Control authority of V-tail vs. Cruciform - Page 5 - RC Groups
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Jun 28, 2012, 01:45 PM
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Freddie B's Avatar
Thank you again Don. I don't want large throws causing drag. On an aerobatic Tug Boat pulling beast not so much of a problem, but I at least understand gliders, low drag, low sink rate, etc. I guess My design could be stepped all over, but I am getting the feeling it is going to be a blast to fly. We will see.
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Jul 01, 2012, 07:59 PM
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Hi Don

I was looking at this tailless using square blunt tip LEs as drag rudders.

What struck me though is how thick the airfoils are. The battery is standing on edge in the wing! I thought that at these REs such a thick wing would not work...I'm confused...
Jul 02, 2012, 08:41 AM
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RE numbers also depend on the wing chord, and these are low aspect ratio delta wings with a fairly large chord for their size. That's one other reason why flying wings are attractive as rc gliders, notwithstanding the stability and trimming issues and the loss of efficiency caused by having to use a reflexed airfoil. Even at larger sizes, the Fauvel flying plank gliders could compete and exceed in performance against contemporary conventional gliders with higher aspect ratios and thinner wing profiles.
Jul 02, 2012, 09:08 AM
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Don Stackhouse's Avatar
The key here is the thickness/chord ratio, not the absolute thickness. With enough chord, you can get a large absolute thickness, and still have an acceptable thickness/chord ratio.

There is also still the perception in much of the model community that thicker airfoils make more lift. While this can be true at higher Re's (such as full scale), at lower Re's (low enough that the flow tends to be laminar rather than turbulent) it typically works the other way.

There are also a lot of folks in the modeling community who still believe a Clark Y is a good airfoil for models.
Jul 02, 2012, 09:38 AM
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Thanks...thickness ratio does look large, the wingspan is 36", chord about 7", that makes it about AR of 5. At 1" thick that will make it about 14% thick. How tall is the battery on its side? If it is 1.25", then it is almost 18% thick.

Talking about airfoils, I need a flat bottom airfoil, anyone have experience with the gottingen 436? I need a low moment flat bottom if such exists.

Regards to all
Jul 02, 2012, 09:54 AM
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Don Stackhouse's Avatar
18% is way too much. You can get more lift coefficient (and therefore lower flying speed for a given weight and wing area), less drag, and a better L/D from something thinner, and without sacrificing stall characteristics to get it.

"Flat bottomed" just means that the camber is about half the thickness, and the distribution of the two along the chord is about the same. Less thickness therefore means less camber, and that reduces the pitching moment. Also, making the max camber and thickness farther forward, and avoiding "aft loading" (a lot of camber near the trailing edge) also helps keep the pitching moment down.

Go 436 doesn't look too bad in the camber department, the pitching moment should be OK, but it's too thick for what you want to do. Might do better if you put it on a diet. I'd slim the camber and the thickness by the same proportion. Max thickness location isn't too bad, but a bit further aft than optimum for our purposes.

All that said, you should not be describing airfoils as just "flat bottomed" or "symmetrical" or "undercambered". There are all sorts of different parameters, all of which play their role. More importantly, the airfoils are only one part of the overall wing and airframe design, and all those other factors need to be included. When I design a wing, the airfoils (note the plural) are one of the last things to be defined in the process. Picking an airfoil and then designing the wing around it is like picking out a pair of shoes without first measuring what size your feet are.
Last edited by Don Stackhouse; Jul 02, 2012 at 10:00 AM.
Jul 02, 2012, 12:46 PM
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That's what I thought but the designer of the assassin flying wing seems prove that it works really well compared with his thinner models. He claims the thicker wing was for durability but was surprised when it performed so well.

Thanks for the advice on the airfoil.

In the case of my attempt at designing a GEV, 'flat bottom' is a requirement because of the Venturi formed between wing and surface when the lower wing surface is convex. Under camber may work but it's generally frowned upon.

The thick wing caught my attention because my last models failure I attributed to too thick a wing and now I'm not sure if it was something(s) else!
Jul 02, 2012, 05:45 PM
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Yep it is 18% thick and a great performer at very low speeds.
Jul 02, 2012, 05:51 PM
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Sorry I copied and pasted but the whole thread came out. What I wanted is on post#51. He says it's similar to a Clark Y but at 18%.
Jul 02, 2012, 11:15 PM
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Don Stackhouse's Avatar
"great" is a relative term.
Jul 03, 2012, 07:04 AM
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Originally Posted by wim5150
Lee, found this thread a week or two ago and have become very curious about a wing without fins. Is the same airfoil used at the root and tip for the Assassin and Pinata? Do you cut any twist/washout into the cores?
The Assassin and the Pinata use the same airfoil from root to the tip. The new Pinata is a more swept version of the Assassin that uses the same airfoil and has been upgraded to the 1.9 lb EPP elevons too. The Pinata looks a little sportier but the Assassin is easier to set up and has more prop clearance and so it is my favorite.

If you look at the airfoil you would not guess the plane would fly like it does. The thick airfoil designed to protect the battery and radio in combat and for beginners has proven to be one of the best performance airfoils at the field. I can fly slower than any of the other combat worthy planes at the field but still have a 70 mph top end which is a great range for combat. We can put big motors and batteries on them and force them to fly 100+ but that is not the goal for this plane. It is designed for small field and low hour pilots that don't want to upset the neighbors and for combat clubs that want an inexpensive durable plane, that will fit in a small trunk, that can take the hits without structural failure.

I did not put any washout or twist in the wings. I designed this plane to be a fully aerobatic plane and washout or twist comes back to haunt you in inverted flight, axle rolls and outside loops.

We have comparing yaw in similar sized planes with and without fins. The Assassin design really is as stable and perform as well as similar planes with fins at all speeds.

Jul 03, 2012, 08:14 AM
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Well, the very thick symmetrical wing was quite common for fun-fly models, to get good upright and inverted stall angles. But it makes for a very draggy wing, that is not very efficient (in the common acception of the term) at all. That said, for that style of flight it's probably better to have a draggier wing with a more powerful motor, to keep the plane relatively slow but still be able to accelerate vertically.
Jul 03, 2012, 08:22 AM
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Agreed, I like thin, just did not realize thick would fly so my book the biggest advantage of thick after strength is space inside.
Jul 03, 2012, 10:31 AM
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Don Stackhouse's Avatar
Anything will fly, including flying slow, if you put a big enough motor on it.

Yes, for aerobatics, drag can be helpful for keeping the speed down on long vertical down lines. However, it means the plane does not retain energy, and has to be dragged back uphil by the motor.
Jul 03, 2012, 11:57 AM
Cognitive dissonance
kcaldwel's Avatar
The Clark YM-18 does appear to work pretty well at low Re for such a thick airfoil. The max Cl is great, and it looks very well behaved despite being 18% thick. It does need quite a bit of reflex to get the Cm less negative for a flying wing - I see the instructions say 1/4" up elevon deflection.

Neat trick of trimming the tips for yaw stability without fins. It is also interesting that you have ended up with negative differential (more down than up elevon) like the rudderless DLGs seem to work best with.

The drag is a higher than thinner airfoils, but it has a great CL range, the upper transition point moves very linearly towards the front over a wide AoA range which should make a very predictable stall. I'm very surprised by how good it looks.

A de-cambered version with maybe half the camber looks like it would retain the good characteristics but require a lot less reflex and have a bit less drag at lower Cl.


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