CEF speed contest Design Discussions (2014)

Thank you all for the kind words guys. Much appreciated. I am really learning as I go along.

Rusty, Thank you for the info on the Rudder. Yes it is sanded on both sides. So it should be the same. But I will ensure that it is not sanded more on the outboard side.

Theo

Theo Kleynhans wrote:Thank you all for the kind words guys. Much appreciated. I am really learning as I go along.

Rusty, Thank you for the info on the Rudder. Yes it is sanded on both sides. So it should be the same. But I will ensure that it is not sanded more on the outboard side.

Theo

Okay good. Just thought I better check. Now another question, being an active component in the airflow, would not there be some drag associated with an airfoil? For speed vs stability, would not a razor edged flat fin that slices straight through the air with little disturbance be faster? I would assume the same of the stab.
Rusty

Hi Rusty

I must say I am not an expert in this field, but I did also raise the question to my mentor and teacher Keith Renecle before starting my build.
He said that a wing with an airfoil is faster than a flat sheet wing. I unfortunately do not know why?? Thus I made the airfoil.

I hope someone that knows the reasoning can help us??

Theo

For a flat wing to generate any lift at all it would have to fly angled slightly up at the leading edge to enable and generation of negative pressure above the wing (lift).  As far as the air is concerned (if air can be concerned   ) the wing appears far thicker than it actually is, creating more drag.  Also, a thick trailing edge would create turbulance behind the wing, which would try to suck it back into the void.
An aerofoiled (the way we of British decent say it) wing with a flat bottom should generate lift with the leading and trailing edges exactly harizontal, so the "thickness" of the wing is only the thickness of the wing.     Well, I know what I mean.
In this way, I can see why a flat wing, even if very thin, would create more drag.

Rod.

Haven't had my coffee so I hope it isn't a bad idea to post.

Rod and Roddie, I understand, but what about the tail fin in question. Is it that a flat blade will "hunt," bouncing back and forth while doing its job of stabilizing the yaw of the plane, thus inducing drag with each bounce? Whereas an airfoil will be more stable being surrounded by a cushion of air and buffered from such disorderly oscillating? I'm Waaay out of my knowledge base here, so I'm guessing and making it up as I go.

The fact that the plane is flying through the air while constantly being dragged off course by the control lines may play into an asymmetrical foil on the rudder, slightly thicker on the outboard side to accommodate less drag for the circular path. Well hell, maybe I've just answered my question. Maybe we actually do want a slight yaw-in on a speed ship. I'm in even deeper now.
Rusty

Eureka! Did I just get that right? We want the speed plane to yaw in slightly for the sake of reduced drag, unlike a sport plane. Seems like a no brainer now.
Rusty

RknRusty wrote:Eureka! Did I just get that right? We want the speed plane to yaw in slightly for the sake of reduced drag, unlike a sport plane. Seems like a no brainer now.
Rusty

er...as long as the yaw in is less than the required turn for the circle...I think having a straight line or even yaw out might be more appropriate than yaw in. Lines tight and all.

Phil

Mmmm! A lot of speed models don't have fins and rudders





Some only have half a wing and tailplane!

RknRusty wrote: Well hell, maybe I've just answered my question. Maybe we actually do want a slight yaw-in on a speed ship. I'm in even deeper now.
Rusty

I wasn't going to divulge any secrets but.... That's why my plane has the rudder air foiled on the outboard side, no tip wieght, and leadouts above the vertical cg that can be moved forward of the horizontal cg. I want it to fly in a counter clockwise circle and rely on centrifugal force to keep the lines tight. Or it may just fly into the center of the circle and try to kill me.  

Jim

JPvelo wrote:

RknRusty wrote: Well hell, maybe I've just answered my question. Maybe we actually do want a slight yaw-in on a speed ship. I'm in even deeper now.
Rusty

I wasn't going to divulge any secrets but.... That's why my plane has the rudder air foiled on the outboard side, no tip wieght, and leadouts above the vertical cg that can be moved forward of the horizontal cg. I want it to fly in a counter clockwise circle and rely on centrifugal force to keep the lines tight. Or it may just fly into the center of the circle and try to kill me.  

Jim

Wow Jim, if you don't mind me saying, it sounds like you're inviting some pretty rowdy guests to your party...

Keep in mind that on 42 foot lines the difference between perpendicular at the leadouts and perpendicular at the fin (say, 8" aft) is less than half a degree. Even if you did the maths and set the fin exactly perpendicular to the center of the circle, a very slight yaw could cause it to go from neutral to a left (or right) turn moment rather quickly!

