Practical Aerodynamics

[mrblucor]

Hello everyone,
I've been looking through this thread and some of the links. What I have found are discussions on specific airfoils and some basic airfoil design information. I'm looking for some practical tips on getting more from ARF's and scratch builts. I'm an engineer but not an aeronautical engineer. I'm looking to get the biggest bang for the buck. I don't want to spend all winter working on a foamy to try to make it perfect. I want to direct my efforts to the biggest problems first. If 5 minutes of sanding will make a noticable difference, then I'll do it. As my modeling skills have gotten better with time, I've done things to my models that are not in the instructions. I hope my efforts are not a waste of time. Here are some of the situations I've encountered and what I've done. I consider you guys to be much more knowledgable than me. I welcome your comments and input.

Shumante F-18: This plane is built of flat foam. I rounded the leading edges and tapered the trailing edges to a fine point. This seems to make the plane go faster and glide longer. Less drag. I've seen guys leave the trailing edge flat. Isn't this a no no? I've also been told not to sand the leading edge to a point because this will cause the wing to hunt. Is this true? After a few flights, I taped the flying surfaces. This made the them smooth and stiff. The plane flew much better after this. I'm not sure if it was because the wings were stiffer or beacuse they were smoother. It is possible that there was some undetected flutter that was solved by the tape. The tape is a lot smoother than the foam, this could mean less drag. On my second F-18, I did all the above plus made the fuse smaller, omitted the canopy, and smoothed the fuse as much as possible. This plane was a fantastic flyer. It also came in an ounce or two lighter than the first one. I figure decreasing the cross sectional area of the fuse has got to decrease drag. It might be that the ounce or so less weight had more to do with its flying characteristics than the smaller fuse. I went head to head with another F-18. Same motor/prop/battery, his came in at 18 oz and mine at 16 oz. He had it painted which left a semi rough surface, better than raw foam but not as smooth as the tape. he also had squared trailing edges. Mine flew faster, slower, and longer per charge. I'm thinking weight, trailing edges, and smoothness were the difference in that order. Does that sound right? What do you think was the biggest factor? Did I get the importance of each factor right?
I've also been toying with the idea of installing a fillet where the wing meets the fuse. I've been told that right angles cause drag. Would this be worth doing? Would some foam triangle stock work OK? does the fillet need to be exactly shaped to make a difference? How about a simple airfoil shape to the wings. Is this more trouble and weight than it's worth?

EPS and EPP foam: Many ARF's are built of these two materials. The manufacturing process leaves moulding injection marks. The EPS foam has large round injection marks and the EPP planes have lots of small dimple like marks. Think GWS and Multiplex. These airplane generally have airfoil shaped wings and some even have a fillet where the wing meets the fuse. It seems to me that the injection marks add drag to the airframe. I sand down the peaks and fill the valleys. I think this is worth doing from a flight characteristic view. I have also tried sanding all the foam parts in an effort to make them smoother. I have no head to head flight data to say wether this is helpful or not. I believe it's worth doing.

I'm sure there are many things I've missed. I'm not trying to make a pylon racer or thermaling machine, I just want to try to get more from the average ARF or flat foam project. I think the comments generated from this could be put together and made into a top 10 list of things to do to improve typical preformance. This could also include weight reduction techniques and how a 5% reduction in weight applies to the flight characteristics of a model. It boils down to the question of wether it's worth doing or not. I know this is subjective. It would be nice to know if a proposed change will most likely make a noticable difference or not. All comments, suggestions, and data are welcome. Remember, I'm looking for simple things that will make the greatest difference.

Thanks in advance, JB

[BMatthews]

The rounding and tapering obviously helped you a lot from the sounds of it and that certainly agrees with the theory of drag and wing sections. Hunting? It may have happened on a few models but for the most part a lot of this often achieves urban legend status. I think you ran into some of it. Your personal findings trumps urban legend any day of the week.

The dimples and molding bumps don't affect things as much as you may think. In some cases they can even help by acting as turbulators to slightly delay the effects of pre stall mushing and the point of the final stalling. They DO add some drag but not as much as you would think. I've seen some of these same models fly with all the junk on the skins and it likely affects how far the can glide but when the pilot turns on the fan up front all is forgotten.

[Brandano]

A sharp leading edge pointing straight forward will make the wing stall at a shallower angle, maybe the hunting referred to is a phugoid oscillation?

[MiseryQ]

I was coming in to start a new thread on the "75mph Barrier" but this is really close.
I've just flew my 1st flat plate last week. It was really over powered but I dopplered it at only 75mph with a pitch speed of 102mph.

I'm doing all you've don hoping it wakes the plane up.
The fuselage will be light spackled.
The wing is getting covered.
But I'm thinking how easy it would be to use a bamboo skewer to "round" the leading edge and glue 3mm foam to the top and maybe bottom before covering to make a "real" airfoil. Tapering the leading edge, which is the aileron, and using the covering as a hinge might also help.

Fillet on the wings is an interesting idea, perhaps a before and after test.
The fuselage, I'm wondering if slightly arced fuselage sides could offer any benefits, instead of slab sides.

