aseaholm

My Feedback: (1)

Does anyone know of a good theoretical/mathematical turning model, preferably dynamic? One that takes into account drag, lift, aspect ratio, induced drag,…etc. I would like to optimize a wing for maximum turning performance.

I read an article years ago along these lines regarding selecting a pylon airfoil. I believe this article was in Model Airplane News, but it’s not in my limited MAN library.

A description of the key variables in turning performance would be of great help also. Greatly appreciated…

WS

By far, the induced drag is the biggest factor in turn performance. If you want small radius, you need slowest stalling speed, which means lightest wing loading. If you want turn rate, you need to fly that circle as fast as possible with maximum lift, so highest aspect ratio as possible with as much thrust as possible. Keep in mind though, that structural integrity will become a serious consideration as you increase wing span.

1Jimbo

My Feedback: (2)

I could sell you one of my newest combat planes.

banktoturn

THE TICK,

I don't know whether there is a turning model per se or not, but there is a pretty good understanding of the trade-offs involved. Beyond wanting maximum turning performance, what exactly are you after? There are a lot of purposes which require good turning performance, but the end goal will dictate what you do to get it. Are you after the ultimate combat plane? Pylon? Funfly?

banktoturn

AQ500

Mr. Tick,
I noticed under your profile that you are an engineer. I'm an ME myself. There are some pretty good books out there that would answer your questions if you had time. I have a couple of suggestions that would help as long as you have a strong math and fluids background. John Anderson has two great books, one called "Fundamentals of Aerodynamics" and he also has one on aircraft design I just can't remember the name. Those are some pretty thick books and just scratch the surface. I spent a lot of time in them back in college. There are probably better books out there. Wells is on the right track.

aseaholm

My Feedback: (1)

Gentlemen,

Thanks for the information. I realize that I was a little vague on the description of “good” turning performance.

My ultimate goal is to put together a program that will calculate the radius of the turn, velocity at the exit of the turn, and other information. I plan to use this to optimize my current RC Combat design and mainly use this model to get a better understanding of the physics involved.

My approach for airfoil selection has been to try and factor in 4 design parameters. I tried to find the best compromise between Cl, Cd, Cm, and percent thickness for strength. I sorted through airfoils with SNACK and chose the Selig 8036. I’ve also examined wind tunnel data from Selig’s site and the 8036 performs well for Re above 300,000. There’s a new combat class in the works with lower Re’s and limited thrust. So this go around I’d like to use turn dynamics to help with design.

AQ500, I’m a ME as well. I’ll look for the Anderson text. I used the Perkins and Hage aerodynamics book in school, but donated it to the university for use in a design competition project I started. Should have kept it around I suppose, that darn 20/20 hindsight thing…

Ollie

See:
http://soaring.cnde.iastate.edu/calcs/frames.shtml
which allows you to compare up to five airfoil polars at a time.

With unidirectional carbon fiber for spar caps and kevlar to control buckling, the strength, stiffness and light weight possible in spars means that airfoil thickness no longer has to be much of a consideration when picking low drag airfoils!!!

In a turn, the centrifugal force is equal to the mass times the velocity squared divided by the radius of the turn. The angle of bank in a turn is the arctangent of the centrifugal force divided by the weight. The lift in a turn is the squareroot of the sum of the squares of the weight and centrifugal force. The lift force in pounds is equal to 0.00119 times the coefficient of lift times the wing area in square feet times the velocity in feet per second squared.

The reynolds number (in a standard atmosphere) is the wing chord in inches times the speed in MPH times 780. The reynolds number, coefficient of lift and the airfoil polars will yield the profile drag coefficient. The Induced drag is the coefficient of lift squared times a planform correction factor divided by the aspect ratio times pi. K is equal to one for an elliptical lift distribution. The parasitic drag coefficient is equal to the coefficients of drag of the fuselage, tail, landing gear, and everything else. The parasitic drag is the hardest to estimate accurately and is best delt with by minimizing it through streamlining, minimizing wetted areas, cowling engines, etc. The total drag coefficient is the sum of the profile, induced and parasitic drag coefficients.

