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Author Topic: vertical stabilizer design  (Read 6324 times)
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turborick
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« on: October 31, 2012, 02:51:31 AM »

anyone have any ideas on vertical stabilizer design?
flat plate style vs airfoil ?
tall skinny vs short fat?
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Rick Yacoucci

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manta22
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« Reply #1 on: October 31, 2012, 11:40:27 AM »

Rick;

An airfoil will have lower drag-- try a NACA 2315 profile. A long thin fin will be better if you can make it stiff enough. Look at glider wings- always long & thin.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091108_1993091108.pdf    http://upload.wikimedia.org/wikipedia/commons/thumb/6/66/NACA_2315.svg/1280px-NACA_2315.svg.png

Regards, Neil
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Regards, Neil  Tucson, AZ
Rex Schimmer
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« Reply #2 on: October 31, 2012, 11:57:17 AM »

Neil,
Looks to me that the NACA 2315 is not a symmetrical airfoil, I would, as I always do, suggest one of the lower NACA 6600 series shapes. They are symmetrical and very low drag. Also, as Neal suggest, shapes from the sail plane people are always very low drag. If you are thinking about something like the piece of 1/2 inch aluminum plate that one of the lakesters ran this year I would not recommend this approach at all, easy to build not very aero. You might look at Bruce Carmichael's book "Personal Aircraft Drag Reduction" as there are several good sections on tail fin design. Just a note regarding some of the info in Carmicheal's book, he writes about a small personel airplane that goes over 300 mph on less than 100 hp, really makes you think about what we do.
Rex
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« Reply #3 on: October 31, 2012, 12:08:06 PM »

You have a choice of evils problem with a tail stabilizer, if you choose a high aspect ratio air foil (ie long and thin) as a tail stabilizer you also will move the center of pressure on that stabilizer up higher above the CG of the car, giving it more over turning moment (tendency to try to turn the car over) if you get seriously sideways or catch a strong gust of side wind.

THe WRC cars use multiple vertical fins on their rear spoiler, which although probably slightly higher drag, has a lower over turning moment when sideways and also will produce much more side force at high yaw angles. If the body shape allows it to be constructed efficiently two moderately short squat tail fins would be better than one very tall thin tail fine in my view.

Larry
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« Reply #4 on: October 31, 2012, 12:21:40 PM »

Rick,
     Here is my two cent's worth. High aspect wings (sailplane) are attempting to create the highest LIFT to drag ratio, not neccessarily what you want. A thin, symetrical low drag shape like the 6600 series Rex cited is a better choice. I would think choosing the lowest frontal area (thinnest cross section) square (perhaps trapizoidal for looks) shape with the required fin area would be the best choice (kinda like what you have now only bigger). Remember this comes from a guy who hung around with aerodynamicists at the old lazy L but isn't one.
Jim
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« Reply #5 on: October 31, 2012, 12:26:46 PM »

Rex, you're right. I wasn't thinking. Of course a NACA 2315 isn't symmetrical-- I was thinking a wing, not a tail fin. Arrgghhh.

Regards, Neil
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Regards, Neil  Tucson, AZ
John Burk
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« Reply #6 on: October 31, 2012, 03:08:33 PM »

Thick and thin airfoils have similar lift up to a certain angle of attack . Thin ones quit at 8 deg . With some thicker ones lift increases up to 16 deg . Straight into the wind the drag of even thick wings is relatively small . Wouldn't a tall tail fin be what you'd want to balance tire friction if you got crossed up . Joe Law's tall fin keep his lakester  on it's wheels during a spin at El Mirage .
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maj
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« Reply #7 on: October 31, 2012, 04:23:00 PM »

i have been using this plotter for getting a general idea of a shape and plotting coordinates
usually just do it in 10:1 scale so it is on one piece of paper
http://windandwet.com/windturbine/airfoil_plotter/index.php
many of the common airfoils have calculated or windtunnel data available online if googled

Whats peoples feelings about vertical stabilisers in a roll incident with narrow vehicles , anything in particular likely to help resist a roll or at least not dig in and make it worse ??
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Dean Los Angeles
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« Reply #8 on: October 31, 2012, 04:47:57 PM »

The NACA 63-012A is used on the Cirrus sail plane vertical stabilizer.
http://airfoiltools.com/airfoil/details?airfoil=n63012a-il

Since this comes under the heading of "emergency Acura saver" you want a low drag number for straight line and enough size to exert authority when it all starts to go wrong.

