I'm working on a new plane meant for speed (100mph+ is the initial target.) I want as clean of an airframe as possible without adding to much weigh or complexity. The plan already includes a gentle sweep from spinner to tail. Would drag or any other aspects that affect flight at high speeds be affected if the fuse cross section were square instead of rounded at the corners or fully circular? How much would it help to fully cowl the engine (not so much as to limit induction or cooling?)

Thanks for any ideas on this.

Do some aerodynamics research on Induced Drag and Parasite Drag. If I recall flight school from 20 some odd yrs ago, parasite drag is YOUR topic. As I recall it decreases as airspeed increases.

Yep.... you want to look into parasite drag. Induced drag goes down as speed goes up. Streamlining the fuse is definitely not necessary, but is definitely recommended!

if your fuselage is the boxy type then rounding off the corners is highly recommended as Kenliko mentioned. There is very little aerodynamic benefit from a circular to a squarish crossection. Basically your "wetted" area is bigger with the squarish fuse than the circular one. A cowling can help you or break you if its poorly designed and creates more parasitic drag. Air should flow from the front of the cowling and exit somewhere on the sides to avoid a stagnation point at the front end.
Although you havent mentioned the wings here, a little swept back on them can be beneficial too. Keep the size of the vertical and horizontal stab to the minimum allow to maintain controllable flight and if you could incorporate a V-tail instead of conventional even better, less wetted surface.

IF you could gaurantee that the airflow would always be inline with the square corners then there would be no advantage to using rounded corners. This would mean that you would have to know exactly what the air is doing at all points as it slides down the length of the fuselage. Since most of us don't know how to figure out the airflow to that degree it is unavoidable that some of the air is going to hit some of those corners at an angle. As it flows around that angle it creates turbulence and drag. Parasitic drag.

But a nicely rounded fuselage will go a long way to reducing the size of any vortices. The rounder the better. And since most of the angular airflow is probably in the vertical plain it usually is more benificial to make the fuselage in a vertically oriented oval shape.

How you do that is up to you but the "classic" method is light wood used for thick sides, even thicker tops and triangle stock in the internal corners. From there you carve and sand it all down so that it is as rounded as practical given the need for structural integrity.

Keeping the frontal area down as much as practical helps a lot too. That means the fuselage needs to be skinny. Or at least as skinny as possible other than the need for those blocks to help shape the roundness. The best of both worlds is a fiberglass fuselage that would be just big enough to hold the engine, tank and gear. Better yet is a high output engine with a rear exhaust venting into a tuned pipe or mousse can muffler in a ducted shape in the fuselage directly behind the engine. All else being equal frontal area is the enemy.

In the end you can't do much better than using some of the electric Speed 400 racer models as a pattern. This shape is often seen in the British Club 20 pylon class as well. You don't see it used in much of our other North American racing classes because of rules governing cross section area, shape and engine/exhaust system selection.

An engine cowl is a good idea but if you have a rear exhaust or a manifold that folds the exhaust around to the rear then a muffler shroud "bump" behind the engine cylinder can offer a lot of assistance in dampening out the turbulence and aiding in going faster.

But all this fades by comparison to the need for an engine that can spin a higher than normal pitched prop as high rpms. If you don't have the power it ain't going fast. That's why a "dirty" planform like the deltas with exposed servos you see in the high speed prop forum can do so well. They often put a fancy engine on the front that just drags the rest along in spite of the protests....

.... so much for aerodynamic finness...

Thanks for your replies:

I looked around for a site and found a good one ( http://142.26.194.131/aerodynamics1/Drag/Page1.html ) that explained different types of drag and their causes. From what I gathered, induced drag will go down with speed (and the decrease in dynamic incidence associated with a lower speed) as Ken and BigPlanePilot said. As I see it, as the speed increases the dynamic incidence of a fully symmetrical airfoil will approach 0 degrees; therefore, the induced drag will approach 0 as well. From what I can tell the biggest portion of the drag at speed would be pressure drag created by blunt surfaces which can be fixed by streamlining. A reduction in cross section would also help as BMatthews said. Could any significant gains be achieved with a smaller "wetted" area?

With a big enough engine, drag really doesn't matter very much. Unfortunately for me this won't help since it'll be a 1/2a.

Thanks

[sm=thumbup.gif] By George, he got it! [sm=thumbup.gif]

Yes, the smaller the wetted area the less parasitic drag you would encounter. Given that building a circular(oval as suggested earlier) crossection wouldnt not bring additional weight then the amount of wetted area would be reduced and with that the parasitic drag.

-in my best Yoda voice....

"If a 1/2A speedster you be wanting then matchlessaero's posts you reading should be."

He did a 1/2A speedster that goes more than 100 and it conforms to what we've been saying here. It's a study all it's own.

Just be sure to incorporate some care to avoid flutter. At those speeds with such small and flexible surfaces it's more than a possibility to the point of becoming a certainty.

BadEnglish,

Cowling the engine would be a huge win, probably the biggest improvement you can make.

banktoturn

You dont have to get this complicated....

Quicky 500's are a square box with a NACA 0012 airfoil and they do ~120+ with a .40!

But look how much faster the Formula 1 models go with the same engines....

Quickie is controlled by rules that limit the fuselage shape IIRC. Otherwise they'd be circular or oval and the engines would be cowled in.

Yes! So this proves the point that the number one item on the recipe list for going fast is HORSEPOWER. Quickies are very inelegant airplanes. There's not much that can be done within the rules that'll clean them up. Oh yes, we do what we can--hidden linkages, gapless/skin hinges on top of the surface and sealing the hinge gap on the underside, sharp trailing edges and the like, but the plane is still just a box. The Bird-of-Prey in my avatar pic will do 110-115 mph with a Thunder Tiger .40 and an APC 9 X 6 prop turning 16.5k. The same airplane will go 150-160 mph with my Jett BSE Q500 motor on an APC 8.8 X 9.25 turning 19.4k

BMatthews too is correct. Take that same motor, (Jett BSE Q500) change the timing and shorten the pipe so it'll swing a smaller prop at 23k to 24k, put it in a Q40, a MUCH cleaner and more streamlined true racing plane, and you'll gain 20 mph.

I should clarify my post above...

The RPM figures I quote are static figures measured on the ground. I haven't yet been able to determine what the actual unloaded RPMs are for each configuration. So before anybody does the math, let me reiterate, RPMs were measured on the ground.

Thats all true, but the heading of this forum is aerodynamics. You can do a lot with horsepower but it gives you a limiting airspeed. Now, if you clean up the airflow around the airplane you can extract another 5-15% extra speed from it. Two birds can attain the same airspeed even if one uses a smaller engine, all you have to do is clean as much of the fuse profile as you can. If all you care is to go fast for a short distance then strap a rocket engine and let it rip. If not why do non-quickie pylon racers are so streamlined?

cappio777,

Without question! I agree completely. That's why racers are always "honing the edge" so to speak. I'm just saying that adding HP is the easiest thing to do and will give the biggest advantage. You can add horsepower and you can pay attention to the aerodynamics of the plane. To do one without the other is self defeating.

I am a firm believer in copying what works. Everyone (repeat everyone) that has ever wanted a model to go fast (except as mandated by rules) has always had a rounded fuselage, streamlined everything, cowled engine, airplane. In the land of the jets the shapes have changed, flatter fuselages to hold two engines, etc. but given a choice it would be the upright oval that was previously mentioned. If the fuselage has any angle of attack it will smooth out the flow around it and have a lower drag. The box fuselage in a high angle of attack, high speed turn is just drag in one of its best (in terms of being worst) forms. At least with drag due to lift you are getting something.