SuperViking

My Feedback: (4)

Can somebody show me a good way to cowl up a webra speed 50 on a flying wing such as a diamond dust, holy smoke etc, I looking for the type the control line guys use on there speed planes..................THANKS

Mike Connor

My Feedback: (5)

Here are a few pictures of what can be done. The first one is a speed plane at the German Speed Cup. The second one was built by one of my customers and there is some discussion of how in this thread.
http://www.rcuniverse.com/forum/m_6034143/tm.htm
It is a long thread and it is in the second half.
The third one is one I built for a kit I sell. It is made of contest balsa and then glassed with .75 cloth.

I have a chapter in PDF format on model airplane cowl design and building written by a well know person. PM if you are interested in a link.

SuperViking

My Feedback: (4)

I have been considering building one of your kits, the delta Im building is my own design also, I may do a build thread but Im going to test the first prototype maybe this weekend, Ive always been a fan of highspeed deltas and decided to do one based on the many Ive had in the past, I noticed yours has a box for radio fuel etc, Ive managed to build mine all inside the airfoil itself and still got a 10 oz hayes inside along with radiogear. Im going to be out of touch but I will show you my design, I think you might like it. It will for sure be the fastest Ive owned even with side exhaust standard muffler Speed 50gt. Next version will be cowled up and piped 55gt webra and a reflexed airfoil at they center section transforms to a very fast glider airfoil at the tips. layout is close to holysmoke wing sweep. may end up calling it a holys**t...........................ED

Mike Connor

My Feedback: (5)

My 6% airfoil is just over 1" thick at the root so the optional raised area is needed for the fuel tank to be inside. Not sure how you would get a 10 oz tank in yours but I would like to see pictures if you could post a few. PM sent with cowl link.

da Rock

There is an interesting bit of detail in the Reducing Drag chapters of model design books.

There is an interesting picture of a number of various fuselages and their drag coefficients in the book "R/C MODEL AIRCRAFT DESIGN" by Andy Lennon. One shows a completely streamlined fuselage with an exposed upright engine sticking out in the breeze. It's just the fuselage, no gear, nothing but fuselage and that engine. No prop. And immediately under that is the exact fuselage shape with that engine enclosed in a speed cowl that actually tapers into the fuselage. Which should be a better situation than having the speed cowl standing out like a fin.

One has a Cd of .237 while the other has a Cd of .225

da Rock

The planform view of a streamlined speed cowl usually shows a streamlined shape that is 5 times longer than it is wide.

Almost all the 20% symmetrical airfoils would work well for this application.

Mike Connor

My Feedback: (5)

I would have expected a much larger difference.

da Rock

Speed cowls certainly look like they'd make a huge difference, don't they.

Years ago a buddy and I added one to a very fast C/L rat racer. We were positive we'd get some major speed boost out of it. What we wound up getting that was obvious and significant was fuel economy. It came from the engine running cooler. No lie. The speed increase wasn't even enough that we knew for a fact it was faster. I was recording temp/pressure/humidity at the time and knew the speeds probably weren't worse, but they sure weren't a lot faster. So I figured I'd screwed up the proportions or something and tore the cowl off and did a more exact one. Same deal.

One very obvious improvement was our pit stops. Rat racers have to pit. The pilot shuts the engine off and lands. Pit guy refuels and restarts. You better get a one hit restart or you're toast. With the cowls, every pit was a one-flip restart. And for sure, we were using less fuel. And we think, but couldn't prove, the engines lasted better. Cooling does all that. But speed.......... We were sure the plane was faster, but were absolutely sure it wasn't much.

C/L is absolutely the best testing environment any amateur team could have. And I'd bet big money those Cd numbers are very good indications of what the cowl does for drag.

