ArmedZagi

My Feedback: (1)

Given the fact you're suggesting pneumatic systems and such I'd guess you're thinking about applying this aproach to a fairly large plane. He's talking about something in the parkflyer size range.. any small amount of weight can make a big difference on a small plane.




Took me a second.. but I got it...

Barfly

Bruce, my friend described the tandem as having positive wing incidence and downthrust in the direction of flight, both ends. He convinced me it could work, but also that it certainly wouldn't work inverted. Maybe you could make one that inverts, but the need to move the C of G bothers me.

Paul, that's a good one. Hehe. Unfortunately I'm not politically aligned so mine will have to go both ways at the same time or I won't stand as a candidate!

ArmedZagi and Bruce, if anything is gonna have to move it will be the battery only, and I am resisting even that. The mechanism will certainly add weight, the process adds problems. Screw jacks or pnuematics might be a bit much for a park flyer, don't you think?

If I could find that unique airfoil, with C of P at or behind 50% chord, all the mechanical problems disappear. Otherwise the plane would be like a flying Transformer toy, where a truck turns into a robot. This is why I am primarily focused on the airfoil.

I might have to make some foils and hold them in front of the table fan and see what happens. I am still looking for more info on the elipse shape, it seems to have the most potential at the moment, considering what NASA has done with Sikorsky and Boeing.

Thanks again for your ideas.

Cheers from the Barfly.

BMatthews

Well, then you may as well give up right now. There's no such thing. It's not the SHAPE that sets the CP point. It's the mere act of the air flowing over the wing. The CP automatically occurs at the 25% point regardless of shape. The only thing the airfoil does is result in varying pitching moments.

Also the Neutral point, and thus the CG location, will always be different for the two directions regardless of planform. This is caused by the fact that the Center of LIft for the surfaces shifts to the 25% chord point from whichever edge is the leading edge. So even your tandem wing will see the neutral point shift by quite a bit. The only way around this is to use something like telescoping wingtips to alter the sizes of the two surfaces in such a way that the Neutral point shifts enough to produce a situation where the CG ends up being in the same physical location for each direction.

My first thought was that you could alter the wing areas and "produce" tail fins by tilting up the wingtips to form fins. But then I realized that you're making the "back wing" size alter the wrong way and so that won't work. To maintain a fixed physical CG location for both directions you need to make the wing sizes alter such that you have a canard with the rear wing of the moment larger. But if you tilt up the tips to do so then your fins are at the wrong end. So you're pretty much stuck with telescoping wing tips. And you still need to figure out a way to make the vertical tails come out and retract.

Barfly

Bruce, the folding wing tips idea is brilliant! I like that. I understand what you are saying about wrong end, but considering I don't yet know the final configuration of the plane I am considering, it's definitely something to keep in mind. I think telescoping wings has been done before, on gliders, but again, is it suitable for a park flyer sized model? Would be a challenge, nonetheless.

When I studied basic aerodynamics so many years ago I was taught that the C of P is variable. For the same airfoil it can move when A of A changes, or when control surfaces or high lift devices are actuated. It will be in a different position for other airfoils, and also variable. But, for most conventional airfoils, it is usually somewhere in the vicinity of quarter chord. In addition to that, there is a torque present in most cases, which tends to rotate the airfoil, and is also variable. This whole issue has been the subject of some debate among knowledgable people in aerodynamics. I now find, on the NASA site, that a datum has been agreed, a standard which by definition considers the C of P to be at 25% for all airfoils, and that torque or pitch moments are measured from that point. This would seem to simplify matters and remove much confusion, and enable direct comparisons of different airfoils against each other. It seems to me to be a good idea.

However, it doesn't mean that the C of P actually IS at 25%. I know there are some shapes where it is close to half chord at around 40% (eg: supersonics), and I see some real world applications which seem to show that closer to 50% can be done (eg: stopped rotors, cylinders). This gives me hope that if C of P at 50% (and therefore reversible) doesn't exist somewhere already, it may be possible to create it.

Bruce, if I am in error in my understanding, please point it out. Thanks.

To be honest, I can't get excited about a model that would have the battery sliding back and forth along the fuselage, folding stabilisers at both ends, and wings that either telescope, move fore and aft on the fuselage or change incidence in flight. I think that would be a terrific achievement in itself, but would be a heavy mechanical monster. Overall, it doesn't appeal to me, sorry. I had thought of this before starting this thread, and I soon realised that the key is that unique reversible airfoil. If that can't be found or created, it follows that the plane can't be made. I hope you can see my point of view.

Cheers from the Barfly.

BMatthews

Yes the folding tip idea is brilliant... if only it produced or reduced the area on the RIGHT END! But that's the problem. It'll put the fins at the "front" and then it'll be like trying to fly an arrow backwards. So that's a non starter.

The old CP travel issue is the old way of looking at and dealing with the pitching moment. The idea being to find a spot where the lift and the pitching torques all cancel. But it has the nasty charactaristic that at the zero lift angle the CP is located infinitly behind the trailing edge. And since math types hate dealing with infinities where possible the method was altered quite a few years back to place the center of lift at the 25% point and deal with the pitching action as a torque that takes place with the CoL as it's axis. You can see why they used the 25% point since any fully symetrical airfoil (as in 0% camber) will have a stable Center of lift with no pitching torque at the 25% spot.

