lee]

Just a question! I have a pic of a flying model that will take off from the ground so is the undercarriage location shown better or acceptable?

Stirring the pot.

Have fun!

 

[Merv]

Stirring the pot.

Im not sure but that look likely a control line plane to me.

 

[SlingShot]

Just a question! I have a pic of a flying model that will take off from the ground so is the undercarriage location shown better or acceptable?

Stirring the pot.

Have fun!

I would say a high wing plane like that, with apparent positive wing incidence, would tend to leap into the air. It's not going to roll up on the mains and run along the ground.

 

[Hai-Lee]

Im not sure but that look likely a control line plane to me.

Nice try but you would not require that level of dihedral for a control line! It is RC with functional rudder and elevator!

Kits are temporarily out of stock according to the website!

have fun!

 

[Userofmuchtape&glue]

You are using rudder to correct for the P factor, yes? And if so is it going the correct way?

 

[Hai-Lee]

I would say a high wing plane like that, with apparent positive wing incidence, would tend to leap into the air. It's not going to roll up on the mains and run along the ground.

Again nice try! Yes it does take off quite rapidly but the rudder is more than enough to keep it straight for a long run take off and a taxi back to the pits if required.

It is rudder control that is the key! Yes the plane is very light and the rudder very effective. In addition the wheels are extremely lightly loaded and hence little friction. My point is simply that if the wheels are aligned in the desired direction of travel and the rudder is effective then you can steer it.

As for the topic of this thread I would like to point you to the analogy of a single axle trailer. As long as the hitch is in front of the axles the trailer will always want to track straight behind the tow hitch, (subject to the alignment of the wheels of course). It the plane has a piece of string tied around the motor shaft and the plane is pulled along the ground does it still want to turn as demonstrated in the video? I severely doubt it!

As the motor thrust is applied in front of the axles then something else is causing the issue. The only remaining things to consider are the spiral of air from the propeller turning and a lack of rudder authority! A steerable tail wheel may be of assistance at very low speed but the real solution is to use proper control inputs, (including throttle usage).

As usual though, Just my opinion and experiences!

Have fun!

 

[d8veh]

As the motor thrust is applied in front of the axles then something else is causing the issue. The only remaining things to consider are the spiral of air from the propeller turning and a lack of rudder authority! A steerable tail wheel may be of assistance at very low speed but the real solution is to use proper control inputs, (including throttle usage).

I think that there are other things going on too, though it's not easy to say which is most significant. In order to make a driving force, there has to be a reaction force, which manifests itself as a twisting force through the motor mounting. that twisting force causes there to be more weight on the left wheel than the right, so more drag. Also, there are strange gyroscopic forces that twist the plane due to slightly different thrust between when the propeller blade is up and down due to the boundary layer effect, and finally, there's a transient torque from the acceleration of the motor while the propeller spins up to full rpm from zero.

So, lots of things going on that steer the plane left as you accelerate up the runway. I guess it helps if we can understand the theory, but all we really need is to know how to deal with it. The problem is that this turning force changes between when you open the throttle and get into the air. The easiest thing you can do is steer the plane with the rudder with or without steerable undercarriage. Next you can counter a proportion of it by mounting your motor with side-thrust, and lastly, you can try and resist it with tricycle undercarriage that can only roll in a straight line.

Tricycle undercarriage can bring other problems, like when your wheel catches a lump and knocks the plane in the wrong direction, you can't easily steer it back on course, and it's more difficult to rotate for take-off.

Side-thrust can't cater for all the changes in torque, though it does deal with the greater part of it. I'm not sure how much it affects the tracking of a plane if you want to do pattern aerobatics.

Steering the plane with the rudder is pretty straight forward, but for a beginner it's a bit like rotating one hand around your belly, while you move the other up and down over your head. It just takes a bit of practice.

It's interesting too how different planes behave. My Sportster tries to turn a right-angle the moment I open the throttle, while as my FT 3D, which is a similar size and weight, and has the same motor and undercarriage, tracks relatively straight. Neither have any side-thrust.

 

[Merv]

I would like to point you to the analogy of a single axle trailer. As long as the hitch is in front of the axles the trailer will always want to track straight behind the tow hitch.

I must disagree with your statement, in a trailer, the weight distribution is critical. If the CG is to far aft of the trailer axels, it will cause uncontrollable fish tailing. As shown in this video and many others. This plane has too little toe in. Possibly wheals that are too far forward of the CG and a high angle of attack while rolling. The high angle of attack pushes the wheals forward, they may be in the correct spot when the plane pivots to rolling level. But it never gets that far before it spins out of control. Adding a tail wheal will lift the tail, reducing the angle of attack. That alone will move the main wheals aft. The tail wheal will also hold the plane straight until it has enough air speed for the rudder to become effective.