Strat2003

My Feedback: (1)

I've read that most of the drag on a control line plane is from the lines. Any drag savings from a cowl would be a relatively small percentage of the total system drag, so you would'nt expect a big jump in speed. That same drag reduction on an R/C plane would be a larger percentage of the total drag, so a noticable increase might be expected.

rmh

yes - the lines make big drag- as a kid we tried nylon lines - steel lines braided steel etc..
on 1/2 A - .008 braided were best - -why? lowest drag. On speed the guys adapted line groupers which hopefully placed one line exactly behind the other- reduced deag thru better fineness ratio.
as forthe cowls--
one of those things which theory seems to say -OH YEH- got to have have em
well you did but the real reason was to insure the air was forced around the fins on the engine
some guys even removed almost all th fins and then fitted the pressure cowls.
On my ZDZ160 I made sure the airpath was around/thru all the fins - makes a huge difference in engine performance as it forces flow thru the fins.
On these flying wings-- same thing-

Mike Connor

My Feedback: (5)

NACA testing int the 1920's showed an engine cowl alone could increase the speed by over 10%. You must consider that most of these were radial engines with multiple cylinders. The round cylinder is very bad for drag so it seems that even adding an extension off the rear of the cylinder, giving it an airfoil shape, would increase speed some.

da Rock

Yeah?

In theory, practice and theory are equal. In practice, they are not. [:-]

Think about the idea that the lines limit the speed. The lines at the handle are creating how much drag? Maybe as much as your arm does? But it's not moving fast at all right. So figure how much of the lines actually affect the airplane's speed, and then start reducing that drag exponentially as you work the theoretical calculations back toward your arm. Yeah, the lines create drag, but it's what it is and the rats were going easily faster than the first R/C racers of that era with the same size engines. If I remember correctly, the junker rat we started with did 160 and everybody passed it but the kid flying the 1/2A stunter. Don't let interesting thoughts that really aren't applicable confuse the issue. Trust me, when we found significant drag reduction we saw measurable improvements.

BTW, keep in mind that the rats didn't really have to deal with the centrifugal forces the R/C racers do. The rat wing never has to contribute drag (lift) like an R/C plane does at least twice a lap and over half it's flight envelope. The rat flight envelope doesn't actually see anything like induced ANYTHING except when it's coming in to land. So let's not try to compare apples with watermellons.

BTW, you might think I'm making fun of your argument. Sorry if you do. I'm not. But it would be good to take it in parts and look closely at it since it seems to refute an earlier suggestion about using C/L observations.
It really wasn't written that most of the drag on a control line plane was from the lines. Actually only the lines out near the plane contribute the drag contributed by the lines. And in actual practice, if you look at the tail rigging on some of the 46size ARFs today that have those wires and such screwed between the vertical fin and the stabs and the stabs and the lower fuselage, think about how much wire and crap that is. And it's out at the airplane, so to speak. Whereas the control lines start at the inside wingtip and head into the center of the circle. And on a rat racer, they were .018 lines and exited the tip one directly in front of the other.

Also, the rat racer wing really had nothing to do compared to an R/C airplane if you considered the L/D deal. The rat flies at basically the zero lift angle. It really doesn't have to carry much weight at all to begin with. And after half a lap it's almost at top speed and has the rest of the time to run almost perfectly level. Even when you have a race winner on the end of your lines, you don't actually change the pitch of the rat more than miniscule bits of degrees to get over the slower plane and then your rat is DIVING..... So don't worry about the induced drag from the rat wing doing any appreciable lifting.

And the rat doesn't even have to carry but half of the weight of the lines. And the lines don't weigh much to begin with. And if the rat pilot was like me, he was "whipping" like a mad man and that unloads all the way to the rat's outboard wingtip.

Actual observation shows that the lines do contribute drag, but nothing really significant or at least more significant than anything else on the airplane. And it also showed that cowling an engine probably produces more benefit from the cooling than from the drag reduction. With C/L it's fairly easy to quantify. With R/C it'd probably be nearly impossible for weekend modelers.

da Rock

Like you noted, the cowl study was for what? a cowl around 9 cylinders? Back in the 20s the cowl studies only dealt with one row also. But the engine still had a frontal area that would have been your delta's frontal area time what? seven?

They studied engines that completely blanketed the front of the airplane, and got 10% improvement? Sounds about right. Divide that by 7.

BTW, the numbers that seem to be in doubt here were actually worked out from NACA studies.

But yeah, the cowling on the Brewster Buffalo or the P-Shooter did wonders for the speed of those old flying barrels. But it really isn't sensible to expect 10% out of cowling just one cylinder that's already half shrouded.

In this case I'd say that you really can't compare rats to barrels.

da Rock

Actually the speed guys simply used one control line. Just one. Called it monoline and it never gave the problems the groupers gave.