My Father was able to get some photos of the plane at almost full speed and I was surprised just how much flexing was in the plane.
The leading edge bent up in the middle the verticals flexed in turns and there was cracks that I didn't notice on the ground.
I wouldn't have thought of any of that but none of it could be good for performance or stability.

[BMatthews]

I thought there was a thread a while back on here about flying fast depron flatties and the issues they have that should be dealt with. You may want to try some searching to see if it comes up.

First thing is to test your prop and motor combo to ensure that the pitch speed is up to at least 100 mph. From there it's a case of reducing drag in more ways than just the airfoil but that's a good place to start.

As for the fuselage there's a lot to do before you begin worrying about a fillet. If this is the typical + shaped fuselage profile with side strakes then there's a lot of drag from the corners of the fuselage and the fact that if the fuselage is not flying at a true 0 degrees all the time then the side strakes will be creating a lot of turbulence and thus drag. A proper rounded fuselage is far better in this way.

A fast foamie is possible but it needs to be made in a way that isn't how most flat foamies are made.

[HugePanic]

i build a lot of foam planes years ago. they had very thick (about 1inch) wings with a handmade profile! i just created sth that looks like a profile.

then i cutted a real airfoil for the plane. it was the very simple Clark-Y.
the gliding angle was muchmuch better.

with the old "airfoil" i was not able to fly loops, the AC just didn't pick up speed.
the clark-Y airfoil plane was much better here.

what i learned:
whenever possible, use clark-Y or better!!!!

[MiseryQ]

In my case the plane was not a profile but triangular ^ on top of a flat wing.
The plane wasn't built for speed, I flew it a couple weeks then threw in a big motor and battery just to see how it would fly.
The replacement has the "standard" box fuselage with a curved top and slightly curved sides. It has forward swept wings just for looks, but I've convinced myself, from a bunch of reading, that planform shape had little to do with drag at our mach speeds, i.e. there's no need for swept wings.

From reading the there's dispute over flying at 0°. Some say at high speeds the angle is so small there's little induced drag. Others say that planes fly at a higher AOA than a foil even at those higher speeds, which is what I noticed the plane doing.
So to fly at 0° we're going to need a semi symmetrical airfoil so my thought for today will be if I should only add 1 3mm layer to the top of the flat wing(?).

As a side note the Don, in the Fastest Foam thread, made a foam plane the broke 100mph with what looks like a flat wing with tapered LE and TE and a slab sided tapered rear fuselage.

[BMatthews]

If you just sand in a leading and trailing edge to the depron and cover it with something that'll stiffen up and smoothen out the "airfoil" you'll have a fine wing.

Assuming 6 mm depron sand an elliptical roundness to the leading edge with around a 1/16 to 3/32 inch diameter nose rounding at the front and taper the elliptical shape from that rounding back for about 3/4 to 1 inch. At the trailing edge just cut a smooth taper down to about 1/32 thick over about a 1.5 inch length. One of the pourous materials like polyspan bonded down with water based polyurethane will stiffen up and strengthen the wing nicely.

From there you STILL need to confirm that the prop has enough pitch and is turning at a high enough RPM to have a pitch speed of over 100 mph. Then if the model is clean enough aerodynamically you can expect perhaps 80 to 85 mph in level flight and that it'll pass 100 in a dive and for a while after you pull to level.

[MiseryQ]

I'm working on maximizing the plane I'm building.
The wing has the layered 3mm sheet as I meantioned, It's rounded and tapered and covered.
The extended leading edge is thinned at the LE although left flat. Verticals knife edge too so there's the question. I'll probably round off the LE of the verticals with sanding and leave the TE sharp but what about the horizontal edges? Any reason to shape that are or is flat good enough there.

[coosbaylumber]

Well, over in yet another forum place, this same subject was brought up via the back door route. One fellow there wanted to know that if keeping everything the same materials wise, would changing the wing angle or incidence then affect the flying attitude and staying power. That set off a racket again.

One noted fellow suggested to take the wing, slot it and fill in the gap with something, but in general change the incidence some. Better or worse? Do same with the rear. Better or worse? I figured he must have a near endless supply of models to cut up or basically ruin with experimentation.

Now, to me that seems like a bunch of fiddleing and experimentation, all on the back of one who really does not know. It seems easier to me to have someone analyze the project just a bit, and say what to be done and where. For most of us it flies OK. But, what if it flew a few percent bettter than OK? Add a little up angle to the wing if if tends to drop off too quickly once power is reduced. Or decrease the tail angle if it looks ...


Wm.

[mrblucor]

WOW! I'm hearing an interest in what I'm trying to do. (a few tricks to make the airframe more aerodynamic without going all out) while at the same time, I'm nor hearing practical tried and true advice. I flew my modified F-18 the other day. It is much better. All I did was to taper the trailing edges. The plane use to whistle during a high speed pass. Now it does not. It's a little quicker and quieter. The pitch speed of the power plant is about 80 mph. The reason I started this thread is beacuse I have the same power plant (batt,motor,prop) on a 1/2A chipmunk. That plane screws!! I'd love to make a foam plane that flys that fast. Is making a true airfoil with 6mm on the bottom and 2mm on the top worthwhile? Will it make a big difference? I guess that's the next step. Any suggestions?

JB