Making a hover more stable?

[Mat]

What would make a hover more stable by changing the frame?
More Wing area, an x-wing or something else?

[raptor22]

having equal drag on all sides of the thrustline is the biggest thing.

[shaneyee]

Stability comes from moving the CG back from the prop. Think of extending your nose so your prop moves forward...
and you'll have to move your battery pack backwards so that the CG is in the same place with respect to your wing.
My 2cent WAG.....

Shane

[BMatthews]

Very rearward CG. But when it's seriously far back the model will be squirly in level flight. Keeping the speed low helps a lot with normal flight. But basically the best hovering comes with having the CG right on or slightly behind the neutral point of the model. That point is determined by the stab size and tail moment. The longer the fuselage tail and larger the stab the further back your model will tolerate the CG.

[raptor22]

and remember that if yo move the tail too far back you lose authority to save botched entries or whatever.

[Salto]

What would make a hover more stable by changing the frame?
More Wing area, an x-wing or something else?

A model with a tractor prop cannot be stable in a hover. There are things you can do to make it less unstable or more controllable, but it can never be stable in the sense that it will fly hands off indefinitely and recover to the hover after being hit with a disturbance or momentary control input.

I recently built a series of models to investigate what influences hover behaviour. Here are some pics.

[Salto]

The one on the left is VERY controllable in a hover but still requires correct control inputs to hold it there. Once in the hover, the CG location does not seem to make it any more or less unstable, but if I've trimmed it for level flight if the CG is not right at the neutral point (giving neutral stability in level flight), then I need to hold in some elevator (or re-trim) in the hover.

Interestingly, this one hovers with no roll movement with the 4 ailerons in line with the wings. Kind of blows away all those prop torque arguments floating around here.

The one on the right was an attempt to try a pusher configuration, which theoretically can be made stable in a hover. This one flew like a dog in level flight no matter where I had the CG, and did not hover any better than the first one.


Then I built a series of more conventional models, but incorporating some of the concepts from the above ones. The result is my Toccata 3D. It has a large fuselage side area, symmetrically disposed above and below a centrally mounted wing. There is plenty of fuse side area up front to get the fuselage neutral point close to that of the wing and tail combination. This approximates the X wing layout. This model hovers extremely well, much better than any of the flat plate foamies I've seen. It also exhibits close to zero yaw to pitch or yaw to roll coupling. But it is nowhere as good in the hover as the X wing tractor above.

Here's a pic.

Graham.

[Mat]

Do the symetrical "airfoil" wings even out the pressure which helps keep the plane stable?

[raptor22]

Thats true. You wan't the center of pressure right on the CG.

[Salto]

Thats true. You wan't the center of pressure right on the CG.
In a hover, the wings are no longer providing any lift, so the normal rules of CG location relative to neutral point are irrelevant. The controlability in the hover is more dependent on control surface sizes, throws and locations.

When starting to move horizontally in a hover, for instance when the hover starts to go all wrong, the angle of attack of the wing is close to 90 Deg. so the centre of pressure of the wing will be close to 50% MAC, not the normal 25%. If you had the CG at this 50% MAC location the model would be unflyable in level flight.

I've "flown" the tractor model above with the CG near this 50% point by taking off vertically and holding it in a hover. Forward flight was impossible. It was not any less unstable in the hover compared to normal flyable CG locations.

Mat, when hovering there is zero airspeed, so airfoil choice for that flight condition isn't too important. The only place where it may have some influence is the very centre section where the prop wash passes over the wing.

Of course, any hovering model must also fly around, enter hovers, do other maneouvres etc. and for these other flight conditions a symmetrical airfoil is an appropriate choice. The zero pitching moment of a symmetrical airfoil can give really nice "on rails" pitch behaviour.

Graham.

[davidleitch]

A lot of traditionally good hovering planes (Diablotin) have relatively long fuselages. This may be because it helps with balancing in a vertical axis. That is once in a hover the plane will tend to hold that attitude. Just like a tight rope walker with a balancing pole.

However the long fuselage is a disadvantage when it comes to other 3D stuff, like tumbles

[the_canuck]

Get you one of these. They hover real well.

[flypaper 2]

This one will stay in a hover hands off. The nose will describe a circle about 6 in. in dia. When you lose altitude slowly, the circle gets bigger so you have to be on your toes. But I have gotten it to sit on it's tail more regularly without falling over. When it's in a hover you have to give down elevator to get out out. CG about 30% which is far back for a delta. Flies on a rail in level flight. No pitch change when inverted. Lots of fun :D