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Old Mar 17, 2011, 09:13 AM
gpw
“There’s no place like Foam”
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I've got a question .... What about using the KF on the "Tail surfaces" ???... Just thinking the KF is more efficient over the common flat plate tails , so we could use smaller more scale size tails and still have stability in our "scale air" ...
On all my average sized Foamies (~36") , no matter the wing airfoil used , I always had to enlarge the tail surfaces beyond the "scale" proportions to ensure a measure of pitch and yaw stability... which to me gave them a different "look"... Might be Nice to be able to get away with scale sized surfaces... Just crumbs for thought
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Old Mar 17, 2011, 09:26 AM
Jack
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I'd probably consider it an unnecessary complication.

The stabs are just that, stabilizers, as long as the surface is flat. Maybe what you want to consider doing is changing the size of the elevator or rudder as far as the percentage of the stabilizer's original surface. Then when that part of the stabilizer is used as a control surface it's larger area will increase it's authority.

Of course, there is no free lunch. The amount of deflection is a factor here as is the power of the servos and the speed of the plane. I have read of high speed planes where the elevator became ineffective at higher speeds. I was never sure if that was a aerodynamics issue, a servo power issue, a control surface size issue, or any or all of those things.

In general I like to think of control surface area as like insurance. It is better to have it and not need it than it is to need it and not have it. I don't generally pursue speed in flight and I play around with control surface deflection amounts when I want more or less response.

Them are my thoughts...

Jack
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Old Mar 17, 2011, 04:03 PM
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Most tails dont do much 'lifting' in fact they are equally likely to produce downforce as upward lift. The most efficient airfoil for most tails is simply a thin symmetrical 'regular' airfoil. Adding thickness with a KF step could only increase drag IMHO.

Also the airfoil used on the tail really makes no difference to how much tail area you need because the lift slope gradient of pretty much all airfoils is just about the same.
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Old Mar 18, 2011, 07:04 AM
gpw
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Well this is just something I'll have to try for myself... These discussions are nice , but the proof is in the flying eh!!! I've had much Luck in the past 50 years getting things to fly that weren't supposed to...
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Old Mar 18, 2011, 07:55 AM
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It will fly all right.. Only i dont think it will be better than a flat plate tail though probably not measurably worse either.
There is little in aerodynamics that hasnt been proved 'in the flying' over the last 100 and odd years

Steve
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Old Mar 18, 2011, 08:44 AM
Jack
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Quote:
Originally Posted by JetPlaneFlyer View Post
It will fly all right.. Only i dont think it will be better than a flat plate tail though probably not measurably worse either.
There is little in aerodynamics that hasnt been proved 'in the flying' over the last 100 and odd years

Steve
Another thought about a hstab that generated lift would be that it's lifting area would only be a smaller percentage of the wing's lifting area. So as the wing lift increased with speed, the lift on the tail surface would increase too but be proportionately less than the wing's lift increase.

Wouldn't that have the effect of the the getting steadily heavier than the wing as the plane flew?

We need to move on to the important questions here, like the number of angels that can fit on the head of a pin...

Jack
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Old Mar 18, 2011, 09:56 AM
gpw
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Gosh , I'm not worried about lift , just about being able to use a "scale" sized tail surface and achieve the stability I usually get with the enlarged (non-scale) flat tails ... Figuring the KF worked Super on the wings , why not give it a try ... I've been wanting another 36" Foamie Profile Spitfire lately ... this time with KF wing , and "scale" tail with KF ... Not meaning to be contrary, I just have to prove it to myself .... you know , old guys ...
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Old Mar 18, 2011, 02:31 PM
just Some Useless Geek
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Well, model rocketeers have been adding clear Lexan extensions to scale fins for decades to achieve stability on scale models where the scale fin wouldn't cut it. Perhaps you could sink to the same level, Jeeper. It would look pretty phunky, but it would work.
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Old Mar 18, 2011, 03:30 PM
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You can get away with a small tail if you use a symmetrical or even slightly reflexed airfoil on the wing.. This works so well that tailless planes dont need any tail at all

Full size planes use the same trick.. The Hawker Hurricane for instance used a Clark-YH which is a slightly reflexed version of the Clark-Y.. This allowed a smaller horizontal stabiliser to be used which produced less drag.

The more camber an airfoil has then the more negative pitching moment the wing produces and the more you need a tail to keep the plane in trim.
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Old Mar 18, 2011, 05:02 PM
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If you try a KF airfoil for your stab, I suggest the KFM1 (step on the bottom). Make the step no more than about 50% of the stab chord. This should provide enough distance between the step and the elevator, that the elevator effectiveness is not compromised. Of course, we are interested in what you try and how it works out for you.

Roger
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Old Mar 19, 2011, 01:08 PM
just Some Useless Geek
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I would think a KFm4 (symmetrical) airfoil on the tail surfaces would prolly be the best solution. Maybe, maybe. Gotta test it out to know for sure, right?
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Old Mar 19, 2011, 01:23 PM
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The stabilizer must provide tail down force to counteract the pitching moment of the wing. The KFM1 would do that. A kfm4 would be symmetrical to be sure, and more like a flat plate, but, in my opinion, it would be awfully thick for a stab.

Roger
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Old Mar 19, 2011, 02:01 PM
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Quote:
Originally Posted by maguro View Post
The stabilizer must provide tail down force to counteract the pitching moment of the wing.
Roger,
That's not always the case.. A tail can provide upward lift, downward lift or can be trimmed neutral. It depends on a number of variable chiefly pitching moment of the wing and CG location. Generally planes with large tails tend to be trimmed so the tail lifts upward. On such planes the pitching of the wing airfoil is balances by the rearward CG location.. this is still stable providing the CG is ahead off the plane's neutral point.

Most 'normally proportioned' planes have the tail trimmed to produce very little lift in either direction when in normal level flight.. That's why a symmetrical airfoil is usually best, drag reduction is the main issue.

Steve
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Old Mar 21, 2011, 06:51 AM
gpw
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Guys , Thanks for all the advice ... We're just going to do a simple build and see if it works ... no troubles , just an experiment , and we Love to build planes anyway ..
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Old Mar 21, 2011, 07:38 PM
treefinder
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