Landracing Forum
Fundraising, For sale, and Wanted => Items Wanted => Topic started by: ggl205 on January 21, 2020, 08:19:02 PM
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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
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John, you ought to be able to find something here in the Air Capital....
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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
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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.
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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 .
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The NACA 4412 is a good general-purpose "low speed" airfoil.
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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
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I believe in long flat fins
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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
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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?
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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 .
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How big was that fin on the Herbert & Steen liner?
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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
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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) :?
:cheers:
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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.
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Something anybody can do to learn about tail fins is bandsaw an 18" wooden model of your car , hang it from various points and blow an air hose at it with small , large and no tail fin to find the dangle point where it points into the air stream . Hang a weighted thread from the nose to keep it level . You may be surprised at how hard (impossible) it is to get the get the aerodynamic center back to a mid engine car's CG . The center of area in side view is close to the aero neutral point when a car is going sideways but pointed down track it's much further forward .
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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.
John - if the cardboard tail extension worked well just make a cardboard fin. All this talk of tubing and fibre glass sounds like hard work :laugh:
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John, that rear extension helped reduce Cd by .3 but did little to move the Cp rearward. Looks like I am stuck trying to affix some sort of fin to this new section. I will swing by Stainless?s place and we can noodle it out.
John
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John,
Duke and I may need to add a fin to our car, depending on what Woody comes up with but I am planning to use a piece of 1/4 inch aluminum plate cut to the area we need and having leading edge that lays back about 60+ degrees, I would also champfer the leading edge and make the the exact center of the champfers be a sharp edge. Read about "vortex lift in "Race Car Aerodynamics" by Joseph Katz which has a great section on it. As the car starts to spin side ways the sharp leading edge develops a vortex that runs down the back side of the plate, a vortex is low pressure so it makes a pressure differential on the tail that is pushing in the opposite direction to the spin. The amount of "lifting force" starts small but increase rapidly as the angle gets larger and will work fairly well at angles higher than 30 deg. This appears to me as being just what we need, easy to fabricate and mount, anti spin force rapidly increases as the car gets more side ways and probably not much more drag than an aerofoil shape.
Rex
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A perhaps not generally considered (and maybe has other disadvantages) method to help the Cp and Cg discrepancy is add weight to the front of the car. Your long lakester would have a big lever. Might not take too much lead.
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Jack:
Probably wouldn?t hurt to have a bit more weight up front anyway. Before making battery and suspension modifications to the front, wind tunnel data said I had around 35 pounds of lift. Given the 525 pounds on front axle, I figured net weight of 490 would be enough to plant the front tires. So, maybe another 30 or 40 pounds of steel (lead is too expensive) along with added weight from battery and axle mods, will help move the Cp back a bit and kill the lift at speed.
John
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Rex:
Food for thought. Mounting a stabilizer was of paramount concern until I went over how Stainless did his. I may need to sharpen up my aluminum welding skills for this exercise, however.
John
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We bought clean, used shot at $1 per pound. Then enclosed it in a steel vessel that fit where I wanted it and bolted it down. Easy to fit, easy to change.
This one is in a 1/2-inch steel enclosure.
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Stan, unless I can get lead locally, shipping would negate any benefit.
There is quite a bit of dirt track racing here in Wichita so I may find what I need in lead.
John
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Find a friendly tire shop and you can get used tire weights.
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There is quite a bit of dirt track racing here in Wichita so I may find what I need in lead.
Tried all the Mexican tire repair places?
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I think the shot conforms and fills the container better than wheel weights. I buy mine locally at a shotgun range that recycles and sells it. It amazed me how clean it was, too.
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Stan, ya gotta melt down the wheel weights... the steel floats the lead is under the scum layer. You get a lotta lead out of a 5 gallon bucket of weights... oops, the damn handle pulled off when you tried to move that bucket...
offer the kid $10 to put it in your car :laugh: :cheers:
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Stainless ? the shot has other values, too. One ? you don't have to melt it. And, Two ? you can easily(?) adjust it.
I made an about 1-inch diameter entry in the containers (not shown) and just closed them off with an 1-1/2" circle of aluminum with a toggle bolt securing it.
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Hmmm, recycled shooting range lead. Never thought of that. We have a few ranges out here in the heartland.
John
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With lead shot you can weld bungs in frame rails & tubing so you can pour in more or drain it out as needed.
Sid.