Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

leading edge?

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Old 11-15-2004, 11:20 PM
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speedy1379
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Default leading edge?

Which would be better for speed,a leading
edge that is rounded or one that is pointed?
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Old 11-16-2004, 10:42 AM
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bob27s
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Default RE: leading edge?

On a wing......Round...........! (And, on tail surfaces for that matter too)

Leading edges have a definate radius. The size of that radius is defined by the airfoil shape, and it is proportion to the thickness and size of the airfoil. The rounded surface permits the oncoming air to smoothly divide between the upper and lower surface of the wing.

A sharp or pointed leading edge provides for indeterminate airflow...... air has to make 'a choice' as to which way to go. This results in unstable and turbulent airflow........ and a poor performing wing. Often, a plane with such an edge will be very pitch sensitive and/or very difficult to trim for level flight.

An example of where a sharp edge is effectively used.... stall strips (full scale aircraft mainly). These are often small pointed edges or metal strips installed on the inboard wing leading edge. Their purpose is to intentionally make the inboard section of the wing stop flying well before the outboard wing. Typically a method of preventing a tip stall.

On a thin wing, it may seem like there is a sharp leading edge..... however it is typically a small radius.

If you in some way try to relate a sharp edge wing like that on a F-104.... that is a very different animal.... designed for high speed transonic and supersonic flight. That airfoil works at a speed far beyond the top speeds we will ever see.
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Old 11-16-2004, 01:11 PM
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Mike Connor
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Default RE: leading edge?

ORIGINAL: bob27s

On a wing......Round...........! (And, on tail surfaces for that matter too)
Hey Bob,

I have heard this many times but wonder why most Q-500 plans suggest the 1/64 ply in the leading edge of the tail surfaces so you can sand to a sharp edge. Also the Shrike has a sharp leading edge wing by default. I left mine that way and the only negative that I noticed is accelerated stalls when turning very sharp.

Keep up the good post.
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Old 11-16-2004, 02:04 PM
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Default RE: leading edge?

Good point Mike.

Yeah, on the tail surfaces, most guys like to make a sharp edge down to that 1/64 ply. They really should be airfoil sections. Sharp edges look cool, but they do nothing for speed/drag.

I try to at least give mine some assemblence of a radius. With a surface say only 3/16" thick, and some sort of airfoil shape, the LE radius ends up being somewhere around .020" So, something other than 'sharp' is ok.

The trailing edges....... very sharp.....you can cut steak with those I started inlaying carbon or making a fiberglass sandwich, and sanding down the TE to the cloth.

That sharp turn (high alpha) departure is exactly one of the issues with a sharp edge. Wing stalls easier.

You might remember the old midwest sweet stick... wing had a 1/4sq stock leading edge, pointy edge forward. I had a guy come out to the field one day, said he couldnt get his plane to trim out. I looked over the plane, and he never bothered to sand the LE to shape. An hour in the shop with some sandpaper, and some monokote work later.... it flew hands off.
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