You have to calculate the lift coefficient at various angles of attack and calculate the moment arm it is exerting on the vehicle.
Since you are above the c/g you are going to exhibit a roll force, but I don't think it is significant, and mostly unavoidable.
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« Reply #9 on: October 31, 2012, 06:45:33 PM »

John Burk has a great point regarding the fact that airfoils do pretty much stop generating lift after a certain angle, which is pretty small and considering that the lift being generated is in the direction that is trying to keep you from spinning maybe the vertical stabilizer should have the front edge swept back at say a 45 or even 60 degree angle. This will make the stabilizer generate vortex lift at even angles as large as 45 degrees and more which is just what you want to prevent a spin. You could still give the cross sectional shape one of the thin NACA symmetrical airfoil shapes and have sufficient area to get the center of lift back from the CG and be able to generate some pretty significant "anti spin" lift if you get her side ways.


Rex
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« Reply #10 on: October 31, 2012, 09:19:12 PM »

Rick,

I'm using a naca 006 wing section on my liner 6% thickness to chord length… drag numbers are very small  mine is a variable length / thickness design..  any questions call 408 206 7050  stabile so far at 350 mph..

Bob Dalton



* quarter.JPG (128.36 KB, 1280x960 - viewed 217 times.)

* tail end.jpg (122.63 KB, 968x1296 - viewed 196 times.)
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Dean Los Angeles
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« Reply #11 on: October 31, 2012, 11:47:37 PM »

Quote
generate vortex lift at even angles as large as 45 degrees and more which is just what you want to prevent a spin

If the wing is designed correctly you won't be able to get to 45 degrees in the first place. I would think that if you are past 10 degrees you've lost the battle.
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Well, it used to be Los Angeles . . . 50 miles north of Fresno now.
Just remember . . . It isn't life or death.
It's bigger than life or death! It's RACING.
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« Reply #12 on: November 01, 2012, 12:00:29 PM »

Rick,

I'm using a naca 006 wing section on my liner 6% thickness to chord length… drag numbers are very small  mine is a variable length / thickness design..  any questions call 408 206 7050  stabile so far at 350 mph..

Bob Dalton


Looks like you swiped it off a Mooney!  shocked cheers
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Jack Iliff
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Dean Los Angeles
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« Reply #13 on: November 01, 2012, 12:15:11 PM »

NACA 0006 details
http://airfoiltools.com/airfoil/details?airfoil=naca0006-il
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Well, it used to be Los Angeles . . . 50 miles north of Fresno now.
Just remember . . . It isn't life or death.
It's bigger than life or death! It's RACING.
turborick
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« Reply #14 on: November 18, 2012, 04:29:37 PM »

Thanks everyone for you answers in the 988 car ( the red one) we added a larger tail fin and it made the car a dream to drive, in a cross wind it would go straight with very little or no steering input.
788 has required constant steering and in a crosswind requires excessive steering input to keep it going straight.
We just put a larger intake scoop on the car and knew it would move the center of pressure back so last week end we tested it and the car was much better to drive, unfortunately there was no strong cross wind to test with but still much better. We had a much more powerful engine  so the increased acceleration could of given me the better stability.
At this point I think we need to test more before we put a larger tail.

In car video of our last run http://www.youtube.com/watch?feature=player_embedded&v=oHfv0Gm1kUQ
don't forget to go full screen

here is a picture of the new scoop (thanks Zane)



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Rick Yacoucci

Nebulous Theorem III #788
http://www.bonnevillestreamliner.com
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