The thrust is equal to the total drag. The total drag in pounds is equal to 0.00119 times the coefficient of drag times the wing area in square feet times the velocity in feet per second squared.

How's that for a short course in aeronautical engineering in only five paragraphs?

banktoturn

TICK & AG500,

John Anderson has another book, called Aerodynamics for Engineers. I have not seen Fundamentals of Aerodynamics, so I can't compare them, but I found Aero. for Engineers to be very readable for a Mech. Engineer.

This is a great discussion. As I think about it, it seems that a big chunk of the problem boils down to getting the lift needed for the high-G turn with minimum drag. My gut tells me that the planes that seem to 'mush' through the turns are the ones that suffer from a big increase in profile drag in the turns, because they need too big an AoA to get enough Gs. It seems that the big payoff is in designing to a target turn radius, as has been mentioned, and focusing on getting enough lift for that radius at minimum AoA. I'm guessing that's the benefit to the 'long wing' pylon racers that were so successful. There might be some benefit in modifying fuselage shape to minimize fuselage drag at moderate AoA, but it seems like a tough one.

I'd love to see what you come up with.

banktoturn

banktoturn

TICK, AQ500 & anyone else,

I had another thought about drag reduction in pylon turns. Do the planes typically have two ailerons, or just one? If they use two, one possibility would be to use them as flaperons. At these speeds, it would not take much deflection at all to get enough camber increase for a meaningful increase in lift at small AoA. With any luck, the increased moment would not require you to give back all of the drag savings in a big elevator deflection. Has anyone tried this? It it legal?

banktoturn

PAINLESS

My Feedback: (19)

banktoturn,

Are you suggesting using flaps and elevator similar to control line stunt planes and current rc fun fly planes?

This would decrease the radius of the turn but with all those surfaces deflected wouldn't the plane really slow down? Not to mention the higher A o A.

One of the keys to success in pylon racing is knowing the limits of your airframe, amoung many other things. I'm basing this on some q500 experience. One of the problems you commonly see is guys have their elevator travel set to high, they pull full stick trying to turn as tight as possible, the AoA goes up and the wing starts to stall which causes a decrease in speed.

Ideally you want to give just enough elevator to turn without losing any speed, and still make a reasonably tight turn around pylon 1. I set my elevator throw to this "sweet spot" when the stick on the tx is all the way back. I believe this is the fastest and most consistant way around the course. It also helps to have dual rates, you will probably need more elevator travel to land. This also applies to aileron travel as well.

banktoturn

PAINLESS,

The AoA that you end up with, and thus the elevator throw you end up with, is determined entirely by the amount of lift you need to generate for the desired turning radius. The problem I am trying to think of solutions for is the situation where you need so much AoA that profile drag goes up as a result. Think of flaperons as a way to have a variable camber wing, which is exactly what flaps are. In a perfect world, you might select an airfoil section that would give you enough lift for level flight at zero AoA. Then, select an amount of camber, or flaperon deflection, which gives the needed lift for the desired turning radius, ideally at zero AoA. You may not hit zero, but by increasing camber, the needed AoA, will be lower than if you left the main wing camber the same. In any case, the required AoA, and elevator throw, should be considerably less than without flap(eron)s. Incidently, independent flaps may end up being the better solution. I initially thought of flaperons for mechanical simplicity.

The plane will certainly slow down, but I think it's a safe assumption that you get less drag by increasing camber than by increasing AoA, especially since it reduces the fuselage AoA during the turn.

The potential problem is that a wing with deflected flaps is not as efficient as a rigid wing with the correct camber. Even so, my gut feel is that you would come out ahead. I assume that one of the common airfoil programs would be able to handle an airfoil section which represents a wing with a flap selected, so it shouldn't be too hard for someone to get a rough idea how this would work.

banktoturn

PAINLESS

My Feedback: (19)

For what you suggest to work the camber change would have to take place over the entire span.

Most pylon racing planes have very short aileron span, 8-10 " is common. If you used them as flaperons, what is the rest of the wing doing?

I'm sure someone could come up with a design for the fastest turning plane, but fitting it into an existing set of racing rules would be hard and it might be at a disadvantge compared to other designs that are optimized for a given set of rules. Just depends on what your after.

