banktoturn

Ollie,

I am at work, and my copy of Simon's book is at home, so I can't look at it right now. I assume you are looking at the drag budget chart which appears earlier in the book than the chart I had originally referred to. I will check it out tonight. For now, I'll make the observation that on the particular diagram I referred to, if we believe Simon, the induced drag is a small fraction of total drag for a pylon racer, even during turns. When I returned to the diagram after your last post, it looked to me more like an eighth than a third. The reason I referred to that particular diagram instead of the drag budget chart that you mention here is that Simon actually places the spectrum of flight regimes for a pylon racer on the chart. If we accept where he puts the pylon racer on this diagram, there isn't a lot more analysis to do, you can just read off the relative magnitudes of the different components of drag.

Having said that, I like to build some intuition about this kind of conclusion. In the case of a pylon racer turning, two things are going on. Certainly, it slows down compared to its straight line speed, but it is still going "fast". Because of its high speed, CL must indeed go up, and induced drag with it, compared to straight flight. The other consequence of its high speed in turns is that profile and parasite drag remain high. The net result is that, even though induced drag increases during turns, it remains small compared to profile and parasite drag for all parts of the race, because the speed is always high enough to cause those components of drag to dominate. Indeed, this is exactly what we see on the diagram. The horizontal axis ( referring to the diagram I brought up a couple posts back ) is velocity, and we can see that the velocity simply never gets down into the range where induced drag dominates. For induced drag to dominate, CL needs to be big, and other components of drag must be small. Low speed flight is the only time that both of these things happen.

Clipping wings is a bit of an ugly expediency, but as a practical matter, its a lot more feasible than reducing thickness enough to maintain frontal area, for structural reasons. I don't know how close to stalling the pylon racers get, but a high aspect ratio wing would be a bit scarier in that regard as well.

I'll check out the drag budget chart tonight.

Thanks,

banktoturn

Ollie

For interesting examples of high speeds in tight turns have a look at all nine pages of:
http://www.shredair.com/album/dsfest.html

I think it is time to get out the cameras and radar guns and measure the angles of bank and the speeds in straights and turns so that we can begin to quantify the advantages and disadvantages of various airfoils and aspect ratios rather than engaging in this,"How many angels can dance on the head of a pin?"type discussion.

PAINLESS

My Feedback: (19)

Had an interesting conversation with my co-worker today. He is the one I mentioned above who crews for Rare Bear, which by the way didn't fly at Reno this year due to lack of funds.

He said they removed the fold up wing tips, about 3 ft each side, and made new tips. They saw a big speed increase but couldn't remember what it was exactly. He said those tips where originally designed to break away in 8g plus dive, also removing some aileron.

He said the plane didn't turn tight enough for the decreased span to cause a higher AoA in a turn, and in his words "it has so much horsepower and torque it wouldn't have mattered".

He also mentioned a term I hadn't heard before, he said the overall "wetted area" (air going around the wing?) was decreased by removing the tips, and this also drecreased drag as does less frontal area.

Based on what he told me and all the above posts, I would conclude that if you wanted to turn as tight as possible without loosing much speed, a high aspect ratio wing would be best. You can see this in the design of the current Q40 pylon planes. If tight turning radius is not important a minimum span would be best for overall top speed. He also mentioned that the Rare Bear makes very "interesting" take offs and landings due to the higher wing loading.

One other note, when I was flying Q500 several years ago, if you banked the plane over and pulled full up elevator you could hear the wing start to stall and see the plane lose speed. If you pulled just enough elevator to keep it from making that sound it was noticably faster in a turn.

matchlessaero

Hey Painless! Good to see you in here from the HalfA forum.

I don't have an Aero or Mech design degree, but I have some experience designing and building pylon racers of all sizes.

For your straight line speed plane, clip the wings. It will go faster, but as mentioned in the posts above, it won't turn as well.

For your pylon plane, you mentioned that you wanted to be able to land the plane on a soccer field. With that problem, I don't think changing the wing loading is the solution. Matter of fact, I think you will probably find that if you bring the weight down on your racer, it will fly, accelerate and turn even a little better.

Instead of changing your wing format, I think you should change the configuration of your plane to make it easier to land, and possibly more resistant to a tough landing. ----Make the plane a high wing. In doing so, you will make the plane easier to land in a short area because the wing tips will be further from the grass when you put it down.

