Barfly

I was chatting with friends about a plane that could fly backwards as well as it flies forwards, led me to wondering about an airfoil for such a thing. Of course, there are airfoils commonly used in modelling that could work going backwards, flat plates for example, but is there an airfoil shape which is symmetrical front to back and has a centre of pressure, the point through which all lift appears to act, very close to 50% chord?

Of course, there are many other things to consider for such a plane, but can we just talk about the airfoil for starters.

Cheers from the Barfly.

Tall Paul

The middle one..... 50 years ago..
Tested on an interesting falling-weight powered rotating arm.

Barfly

Thanks, Tall Paul. I see that the thick flat plate has different shape leading and trailing edges, so it's not quite symmetrical front to back. Of course, that can be fixed. The thin flat plate is very familiar in modelling, but from my experience the centre of pressure on these is toward the leading edge, maybe 25 to 35%. I don't see any information on centre of pressure on the graphs, or pitching moment, and that's what I'm really searching for.

Just to clarify, I am thinking of a model that can reverse it's direction in flight, without changing the position of the C of G, without changing wing incidence, and without moving the wing forwards or backwards along the fuselage. Basically, just hit reverse thrust and fly away! I'm not sure it's even possible. Finding a suitable airfoil is the first step, and a possible showstopper. Thanks again.

Cheers from the Barfly.

BMatthews

OK, the center of pressure concept was dropped some years back due to a pesky matter of the Cp being located at infinity behind the trailing edge when you run the airfoil at the zero lift angle. The current method of describing the twisting action is that the center of lift is located at the 25% point of the airfoil and that any twisting force is called the pitching moment.

And with any airfoil it won't matter if you operate it backwards or not the center of lift will always be at the 25% point behind whichever end is the leading edge of the moment. What would change is the pitching moment since it's largely related to the camber value.

You're not going to find any airfoil that is optimized for both directions. Low drag requires a sharp trailing edge. But to soften the stall and avoid other issues you want a rounded leading edge. The one area where this alters and a truely symetrical airfoil would work is at very, very low Reynolds numbers like with indoor or slow flying free flight models where a simple arc curved flat plate works fine and is the airfoil of choice like with indoor models. But for general RC flying and dispite Tall Paul putting his wing on backwards as a giggle it's generally not a great idea.

B.L.E.

You can easily demonstrate the effect that Bruce referred to by taking a business card out of your wallet and letting it fall to the floor. The CG is in the center but the CL as an airfoil is towards whichever edge happens to be the leading edge. This results in a continuous tumbling as the card falls. When I was a kid, throwing a yardstick as high in the air as I could and watching it spin at a high speed as it floated to the ground provided endless amusement.

Barfly

Okay, thanks guys. I was not aware of the 25% standard, although I did know there were isues with pitch moments. I am familiar with the flat plate airfoil, and I'd be happy with it too, but of course it won't provide the solution I seek. I also used to do that thing you do with the yardstick, at the time I didn't know why. Do now.

I had a feeling this wasn't possible, but you never know. Even if we could find a suitable airfoil, designing a stable plane around it would be a challenge. So, I think this is a non-starter.

Thanks for your input. Brainstorms still welcome!

Cheers from the Barfly.

BMatthews

It's very much a starter but you just got to start it at the right size range. Think slow parkflyer speeds and size.

But you'd have to have some way of shifting the CG and tail areas around so it's not something that would be easy to do in flight.

Barfly

Yes, I was thinking small. Depron, electric, etc.. But for me, if I couldn't reverse it in flight without reconfiguring the plane I don't think I'd have much fun with it. I had considered using one of those 4D variable pitch props that we see on Flatouts and ShockFlyers. Stability and C of G is one of those "other things to consider" that I have no solutions for at this time. I figured if a canard can fly 'tail' first, then it shouldn't be too hard to make it fly backwards. A normal aft mounted tail doesn't like going backwards. A friend suggested a tandem wing layout with no tail surfaces. I wonder.....?

Something else just hit me, can a canard fly inverted? Sustained negative 'G'? I never thought about that before. If so, it might be a starting point.