If you look at Q40 planes and watch them fly, I believe they are as optimized as they can get, while still meeting all the class rules.

banktoturn

PAINLESS,

I wasn't assuming that we would necessarily be limited to using existing ailerons. Even so, it may well be possible to get enough extra lift to be worthwhile even from a partial-span flaperon. My main worry with that would be causing a problematical early stall toward the tip, where ailerons are most likely to be. Certainly, it would be ideal to have more span to work with, and more toward the root. If there is no problem with aileron authority, I would be tempted to place the flaperons near the root, like traditional flaps, and leave the tip alone, to get some natural tip-stall protection.

I don't know anything about the rules, that's why I asked whether this idea would be legal. I would not assume that current designs are as optimal as possible under the current rules, though. There is usually some kind of improvement that can be made, unless they completely lock down the airframe you can use.

banktoturn

PAINLESS

My Feedback: (19)

I haven't looked at the AMA rule book lately but I don't recall anyhting about flaps being illegal. It is on their web page if you want to take a quick look.
They do have a minimum weight though for all classes. The advantages of what you suggest may be lost to the added weight and complexity.

banktoturn

PAINLESS,

I took a quick look at the rules on the AMA web site. I didn't find any mention of any restrictions regarding ailerons or flaps for any of the classes. It is possible that scale considerations could enter in for some of the classes, but it didn't seem like it.

Certainly, weight could be an issue. Most of the classes had min. and max. weights. I am assuming that most serious competitors come in right at the minimum weight, and could lighten further. If that is the case, then I would expect that using some of the min. weight allotment on variable camber would have good payback. The total weight would be one servo & odd linkages. I don't think the complexity involved would be an issue for any of the serious competitors.

It would be great if someone who is active in pylon racing would fool around with it. I'm sure it won't be me, as I am still working on getting my Goldberg Eagle back in the air to show my 7 year old son. Maybe THE TICK??

Regards,

banktoturn

PAINLESS

My Feedback: (19)

banktoturn,

There is no racing in my area anymore, I wish there was, I really enjoyed it.

You might try posting over in the pylon world forum to generate some interest in your ideas. You would atleast get some good feed back on whats allowed and whats been tried leading up to current designs.

Also take a look at the Pylon world web page, especially the Q40 planes, I think the are averaging 180 MPH in a 10 lap heat.

PAINLESS

Ollie

The radius of a turn is equal to the mass times the velocity squared divided by the force. So there are three basic ways to decrease the minimum turning radius of a model. Decrease the mass (W/g). Increase the force(lift). Decrease the velocity. There are limits to how much one can decrease the weight, increase the lift and decrease the velocity. Measures that improve one of the three often adversely affect another of the three. This leads to a conflict of objectives. That is one reason why design is a creative synthesis rather than an analysis that can be accomplished with one pass through a spread sheet. A spread sheet can be very useful in analyzing a candidate design. Then its performance can be compared to another candidate design to see which balance of conflicting objectives fulfills the mission best or is the most pleasing to the designer. The process of design modification, analysis and comparison can be iterated until the designer is satisfied or, tires of the process.

Cajun

I'd like to return to this thread for a couple of comments. I'm working on a plane for the same class as The Tick,,,,,,,,only I'm not an engineer.

Specifically, I'd like to see some reccomended airfoils.

The plane is A class, slow combat, .15 engines (box stock), 40oz. Minimum weight, and minimum 400 sq. in. wing area.

We are looking for adequate speed, with maximum turning ability while holding the speed thru the turns. But we need something that does more than turn left and go fast. I am interested in a high aspect ratio wing with a span of 54" to 60".

I have looked at S8036, S8037, E203, E197, and E209, and combinations of some. I haven't found the performance I am looking for.

Suggestions. I have Profili and cut my own foam.

Cajun

Ollie

What you need is a variabl camber airfoil (flaps). If flaps are permitted then a relatively thin airfoil with flaps mixed to the elevator will extend the range of coefficients of lift and with the low drag you are looking for. To keep from loosing too much speed in the turns, the flap travel should be limited to about 5 degrees. With careful design of the hinge line and a thin airfoil, the drag won't go up much with so little flap but the increase in lift will be substantial. Go with the MH33 or S6063 and 22% flap width. Use full span flapperons. Don't make the tip chord less than about 75% of the root chord unless you want easy spin entry and snappy snap rolls.