If you go to a triple tapered wing, look at your airfoils, and what combinations of washout you can use. I have been experimenting with different amounts of washout to- allow the plane to be flown slower for landing without tip-stalling, and not drastically affect the top speed performance.

When you get your next one done, you need to come on down to Memphis and put it on the course!!!!

PAINLESS

My Feedback: (19)

Matchless,
Congrats on that new 1/2a pylon racer, looks really good. Did you find a source for those Fourmost mounts? Tower use to carry them but I looked tonight and couldn't find them.

My plane is about 14OZ, I did have to add about 1/4 oz of lead to balance it, the next one will have a slightly longer nose moment. The only other thing that would help is a smaller Rx , I have a standard r127 futaba in it now.

I did cut a new wing and buy some wood for the next one, but I need to finish two control line combat planes first. As far as coming to Memphis, its more like 1200 miles across instead of down, I'm closer to the left coast than the right one. Pylon racing in my area has died. So lately I have been more interested in all out straight line speed planes. I've been designing a plane similar to a Weston Magnum I hope to start on soon also. Thanks for the invite though.

banktoturn

Ollie,

My mistake, you were looking at the same diagram I was. Revisiting it, I don't really see any ambiguity about which components of drag are significant for pylon racers, while going straight and turning. It doesn't seem to me that we are at the 'angels-on-pinheads' stage, but we can certainly drop the discussion. I don't have access to cameras and radar guns, but would love to hear what you find out. I will note that in the original post in this thread, PAINLESS mentioned a desire for 'all out speed', not faster times in a pylon event.

As a practical matter, I think that matchlessaero has hit it on the head: best straight line speed favors clipping the wings, but turns will suffer to some extent. Where the ideal trade-off is is a tougher question, but the data we are looking at, Simon's chart, does not indicate that there is an onerous induced drag penalty to consider, even in turns.

Regards,

banktoturn

banktoturn

PAINLESS,

Thanks for the info. from the real pylon racers, and your own experience with stalling in turns. Wetted area is simply the total external surface area of the airplane. One important component of drag is skin friction, and it depends very strongly on the total surface area over which the air flows, or the wetted area. This is the other reason that wings, and other components, should be as small as allowable for minimum drag. The other factor that strongly affects skin friction drag is whether the flow is turbulent or laminar. I suppose this is the only aerodynamic advantage that our models have over full scale airplanes, since we fly at much lower Reynolds numbers, and, in principle, have a better chance of maintaining laminar flow.

banktoturn

rmh

I see little comment on weight and loading -
The theory stuff is fun reading.
My practical experience on this same problem showed a couple of things:
1 youjust can't make a 1/2 a model too light - impossible - too fragile -yes -too light no.
shoot for 10 ozs.
make the wing a thin as possible whilst holding strength
The lower AOA possible with reduced weight reduces induced drag.
Thinner wings have more lift -at lower AOA-BUT nasty drag at higher angles.
eliminate all 90 degree intersections with wing to fuselage -make the wing fair into the top or bottom of fuselage
save weight wherever possible.
Use a low aspect ratio wing -on a 1/2 A - the RN is so small that the extra chord helps.
This setup will land slowly -as you can pitch it as req'd to induce drag-
a more aft c/g can be used -but elevator control becomes more critical-
with a aspect ratio of 3-1 you should be about right for *****g the setup.
One of the best full scale designers -ever - was Steve Wittman- designer of the Buster and Bonzo and Tailwind -etc..
The EFFICIENCY in the designs was what made em work -
Look at em and you will see the best layout for a good 1/2 A

banktoturn

Dick,

I think your conclusions from practical experience are just about right on. Lower weight is definitely the big win from every angle. Thin wings are definitely lower drag at low AoA, and low AoA is the rule at high speed. I don't know that thin wings are generally considered to generate higher lift, but high lift is not an issue. You make an interesting point about RN & chord. Do you happen to know the typical straight-away speed of these planes? I'm curious about the chord-based RN range.

Thanks,

banktoturn

rmh

Ours were only a little over a hundred - sounded like 200 tho .
The very thin wing lift is a function of AOA .
use a symmetrical foil-
This is more or less a critical setup -that is - a snap can be easily induced -at about 10-12 degrees of attack-
so the trick is get a bank and hold it! no dives and pulls and such for best lap times.
a good expo setup for elevator is a must.
I am doing my new aerobat with a thin wing (12/10%)- and 19 oz ft wing loading -on 1280 squares ZDZ40 - prototyps show that the concept works - fast smooth - INSTANT snap entry and recoveries.
But not at all goosey - slow speed stuff is like a trainer -
Also excellent 3D-
It is all in the very low loading and a wing which will stall upon command - the std 3D stuf with fat foils is no good for snap maneuvers.