@openatheclose you might try some adding camber to your main wheals. When the plane is level, make sure the bottoms of the wheals are closer together that the tops. Your plane needs quite a lot of rotation to go from sitting, tail on the ground, to a level roll. A lot of rotation means, what is camber when the plane is level becomes toe in when the tail is on the ground. You may need both camber & toe in.

 

[makattack]

Here's a good video about weight distribution and oversteer/understeer on cars:

 

[Hai-Lee]

I must disagree with your statement, in a trailer, the weight distribution is critical. If the CG is to far aft of the trailer axels, it will cause uncontrollable fish tailing. As shown in this video and many others. This plane has too little toe in. Possibly wheals that are too far forward of the CG and a high angle of attack while rolling. The high angle of attack pushes the wheals forward, they may be in the correct spot when the plane pivots to rolling level. But it never gets that far before it spins out of control. Adding a tail wheal will lift the tail, reducing the angle of attack. That alone will move the main wheals aft. The tail wheal will also hold the plane straight until it has enough air speed for the rudder to become effective.

@openatheclose you might try some adding camber to your main wheals. When the plane is level, make sure the bottoms of the wheals are closer together that the tops. Your plane needs quite a lot of rotation to go from sitting, tail on the ground, to a level roll. A lot of rotation means, what is camber when the plane is level becomes toe in when the tail is on the ground. You may need both camber & toe in.

The weight distribution argument is off as the original video showed the problem whilst the tail was still in contact with the ground and hence fully stable. If and when the tail raises the plane is balanced around the undercarriage so the issue of weight is actually less relevant than many seem to think.

Continue on with the various arguments as to what and why!

Have fun!

 

[d8veh]

One other thing i can think of. It's worth eliminating it as a cause, but I'm not convinced this is it. The wheels need to spin freely, which they most likely do when the plane is stationary, but when rolling, does one move to the side so that it jams on the wire?

 

[reburg99]

I've had similar experiences in the past, what I've found is something is typically out of aliment or one of the wheels is dragging. From what I'm seeing in your videos, I suspect your left wheel (left side looking from the rear of the plane, the direction you're veering off.) is dragging or binding on it's axle. Do both wheels spin freely? Does the left side gear move or twist during travel?
Try this, roll the plane by pushing it, no power applied. Does it roll straight or veer off to the left? Adjust the landing gear (including the tail wheel) until the plane rolls straight without applying power. Once it roll straight, start trying to taxi the plane. In order to taxi a tail dragger, you need to apply up elevator, this keeps pressure on the rear wheel allowing you to steer the plan on the ground. The faster you're going the less up elevator is required until you reach flight speed, then increase up elevator until your in the air. No it's not easy task and much more difficult in real life then on simulators IMHO. Hope this helps get this straightened out for you!

 

[Tench745]

My newest racer had this same tendency when I taxi tested it. I ended up forgoing a ROG takeoff and hand launched as I'm comfortable with the procedure. The maiden ended poorly, requiring a rebuild. Version 2 had no problem tracking straight even though I'd made virtually no changes, CG and gear placement were the same, same landing gear, same aircraft geometry, etc. From this experience I would guess that weight, gear placement, and CG are less of a factor than others think.

Things to check:
-Are your wheels parallel to each other? Toe-in, the fronts of the wheels angled slightly towards each other, helps straighten the plane. Toe-out, the front of the wheels angled away from each other, will make any turn worse quickly.
-If you put some pressure on them, like the plane is rolling, do the wheels stay that way?
-Do both wheels turn freely, especially with a little weight/pressure on them?
-Rudder going the correct direction?
-If you give full up elevator to "plant" the tail does it still spin out as easily?
-If you try this on carpet or turf, something more "draggy" than pavement, does it still spin out as easily?
-Is there a specific speed where it begins to worsen?

Possible fixes:
-Steerable tailwheel
-Add toe-in
-Stiffen landing gear
-Enlarge vertical fin and Rudder (personally, I think this will do a lot for you)
-Practice your taxi.
When I was first learning to fly I would take my little Hobbyzone Champ out and burn a battery just taxiing around seeing how it behaved. Play with rudder inputs, deflection, speeds, amount of elevator, etc to see how they all effect one-another. Practice just driving slowly in a straight line. Do it again but faster. Do it over and over, speeding up your taxi until you're near takeoff speed. Blip the throttle when it starts to veer off to give yourself increased rudder authority without speeding up too much. You'll feel a lot more confident in your aircraft and your skills without ever having to leave the ground.
-Best of luck!

 

[Merv]

I can't seem to get my custom plane off the ground.

Any update?