Cheers from the Barfly.

jmb52760

I can't resist throwing something into the mix here. I have seen articles discussing a NASA project for high speed helicopters. I believe the concept is called X Wing. http://en.wikipedia.org/wiki/Sikorsky_S-72

The rotor blades stop in flight and become wings. This means that the blade on the retreating side reverses direction. They accomplish this by using an oval shaped blade and blowing air out of the trailing edge to form a virtual airfoil.

I know that this is much to complex for RC. Just thought I'd help with the brainstorming.

Interesting topic.

edited to add this link to a UAV that uses this technology http://www.answers.com/topic/boeing-x-50

B.L.E.

An airfoil like the well known Clark Y works best when moving forward and right side up, yet planes with this airfoil will fly inverted. I'm sure that it will also lift backwards and even inverted backwards. The shape is just not optimum in that configuration.
When I windsurfed a lot, I would sometimes jibe the board but not flip the sail, sailing off with the sail "clew first". This was mostly done on light wind days when you needed to do tricks to make it interesting. The ease with which windsurfers can be sailed backwards and the cluster-****s that this caused at the starting lines of races lead to the creation of a special sailing rule just for windsurfers. "A boat clear ahead no longer has the right of way if it is sailing backwards."

Tall Paul

The 3D fliers with their reversible props are having ordinary planes fly backwards.
How they do that, I don't know.
The guy that tries it locally hasn't managed it successfully yet.

jmb52760

I haven't seen this but I have to ask:

How long does it last. Surely they cant do this for very long. Do they use gyros?

I've seen the helicopter videos (szabo!!!!!!) but I think RC helicopters have a special exception to all physical laws.

We don't get one of those in aircraft now do we?

B.L.E.

I haven't seen one fly backward for very far, about the distance of a tailslide. This in a video that's about 2 years old, maybe pilots have had a chance to learn more since then. I would guess that backwards, the plane is so unstable that it's like flying a unicycle. Yet, there are people who can ride unicycles so who knows?

BMatthews

IF you can rig up some form of switchable verticals and go for an all flying tail and IF you can rig up the fin swap with some form of traveling weight or wing to radically alter the CG then you could do an inflight conversion from conventional to canard. And wonder of wonders the UP elevator would work the same way both forward as well as backwards. Ailerons would need some fiddling to figure out or if you don't mind using rudder control only that would be best.

I think it's actually doable but would require a LOT of engineering and gimmickry to keep it light enough and yet strong enough. But the looks of the faces in the peanut gallery would be worth TWICE the efforts....

Barfly

jmb52760, I don't know enough about the S72, but that's interesting about the 'virtual' airfoil. I had considered an oval shaped airfoil, as an evolution of a round airfoil, or a double ended symmetrical airfoil, which would seem to answer the requirement with one important exception:- The round one doesn't generate any lift! Is there any wind tunnel data on oval airfoils?

By the way, don't think I didn't notice the Boeing Dragonfly. I use it as my avatar on another forum!

B.L.E., I agree that virtually any airfoil shape, even a clark Y, will lift forwards, backwards, upright or inverted, but center of pressure and pitch moments would change so dramatically that stabilising it would be a nightmare. That is part of the reason for searching for a unique airfoil which would offer consistant behaviour in those differing flight regimes.

Yep, the 3D guys go backwards, briefly! I haven't yet seen one that can sustain reversed flight and maneuvre as it does in forward flight. I reckon this would take it to the next level, if it can be accomplished.

jmb52760, Let's not bring helis into this. Anyhow, I don't think the Szabo's have any respect for physics! Hehe.

BMatthews, Control surfaces are another of those "other things to consider" that I mentioned in my first post. All flying stabilisers sounds good to me, but again, airfoil selection for these will be important for proper balance. I feel reluctant to attempt changing C of G, or wing position, in flight. My gut tells me to avoid it. In the end, it may be unavoidable.

I imagine flying toward a mate for a Zagi catch and just before he grabs it, slam it into reverse and fly away backwards, with a roll and a loop, just to stamp your authority on proceedings. Should be a hoot! Flying and maneuvring a plane backwards would be challenging, I think. Remember the old days when helis used the 'invert switch'? Nowadays, pilots just do it, switchless. Flying planes in reverse switchless should add an interesting new challenge to 3D flying.

