Rear Stabilizer Fin

[ggl205]

Looking for a strong, mountable rear stabilizer fin for my lakester. Tried building one but did not like how it turned out. Hope to find something that works.

John

[aircap]

John, you ought to be able to find something here in the Air Capital....

[ggl205]

One would think but anything from an aircraft is too big. Stainless said stabilizers on some missles are right sized. Anyone have have a spare missle taking up space?

John

[manta22]

Well, John- I do have some pop-out fins from a smart bomb. They are carbon fiber, about 12" long and about 4" chord. I doubt if they would be big enough for you, though.

[John Burk]

Some thoughts on tail fins :

Spins are usually caused by a loss of lateral traction due to wheel spin which a G/GL may not have at high speed .

It takes a really big tail fin to make a conventional lakester directionally stable .

Tail fins do 2 things . Reduce directionally instability . Raise the aerodynamic center when going sideways so air pressure keeps a slide from becoming a roll (the taller the better) .

An airfoil shaped fin does a better job of countering a yaw than a flat one . 2 vertical tubes and lengthwise formers with an airfoil profile to rivet the skin to . Theory of wing sections (book) is a good source of wing profiles . Let me know if you need help on which profile etc .

Airfoils stall at not much angle of attack . If the leading edge slopes back it raises the stall angle .

[manta22]

The NACA 4412 is a good general-purpose "low speed" airfoil.

[TheBaron]

Let us be careful here with selecting an airfoil shape for a vertical fin.....

A wing airfoil will generate lift in one direction relative to the airflow and it doesn't much care as to its orientation to the earth,,,,It mainly cares about its angle of attack and the velocity of the air moving over it....  This could cause a big time yaw toward the curved side of the fin as as velocity increases and lateral "lift" starts being generated........

Now a symmetrical airfoil shape ( both upper and lower sides curved exactly the same ) who's center line is aligned with the vehicle  center line might be a good place to start......

In addition, the vehicle will not be as stable in cross-wind conditions with a fin as compared to fin-less as the fin will try to swing the nose of the car into the cross-wind.....

Main point is to take your time, be careful, and test the car carefully and properly...

Good luck and have fun,
Smitty
Red Baron Race Team

[SPARKY]

I believe in long flat fins

[ggl205]

And I thought this was going to be easy (LOL)!

My lakester did spin on its last run in 2017. Still not entirely sure why, even after a good deal of inspection and discovery. I was fortunate to have put the car in the Darko tunnel on my way back home from World of Speed in 2017 and did find a few things out. But it was Tom Burkland that found my Cp a couple inches ahead of Cg so we added three foot of cardboard to extend the rear and that helped. I figured adding a stabilizer fin would be be additive to straight line stability.

Neil:

Thank you for the offer but you are correct, those smart bomb fins may be a bit too small.

John

[tortoise]

In addition, the vehicle will not be as stable in cross-wind conditions with a fin as compared to fin-less as the fin will try to swing the nose of the car into the cross-wind.....

So, say a cross-wind from the left pushes the car to the right, and the nose of the car moves left under the influence of the fin, steering the car to the left, then this is less stable than no fin?

[John Burk]

NACA 4412 is not symmetrical so wouldn't make a good tail fin . Thin wings quit at 14 deg. yaw . Fat ones work at higher angles . NACA 66-021 (thickness is 21% of length) looks too fat but would work at 24 deg. yaw , has low drag going forward and high drag when you need drag .

[aircap]

How big was that fin on the Herbert & Steen liner?

[ggl205]

JB, I think a flat fin will work for me. I hope to use this device to keep the car straight in normal running so control at 14 degrees of yaw is plenty.

John

[Stainless1]

John, I've got an idea...make a 1 inch wide U... maybe _U with bottom material about 5/16 thick threaded 1/4 20 every 1.5.  Fill center with flat blue foam and sand leading and trailing edge pieces also made from blue foam.  Glue all pieces to metal _U... cover with glass.  Drop by again and we can sketch something up.... Surly you can do most of the work in the house where it is warm... although someone might disagree... (pun intended) 
 

[manta22]

NACA 4412 is not symmetrical so wouldn't make a good tail fin . Thin wings quit at 14 deg. yaw . Fat ones work at higher angles . NACA 66-021 (thickness is 21% of length) looks too fat but would work at 24 deg. yaw , has low drag going forward and high drag when you need drag .

You're right, John. I was thinking of a rear wing.