Cajun

Thanks Ollie. That last sentence was one I was wondering about, and didn't have the answer.

I'll try the flaperons.

Appreciate the help.

Cajun

aseaholm

My Feedback: (1)

Cajun,

Dang it, I was hoping to keep these guys all to myself. There is a wealth of information on this site. Keep up the good work gentlemen.

The mixed flap and elevator input should work very well at the speeds we're at with the NERF planes. People have tried this idea with the faster (65-75 mph) B class planes with minimal success.

With the longer nose moments prevelant with the lighter engines, I may have to bring back my canard experiments. The longer moment may give a little more authority. Cobra maneuvers like the Russian Sukoui performs at airshows would be fun to watch if nothing else.

Ollie

I suspect that the flapperon idea didn't work because they were using too much flap deflection which caused a drastic decrease in speed. If you want to use flapperons and maintain speed well, it is very important for the hinge line to be very smooth on the top surface (tape or skin hinges) and for the deflection to be limited to about 5 degrees so that the flow does not seperate over the flap causing a big increase in drag. Flapperons work better, for turning with minimum speed loss, the higher the aspect ratio.

rmh

Theory is fun to read -but in application -on this small stuff I truly believe that max performance -in this case - the tightest turn with least speed loss possible, boils down quickly -
First second and third - is wing loading.
The theory here is that tiny wings simply slip/fall/ slide thru the air-the air never gets a chance to to compress under the foil-so little or no pressure difference
The only way to get the lightest loading -is to build the lightest shape, capable of doing the job-
So here we have two basic approachs at work:
!- turning smallest radius as possible whilst not stalling-OR-
2 turning radious as fast as possible without loosing momentum.
Now ask yourself - how can one execute the turn (180 turn) -in less TIME-loosing less SPEED, if induced drag increases?
Maybe- vectored thrust might really produce the best results.
What if one greatly increased the AOA of the prop disc to assist in the turn?
One might couple the "elevator?" to change wing to fuselage relationship-for the turn .
By the way
I do not have a degree in engineering.
So keep your answers as simple as possible -for me.

Cajun

Tick, I love this forum. I don't post much here, as I am not litterate enough in engineering to talk the "language".

But, I sure appreciate the advice of the experts who congregate here. I usually just come in and rummage around for a while until I find what I am looking for. There is a lot of good information here, just for the reading.

I have a wing ready for the two servos and will try the flaperon idea today. Thanks for the help, Ollie

Cajun

banktoturn

Dick,

There is no doubt, you want to keep weight to a minimum. Once you've done that, you want to look at how to minimize the drag increase you suffer in a turn. While induced drag does increase during a turn, it stays small compared to the other components of drag. One component of drag that gets bigger is profile drag, due to the plane flying at a higher angle of attack. The rationale behind coupling the elevator to flaperons, or flaps, is to get the lift needed for the turn with a smaller increase in the airplane's angle of attack ( or maybe no increase ). You mention coupling the elevator to the wing incidence. That is exactly what coupling the elevator to flaperonse does, among other things. When flaps/flaperons are deployed, the camber increases, and the angle of attack of the wing section increases. Both of these effects increase the lift without requiring the airframe's AoA to increase.

I prefer to think in terms of a variable camber ( & variable incidence, as you point out ) wing than flaperons. If you had access to the necessary data, you could design the wing to generate enough lift for level flight at target straightaway speed without flaps deployed, and enough lift for the target turn radius with the flaps deployed. A wing with flaps deployed is simply a wing with a funny looking camber line, and can be analyzed with Xfoil or other tool. Lacking this data, one could build a variable camber wing, with small throws as Ollie points out, and find the best turning deflection by trial and error, as a kind of trimming problem.

The small wings behave differently due to their small reynolds numbers, but not that differently. I think most of the design rules are similar, in general terms.

Cajun,

I am anxious to hear how your tests go!

banktotturn