Absolutely Symmetric Airplane

[Lon Enloe]

OK, now suppose I go and build an absolutely symmetric airplane: mid-wing, so the wing and horizontal stabilizer and the thrust line are all lined up, rudder symmetric with the thrust line as well, symmetric airfoil, equal throws up/down on the ailerons--will there be any coupling between the axes?

Now, I know I can't quite build something like that, 'cause it does have to land on the ground and the rudder will need to be taller on the top--should I try to balance the rudder area, or the rudder moment, around the thrust line?

Finally, assuming that there are fixed gear on the beast, it there any rule of thumb for getting the fuse and/or rudder area above the thrustline set to counter the torque from the drag on the wheels? Calculating the drag of the wheels (with or without pants) and that of the fuse seems like a tricky proposition.

Thanks!

[Sparky Paul]

Symmetrical planes show up at many pattern meets... but you have to go to the meets to see them, since they never win anything.

[space_case]

Yes, there will always be some amount of coupling.

Remember, the wing will have to fly at some + AOA to stay in the air. Also think about decalage and why the horizontal stab is there in the first place.

[Salto]

Here's mine. http://www.rcgroups.com/forums/showthread.php?t=275295&highlight=toccata

The Toccata is fully symmetrical in the wing, fuse, tails, thrust lines, vertical CG, etc. It has very neutral behaviour, which is absolutely identical inverted, and knife edge flight is clean with negligible yaw to roll coupling.

On such a model, there are still three assymetrical forces that are difficult to avoid. The first is the swirling prop wash which hits the wing and tails assymetrically, the second is the precession torque from the spinning motor and prop, and third is the "P" effect where a yawed or pitched prop produces an offset thrust.

The effect of these remaining assymetric forces can be reduced by using a small prop spinning fast, or even better would be counter-rotating props.

I don't disagree with Sparky's implied statement that such models are not the ultimate answer to aerobatic purity, but I'd be interested to learn exactly why in practice other solutions are better.

Graham.

[Salto]

Remember, the wing will have to fly at some + AOA to stay in the air. Also think about decalage and why the horizontal stab is there in the first place.
I've built a number of symmetrical airfoiled models which you can trim to fly level and hands-off right way up and inverted. The zero pitching moment of symmetrical airfoils helps here, as does a very low static margin. Some residual pitch stability, and some +ve angle of attack on the wing is provided by the wing downwash influencing the flow over the horizontal stab, even with zero geometric decalage. This effect works the same both ways up.

Graham.

[Bill Mixon]

Symmetrical planes show up at many pattern meets... but you have to go to the meets to see them, since they never win anything.
You know I wonder if a lot of that has to do with the pilots of these planes being better builders than pilots.
I'm not saying that there aren't issues with symmetrical designs or that the pilots of them can't fly.
I think if they were not so "odd" more people would contribute to making design improvements, and with hundreds, or thousands of hours of practice by top level competitors it might have a lot of potential.
OK, now suppose I go and build an absolutely symmetric airplane: mid-wing, so the wing and horizontal stabilizer and the thrust line are all lined up, rudder symmetric with the thrust line as well, symmetric airfoil, equal throws up/down on the ailerons--will there be any coupling between the axes? Yes coupling will be there, but in my experience it is much less, and is more manageable throughout the maneuvering range.
I have yet to try a symmetrically arranged pattern design, but have done many 3D aerobatic models and they just fly great IMHO.
On mine I use a minimal landing gear or none at all.
Below are some pictures. Note the single wheel landing gear on the bottom of the vertical wing (SFG) on the Symmetry 3D.
The symmetrical foamy bipe has minimal coupling issues for a biplane.

[BMatthews]

....The zero pitching moment of symmetrical airfoils helps here, as does a very low static margin.....

If you truly want symetrical performance the CGwill need to be pretty much right on the neutral point. That would be a very large part of the equation.

[Sparky Paul]

You know I wonder if a lot of that has to do with the pilots of these planes being better builders than pilots.
...
.
Yes, basically that's the situation. Too much tme spent designing and building, not enough time flying.
Many of the top fliers don't build, they buy.

[DeaninMilwaukee]

I built this one from pink foam using an endoplasma about 4 years ago, a true zero zero zero plane inspired by the ripmax alliance. Thrust/weight was near 1:1, flew VERY well.

CG was tricky to get exactly right, but the final adjustment had it flying top up or down with no elevator input at all, and the fairly thick selig 1016 airfoil did a nice job of keeping speed down on downlines. Overall a pretty easy plane to fly.

Design notes done right:

Equal area of rudder and fuse above and below thrust line.
Zero zero zero setup makes it VERY neutral handling and predictable.
Thick wing keeps drag high.

Design notes done wrong:

Too much fuse area near tail made knife edge tough to hold altitude.
Needs a tapered wingform, simply won't snap properly, made worse by low wing loading. ( 480 in/sq, 48 oz RTF weight).

[BMatthews]

Dean, a larger rudder would have helped. If your rudder is only the white part then it's pretty small by today's stunt, pattern and 3D standards. But you're right, it looks like it could have used a little more side area forward.

Some 3D designers related in a post a couple of years back that the key to good knife edge and yaw looping manuevers is to set up the fuselage so that the total side area of the fuselage AND fin with rudder has 25% of the area forward of the CG point on the wing. If you follow that then the yaw reaction will be far more neutral and you can get knife edge balance with less rudder.