Mike James

You're right about the airfoil thickness, Dick.

I remember that when Pattern planes were ".60 size", we used to think that 15% thickness was normal. A few years later, I started using Eppler airfoils (E168) in the 12% to 13% range. Now, having recently measured some modern Pattern planes myself, I find that thicknesses are typically around 10%.

I assume this is a tribute to airfoil precision, light construction techniques, and the changes in maneuvers we've seen over the past few years.

rmh

Yep -the old pattern designs were typically very heavy for their size and the thick wings operated easily over a wider AOA.
They were less efficient tho -
Some guys insisted stall strips were needed for snaps --- lots of poor engineering - all part of a learning curve.
I started thinning wings way back when -but also dropped the wing loading at the same time

banktoturn

Dick & Mike,

This is really interesting to me. My background is kind of 'book learnin' aerodynamics, and the notion of 'beneficial' stall for aerobatics is new to me. In that light, the comments you make about thin wings for pattern planes are certainly consistent with the stall behavior of thin wings that has been understood for a long time. Are all the maneuvers the same for full size aerobatic aircraft? Do they also use thin wings for the same reason?

Thanks,

banktoturn

Mike James

Hi banktoturn,

My experience with aerodynamics, airfoils, and design is exclusively with models, so I don't know the current numbers for full-size aircraft. (I have a private pilot's license, but don't do full-size aerobatics.)

"Beneficial stall" is handy for producing snap rolls quickly, on command... a trait not so desirable on full-size aircraft, except maybe for a few aerobatic ships.

Ollie

It's time for me to eat humble pie. High aspect ratio in a pylon racer is not as important as I thought.

I designed a trial 1/2 A pylon racer on paper. I selected the S6063 wind tunnel test model airfoil which had the lowest coefficient of drag in the Selig series of tests at UIUC. It has a low drag bucket from Cl=-0.05 to Cl=0.4. It doesn't stall until Cl=0.8. The Cd minimum at R==300,000 is 0.006.

I assumed a 36 inch span, 4inch average chord, one pound flying weight and a speed of 100MPH. The airfoil reynolds number under these conditions is 312,000 which allows the use of the windtunnel test data. In straight, level flight, the Cl computes to 0.04. In a ten G turn, the coefficient of lift would be 0.4 which is just within the low drag bucket. The Cd=0.0075 in a 10G turn and the bank angle is 84 degrees. The radius of a 10 G turn is 67 feet. Does this sound like it is tight enough to win races?

Since the S6063 is ony 7% thick that would be 0.28 inches on a 4 inch chord. This raises the question of spar strength, stiffness and weight. Unicarbon pultrusions are available with a compression strength of 275,000 PSI. A standard size is 0.034 x 0.121. Two spar caps of this stock weigh 7 grams. With 0.2 inch high, end grain balsa as a shear web and kevlar thread wrap such a spar would weigh about 1/2 ounce and be able to carry a 50 G load in this application.

The total drag is hard to estimate because of the unknown parasitic drag. However, assuming a parasitic drag coefficient of 0.01 and adding the induced drag coefficient of 0.0057 in a 10 G turn plus the profile drag of 0.0075 for a total drag coefficient around 0.0232. From this I calculated an estimated drag of 9.5 ounces. This seems in the ball park for the thrust of a 1/2 A engine.

The conclusion is that the speed advantage of the aspect ratio 9 wing over an aspect ratio 3 wing would be roughly 10% in the turns. Also, with either wing the speed in the turns would be about ten percent slower than in the straights.

rmh

The wind tunnel stuff is quite interesting .
The 50 G tho--- I don't see the racer ever pulling over - oh - 15 G- tops
The why is that it simply falls thru the air molecules too easily.
I have no wind tunnel experience but a fair bit of shade tree engineering experience.
One of the reasons I don't like high aspect ratio wings for racing models is that you can't control slip or yaw easily - and high aspect ratio wings--when slipping sideways can easily get one wing working better than the other- with very predictable results.
The low aspect ratio wing - when mushing - (3-1 aspect ratio -- has a nice tendency to allow more spanwise flow - which does tend to stabilize .
The wind tunnel wing does not allow for corrections in flight , which can -to put it simply , create a "over the edge " condition.
This is why I opt for the most simple - easy to fly setup - which is an extremely light model with the shortest wing that will fly fast.
My last pylon 1/2 A looked just like a stubby winged Ugly Stick--This made for a lifting fuselage to boot.
It was 12 oz all up .
I also did a model which looked like a Phantom fighter - with the only controls being the inverted VTail stabs -hooked up as elevons.
additional stability was done using small wing tip 45% winglets.
It was fun -but not fast enough - too much wetted area.
I am now getting ready to do a new model for my ZDZ210 -and am planning on staying well inside 40 lbs -and under 3000 squares.