I'm encouraged by all the interest, thanks, but I think we still need that unique airfoil. I'm all ears!

Cheers from the Barfly.

BMatthews

Because the center of lift would jump from the 25% point from one leading edge to the 25% point from the NEW leading edge at the point of reversal you had better believe that the CG would have to move, and move a LOT. It would be worse than you think too since the CG position for what would suddenly be a canard would need to be closer to the new leading edge or even in front of the new leading edge. So some way to slide the main wing back and forth or some way to shift a lot of the heavy components and alter the CG that way is a must. At the same time the old vertical would have to snap into hiding and the new vertical(s) snap out and into place. Ideally this would be done at a low airspeed or right at the stall point otherwise the new configuration would be like flying an arrow backwards all of a sudden. Just think of a standard model noseing up into a stall and then BAM! the fin on the tail snaps into hiding and the wingtips snap up and back to form new fins while the innards run backwards on rails inside the fuselage to move the CG from the 35% mark from the new trailing edge to some point just in front of the new leading edge so you have a suddenly functional canard form.

You could actually jig all this up with a little all balsa test glider in a few minutes and see how much it would have to move.

jmb52760

first try with google "airfoil data x-wing" produces:

http://www.fluent.com/solutions/articles/ja153.pdf

for what it's worth

and this patent:

http://www.freepatentsonline.com/5405104.html

I didn't pull-up the entire patent so I don't know if it includes the data.

nothing that I could find here: http://www.ae.uiuc.edu/m-selig/ads/coord_database.html

but the link is included because it is such a valuable database for the designer

By the way, a round airfoil will generate lift if you spin it. Of course, you would have to be able to instanly reverse the spin.

Barfly

jmb52760, good links, thanks. I should spend some time with foilsim or similar before proceeding. It looks to me that both the Sikorsky S72 and the Boeing X50 are using oval shaped airfoils, and at least one, if not both are using Coanda effect. I don't think that is practical for a park flyer, but the shape is interesting and I want to learn more about that. The fact that the oval airfoil is used on a helicopter rotor blade suggest the C of P and pitch moments are quite different to normal foils, and may be close to my ideal.

Cheers from the Barfly.

ArmedZagi

My Feedback: (1)

Ok bad (very bad) drawing... but here's a way the vertical stabs could be changed as the prop changed pitch. As the servo for the prop moves it would raise one stab while dropping the other. Tandem wings at ethier end of the fues. Maybe something along those lines?

Tall Paul

Neat!
The goal should be taking all the area off the back when it becomes the front, and adding that same amount of area to the front when it becomes the rear.

Barfly

ArmedZagi, thanks, I'll keep your idea on record, 'cos I don't have a better one. Yaw stability is a big question mark. But then, so is pitch stability! My friend here already suggested a tandem wing layout. After he described how it would work forwards and backwards, I asked him what it would be like inverted, and he said "No way!".

Tall Paul, that's right, but how? I think I can see why a reversing plane hasn't been done before (and may never be done!).

I spent some time with FoilSim today, specifically the elipse shape and the flat plate. I couldn't find any specific info on the C of P position, or the pitch torque or moment, for the airfoils in the graphic.

Does anyone know how I can get that info? Thanks.

Cheers from the Barfly.

BMatthews

Nonsense. A tandem will fly upside down just fine. But it will NOT fly both forward and back with the CG in one place It would still have to shift by quite a bit to maintain stability.

Tall Paul

.
In "Car & Driver" 30 or so years back Bruce McCall had a feature on fancy airplanes... one of them was a Italian fighter which could fly in either direction.. depending on who the Italian government was aligned with that day.

ArmedZagi

My Feedback: (1)

Back to the picture I drew earlier... If a weight was attached to one of the pushrods... As the rod and weight moved it would shift the CG of the plane.

BMatthews

I doubt you could put enough weight onto a simple pushrod to move the CG far enough even if it was literally a brick. The travel is too short.

You'd need a long threaded rod and motor to turn it and limit screws to shift a smaller weight pretty much from one end of the model to the other. Remember that mass times distance is the equation and to keep the overall flying weight down you need a lot of travel.

Or if the jack screw deal is too slow a tube with a pneumatically driven weight with controllable locks at each end would be quicker.