[DeaninMilwaukee]

Yup, I know it needed motre area up front. The ruder while smaller did have a fair bit of power. It could hold the nose up , but nonetheless the plane would decend.

[Lon Enloe]

Thanks for all the inputs so far--and more are obviously welcome! After having struggled in vain with a flying wing design and crunching that prototype this weekend, I've decided to sit that one on the shelf and, while watching the Super Bowl this weekend, started framing up the wing on a "symmetric as possible" design.

It's still a design in progress, but the specs so far are:

36 in. span
240 sq. in. area
Modest taper: 8.5 in at the root, 6.5 in at the tip
Eppler 168 airfoil (I've used that one before on a successful first pattern ship)
Ailerons are 28% of the chord throughout

Power plant will be a BP-12 currently sitting on the failed wing.

Given that this is going to be a mid-wing design, and fairly small to boot, I'm going to make the wing integral to the fuselage structure rather than having it removable. I'm using a couple of carbon rods in place of a plywood wing spar, for minimal intrusion in the fuselage space.

Everything else is TBD at this point, so more inputs are always welcome.

I'm a believer in "form follows function," so the goals of this project are:

Have something I can learn to knife-edge with.

Have something I can fly off pavement or grass (I've got access to a really nice grass field for flying over lunch, but it's almost too well-maintained: The grass is so long it catches and flips my planes much of the time.)

Have a little better power-to-weight ratio than in my current ships.

I'll let you know how things progress.

[portablevcb]

If you want to see this concept in the air look at all the new flat foam 3D planes. You almost can't tell which way is up unless they have some LG or are painted differently.

For true symmetry why not a box? Missiles have been designed for years as being fully symmetrical. There were some designs in the late 40's and 50's that had 4 equal wings. Didn't go very far because of landing, takeoff, and other mundane aspects of aircraft operation. One of the 'other' reasons these aircraft didn't go forward is because of the pilot. Why have a plane that can do 9g's "sideways" when the pilot can't take more than 3g's.

Why not symmetric pattern planes? Not sure. But, could be because the judges don't like them, the pilots have a hard time keeping orientation, or, they just don't fly as well as the designers think.

Good luck on your quest!


charlie

[Lon Enloe]

OK, well, here's my attempt at a maximum symmetry aircraft framed up and awaiting covering. It's not as absolutely symmetrical as Graham's or Bill's designs, but it's a true mid-wing with symmetry in the main body of the fuselage, broken minimally by the gear and the canopy.

The vertical stab is about a 1/3 to 2/3 split in area about the mid-line. One of the goals of this aircraft is to give me something I can fly in the beautiful grass field by my office without flipping over on landing. To this end, the lowest part of the vertical stab is even with the lowest part of the fuselage, and the gear are removable (main gear and tail wheel, too.)

I'll let you all know how she flies when she's done.

[vintage1]

I himnk you will have a VERY neutral model there..only thing I think may not be optimum is the fuselage side area..my guess is she will be 'nose heavy' in knife edge..a longer deeper nose with a light motor might be preferable.

But she should be an absolute joy to fly..

[Salto]

It's not as absolutely symmetrical as Graham's or Bill's designs ....
But it's much better looking!!

Well done with the build, and yes, please let us know how she flies.

Graham.

[Jim_Marconnet]

If you want to see this concept in the air look at all the new flat foam 3D planes. You almost can't tell which way is up unless they have some LG or are painted differently.....
The Atmopod is about as symetrical as you can get!

I'm still learning to hover, transition, and fly mine, so I cannot talk about coupling.

Jim

[Lon Enloe]

I finished the covering this afternoon. All that's left is installing the receiver and servos.

Unfortunately, it's cold and windy here, and looks like it will be for a while, so there's no rush. :(

[Lon Enloe]

Well, I made a last push and got the servos and linkages installed in "MaxSymm" last night. The weather report on the evening news said that today would be windy (30 mph) but I took a chance and threw the plane in the car when I took my son to school this morning. There's a (potentially) major snowstorm headed our way this weekend, so I figured this would be the last chance in a while for a maiden flight.

It turned out to be a beautifully calm and sunny (if a bit chilly) morning. I got three flights in and I am pleased to say that "MaxSymm" met all my expectations.

Gound handling was a bit wierd--given that I tried to balance the rudder area above and below the thrust line, and minimize the moment arm on the main gear, there was not a lot in the way of angle of attack as it rolled down the runway, so takeoffs were a matter of jerking it into the air with the tail dragging some--but once it got aloft, it was another story.

It required minimal trim to track straight and true--there were several moments when I let it fly "hands off" down the length of our field while I was warming my hands!

Inside and outside loops felt identical. Rolls were as crisp and axial as on any plane I've built. There was no tendency to drop a wing that I could detect; I took it up high, cut the throttle, and played glider for a while--that was kind of fun. It even knife-edged OK! (I'm not very good at this, but it seemed to hold altitude as I waddled across the field.)

It's not a 3-D ship by any stretch of the imagination, but it came in at 12-1/2 oz dry, 16 oz wet with a 1300 mAh battery, the BP-12 motor was pulling 10 A with an 8x6 prop, and it could hang on the prop for a while if I pulled it straight up, so thrust-to-weight was about 1:1 and entirely acceptable for pattern flying.

All in all, I'm a happy camper. I'll be in a much better mood watching the snow this weekend since I've had a chance to fly this morning.