Also, the fin isn't just operating in clear air as it moves around the circle. It is subject to the wind (if any) shifting 360 degrees each lap and is also operating in the swirling wash of the propeller. With a right-hand prop the wash swirls in a spiral around the fuse ultimately hitting the fin on the left side causing a tendency to veer to the left. That is why several of the Proto-speed designs put the fin on the bottom of the fuse where that same swirl effect tends to cause a right turn. Even full size single engine airplanes have the fin rigged with a slight offset to help compensate for this effect. Of course with a left-hand prop the effect is reversed and works FOR us in a counter-clockwise circle.

Combine all that with high leadouts (inward rolling moment), no tip weight, leadouts forward of the CG and it all sounds a bit too much.

Of course, you are a pretty sharp guy and have probably thought of all this already, so I can't wait to see what you are REALLY up to!

Still, wear a helmet please...   

Mike

Not by posting, just because you never flew it...  

I see, I was only pulling your leg. I gave you a + rep point for posting.

That plane of yours looks interesting Roddie. You should maybe try and fly it now that you are experienced.

All of this talk regarding the rudder and airfoils is a lot to take in.

I have another question regarding engine position. If you have a look at the picture below of the Nemesis, you will see that the engine/prop is not situated in the middle vertically at the nose. It is actually higher than the centre line of the nose. Can I mount my engine the same way? Higher up like the drawing. Will this make a difference?


Theo

Theo Kleynhans wrote:That plane of yours looks interesting Roddie. You should maybe try and fly it now that you are experienced.

All of this talk regarding the rudder and airfoils is a lot to take in.

I have another question regarding engine position. If you have a look at the picture below of the Nemesis, you will see that the engine/prop is not situated in the middle vertically at the nose. It is actually higher than the centre line of the nose. Can I mount my engine the same way? Higher up like the drawing. Will this make a difference?


Theo

Theo, the reason the engine in Nemesis is offset like that instead of centerline is because all the fiddly bits of the engine are hanging below the case.  The carb and oil pan are down there, so the cowling has to be offset to keep everything nice and tight.  I don't know if Nemesis uses an alternator, or just relies on battery power for such short race runs, but there could be electrical system components hanging below the engine, too.   I'm sure if they could, Team Nemesis would love to have it centerline with minimal cowling area and drag.  

My personal opinion is that it won't make much of a difference for our contest.  If you could get your plane 0-0-0 with everything in-line, you'd have less drag on it.  But I don't think engine above centerline is really a meaningful concern.

The Blasé Mark

Hi everybody,

I just re-read the post I put up regarding Jim's plans, rudders and such and I feel I need to apologize.

When I read it this morning, I felt it came across as somewhat snarky and sarcastic, and that was NOT AT ALL what I intended!

Jim, your planes are very clever, well designed and built. When I said I felt that you are a sharp guy and that I can't wait to see what you are up to, I meant it very sincerely, but the way it was written it could be perceived as being sarcastic. That is in NO WAY what I intended, and if it did come across that way to anyone, especially Jim, I sincerely apologize.

Mike

Theo Kleynhans wrote:That plane of yours looks interesting Roddie. You should maybe try and fly it now that you are experienced.

All of this talk regarding the rudder and airfoils is a lot to take in.

I have another question regarding engine position. If you have a look at the picture below of the Nemesis, you will see that the engine/prop is not situated in the middle vertically at the nose. It is actually higher than the centre line of the nose. Can I mount my engine the same way? Higher up like the drawing. Will this make a difference?

Theo

As long as your thrust line stays the same you can mount above or below centerline. It really only poses a problem if you build in up or down thrust .

For small offsets using a straight thrust line works well. Offsetting the thrust line somewhat significantly is one way of handling prop rotation effects. In order to fully account for engine offset drag of all parts of the plane must be accounted for.

There is reasonable preference to having everything in line.

Hang your engine inverted and run with it for the scale look.

Phil

roddie wrote:

pkrankow wrote:

Hang your engine inverted and run with it for the scale look.

Phil

I agree with Phil.. and utilizing the scale "air-scoop" would help when you cowl-in an inverted cylinder. You could put louvers.. or some other venting rearward in your cowl, to scavenge the exhaust-wastes out.. and insure good airflow/cooling of the engine. A side-hole in the cowl for an exhaust-prime might be a good idea to help start the engine. Leave just enough of the glow-head exposed out the bottom to attach your battery clip.

The thumb says minimum 2x the exit area as the intake area for the vents.
Phil