Ollie

Dick,
The maximum G's that my example could pull at 100 MPH are about 20 G's and after that the wing stalls. the only reason to mention 50 G's of wing spar strength is to establish that a strong, stiff wing spar doesn't have to weigh much in this size model even with a very thin airfoil and a higher than normal aspect raiio. In other words, structural considerations are not necessarily limiting. The reason to limit the turns to a coefficient of lift of 0.4, corresponding to 10 G's, is to keep the profile drag in the low drag range and to provide plenty of stall margin.

rmh

OK- tho I am surprised it will calulate to 20 G possibility - That is far higher than we ever considered our very big models could pull- with chords avging 18"
Hmmm.

Ollie

Dick,
Did you check out the dynamic soaring link in one of my earlier posts? The DS's pull 40 to 50 G's at 170 to 180 MPH. The F3F slope racers pull similar G's.

matchlessaero

Ollie, thanks for 'doing the math' there. Pretty interesting stuff. That is the first time in a while that I have seen anyone go at this problem (fast HalfA's) using 'true' engineering principles.

I have been building half-a pylon racers since the late eighties (took a long break off in the late 90's), and fly with a good friend (Spindoctor) who has built many more halfA pylon racers than me, and build some of the best I have ever seen. (The planes in his HalfAst series are something to behold)
We got to talking about the High vs low aspect ratio wing idea last weekend. We talked/debated which was best-high or low aspect ratio. We were both at the first pylon (40size) events when the first 'long' wing planes came out from guys like Jim Katz and others. We talked about how well they flew then. Now all .40 size planes use a relatively high aspect ratio wing.

I based my latest attempt on those ideas. A picture is below. Though this plane looks large (34" span), it has a very thin wing at 6% and a pretty high aspect ratio wing for a half A. I built it this way even though it was completely alter to my prior experiences/thoughts.
Well, what is my point? ---This plane is much faster than any HalfA pylon racer I have ever seen before. The difference is striking. Additionally, the airplane flies through the turns instead of mushing. Speed loss is negligible.
There are other more practical benefits also. Smaller spans produce harder-to-see airplanes. Oftentimes, they are also twitchier on the roll axis. High aspect wings seem to be groovier, and smoother overall.

These are just my experiences, but I feel pretty strongly that they have pointed me in the direction I will go for a while with my (pylon) designs.

Best of luck with your next design Painless

Jeff Leavitt

Ollie,

Thanks for sharing the work up you did on the 1/2a racer. On the turn radius you mention at 67 feet, I'd love to be able to hit that number consistantly! It sounds easy, but it just dosen't work out that way. If a pilot could hit in and out of the turn inside of 40 feet and retain 90% of his speed he'd be awesome! I hear people say all the time "hey! what's so tough about pylon? all you gotta do is go fast and turn left!" I'd love to see some of these guy's just try to fly the course... Sounds a lot easier than it is. The first thing that a pylon ship has to be is a very good flyer. So, if you can fly it really good, you'll beat the pants off a guy who has a rocket that he can't keep on the course. Next item up is speed. I worked up these numbers up from motor calc and it's been a while. As I recall, a 3" pitch prop at 30,000 rpm works out to be 87 mph. And, that number assumes 100% effeciency. The prop and rpm values come from my experience so I'm sure were in the real world here. So, I'm not sure what this 10 mph difference would have on your findings but I do believe that 90 is a much more realistic speed.

If I could, I'd like to ask you to use these numbers and run your model again. (For my own selfish reasons of course!). Your choice of airfoil is great. Let's use a 5.5" cord. Span, 36.5". Weight at 12 oz. And the 90 mph. speed, and see how that comes out.

These little racers make for some fast and furious racing. They are the best kept secret in the RC Hobby! Best Regards,.... jeff

Jeff Leavitt

This is a picture of my racers from earlier in the season... Only the shadows are original! Jeff.....