But even if you look at the old CP method where it moves all over there's no shape that will give you the same CP range for the same angle of attack variations in both the forward and reverse flow states. The CP will always hover around the leading edge of the moment just like the Center of Lift moves to assume it's location at the 25% point behind the leading edge of the moment.

But airfoils aside you also need to deal with the Neutral point movement of the overall aircraft. The NP will move based on both the Center of Lift movement of the surfaces as the airflow reverses as well as the planform alterations that will be required to flip from one direction of flight to the other.

It's an intriguing exercise obviously or I wouldn't still be typing this. By far the easiest way is to alter the geometry of the craft in flight or perhaps alter the geometry along with a smaller mass shift of something like the battery pack. But however it happens there's going to be a lot of mechanical trickery at work. I'm not sure this fits in with the need for a parkflyer model to keep it's weight down.

Barfly

Bruce, I'm glad you're still typing. I never expected this thread to go in this direction, but it's got me thinking, which leads to learning, which.......... is a good thing.

Park flyer? I build 'em and fly 'em, have all the stuff to prototype 'em. It's an obvious place for me to start. But it does imply some limitations. Weight and complexity are important considerations here. I have to go with the flow on that.

Although I don't agree with you 100% on the C of P bit, I think I understand where you are coming from. I have to admit that I don't know enough to take the issue any further. I need a little (a lot!) more homework.

I had an intriguing thought today. I used to work on General Dynamics F1-11's, and I wondered if swing wings might be usefull here?

I feel that for the most part the balance issue has never been particularly critical in airplane design, it's more important to focus on other parameters such as lift and drag. A designer will arrange the stabilizing surfaces to suit the characteristics of the chosen airfoil and airplane configuration. But now I come along with this crazy idea of flying backwards and suddenly the balance becomes one of the most critical factors. I can't help thinking it IS possible, it hasn't been done simply because it was never needed before. Now it is. I need it! So I live in hope.

I would love it if some more aerodynamicists, qualified or otherwise, would take a moment to express an opinion here.

Cheers from the Barfly.

BMatthews

Swingwing? I have to admit that I didn't think of that aspect. You may have the answer there. It still doesn't solve the vertical fin needing to move from one end of the model to the other in some way though.

Going back to the CP deal for a moment. Even if you look at the old NACA airfoil charts you'll notice that the CP always tended to stay around the forward portion of the wing chord. Only as the angle of attack approached the zero lift angle did it migrate strongly to the trailing edge. But at any angle that produces significant lift the CP was always in teh forward third of the airfoil. Again this is because of the nature of air flowing over any airfoil be it a flat plate to a Clark Y. It even applies to a Clark Y be it right side up or upside down..... or even if you turn it around and fly it backwards. The CP or CL will both move towards the new leading edge and totally ignore the actual shape of the airfoil change other than in minor ways. Nothing you do to the airfoil shape will either move the Center of Lift to the middle point or encourage the old style CP to center it's range of travel to the mid point of the airfoil.

So.... it means that you just need to live with that fact and compensate for it.

Barfly

Well Bruce (et al), it looks like the reversible airfoil is in fact a showstopper at this point, as I guessed at the beginning of the thread. And the 'transformer' plane is a non-starter for me. Maybe someone else will eventually do it.

Until I find time to do more searching, and McGyver up a wind tunnel on the kitchen table, we seem to have stalemate. So I'm gonna step down now, but I'll keep a sleeping dog's eye on it.

I appreciate your contributions, thanks.

Cheers from the Barfly.

williesims

perhaps this idea We all know of the concept planes with the wing mounted in the back and the fins up front now the airfoil what if a roller rod was mounted under the covering this rod when it moves to the back would move the curve from the front to the back?

williesims

perhaps this will give you a better idea

da Rock

It's not the wing's airfoil that's the problem. Read over the existing discussion again.

simhatus

Hi

How about somthing like this when flying backwards it would be inverted and the weightslides down a tube to alter the cg some way of locking the weight at either end of the tube would be needed if the plane was to do any sort of aerobatics.

The tailplane would be all moving.

It could use spoilers instead of ailerons going either direction.

Just thought slight mod from the pic : 2 fins free to pivot the one on the back for normal flight drops out from underneath and drops back in when inverted the one on the front is on top drops out when inverted and back in when right way up

Simon

victorzamora

I didn't read all the way through this forum (forgive my laziness), but I read most of it. Well, I asked myself the same question....so I sat down one day with my 3D graphics calculator to crunch some numbers to plug in to a simulator, and eventually run through a wind-tunnel. The best design to fly backwards and forwards is an eliptical-profiled wing. I don't know how to explain it, and I don't know all the terms...but if you cut the wing down the cord-line it should be an ellipses. The control-surface hinges should be sealed. Also, for a 40-sized plane, the cord length should be approximately 8"-10" long with a 55" wingspan and about 1.5" thick at the center. The thickness two inches from both the TE and LE should be .75". That will make a perfectly flying forward-backward plane, but be careful because elliptical wings tend to tip-stall like a b*t*h. To help with that, add FuntanaX-style SFG's and do NOT sweep the wings at ALL. That should about do it, if anyone tries it I'd love to hear results. I'd also love to hear opinions, I've put hours into this...so any criticism is constructive!!!

Make sure that the curve is mostly even, like don't make the whole wing nearly flat and curve sharply near the ends.

da Rock

It's not the wing's airfoil that's the problem. And it's not the wing's planform that's the problem. Read over the existing discussion again.