Yeah, just one line.

rmh

I should not have even mentioned lines --but groupers were NOT a precurser of the single monoline - those things twisted to operate a dingus in the fuselage
different ballgame.
I did fly some class A profile scale speed - no groupers allowed. long ago -
those housings shown on some engines above.--- add frontal area. and on speed stuff-- that shows up

HighPlains

My Feedback: (1)

A 5:1 ratio (20%) for the cowl is too long for maximum drag reduction. A fineness ratio of 3.33:1 (30%) has less drag.

Control line speed uses line size to control speed. When engine and airplane improvements reach the speed trigger point, the line size is increased in the next rule cycle. It's been done this way for decades.

Cooling drag on full scale aircraft is significant, generally 10% or more. The theory is to slow the air down inside the cowl (which is done with expansion) so the air has a chance to gain a greater delta T. Then the heated air needs to be accelerated to the free air stream with as little turbulance as possible.

iron eagel

"... Then the heated air needs to be accelerated to the free air stream with as little turbulance as possible."

If I may ask what do you think is the best way to do this?
I subscribe to the 1:1.4 ratio of intake to exhaust. But lately I am thinking that how and where the exit air is blended with the airflow around the plane is just as important.
Do I need the lower air stream to stay attached to the rear of the fuselage? I want to accelerate the heated air, by using the step concept used in float design. By that I mean about venting your hot high temp/pressure air into a high pressure area by creating a relative low pressure zone in a high pressure area, do you think there is any merit to this approach?

da Rock

Line sizes have been increased over the years for a simpler reason. When the speed goes up, the centrifugal force increased the load on the lines. They've increased the line sizes every time they felt the lines needed to be increased to keep the model going around in the circle. They increase the line size to make the lines STRONGER. And they basically do that about the same time they increase the PULL TEST weights. Which is often what suggests to them the line sizes need to be increased. And this isn't just my guess on why it was done. I've worked in the process and with the Control Line Contest Board. During that time, there was never any thought at all to limit speed, only to keep safety abreast of the speeds. Everyone considered very seriously keeping interest in speed alive, and a major consideration was to allow speeds to continually increase if possible. Jeezus, why do you think we started calling for the center posts? They were a huge effort for most flyers, but they were necessary if the speeds were to increase. To think that the AMA was trying to limit the speeds is just simply BS. BTW, you do know about the speed posts, don't you? They were a pipe sunk in the ground at the center of the circle. They had a swivelling keeper at the top that the flyer had to hook his control handle into. That way the flyer didn't have to be strong enough, and heavy enough, to hold onto the airplane when it started pulling. And even the little ones put a hurting on you with their pull. Get anything up to 200mph or more and it's going to PULL. Those Contest Board guys sweated bullets everytime they considered line size changes. They really, really did not want to affect interest in the sport. And the really had no wish to ever stop the speeds from increasing. That's NASCAR that works that way, not AMA's CLCB guys.

da Rock

Since pressure cowls work on the theory that you give the cooling air no place to go on entry other than into the cooling fins or against whatever it is you want to cool, and then to give it as unrestricted exit path as possible, and then to help coax it out with negative pressure at the exit opening, I certainly would have to guess how you'd do otherwise to "accelerate" it..... if you plan to accelerate you'd have to have something doing the accelerating. Locate the exhaust opening in an area of lower pressure and nature does the work for you and provides "acceleration". More than that?????

That's actually already a part of the pressure cowl theory. Place the exhaust opening where it'll naturally experience negative pressure. Or create the area. As the flaps that originally completely surrounded the Corsair's cowling and opened when needed were.. Or the flap on the back of the P51's radiator bulge. The bulge created an area of negative pressure so the flap didn't need to be very far open until the cooling requirements were heavy, then the flap opened and created even more negative pressure.

But you're right to look at where the hot air comes out for help.

iron eagel

DA
Given that we are talking about a "Speed Cowl" for a Delta airplane in this thread, I have to ask about one consideation.
Given that your exit air is going to create a "seperation bubble" and therby cause induced drag would it not be best to duct the heated air as far to the rear as possibe?
I know that most cowl designs simply dump the air out the sides or rear of the cowl on top of the wing, doesn't that cause a disruption of the airflow over the top of the delta plane form?
Where you have a airplane that has a high induced drag isn't this a problem, that you could reduce by ducting the heated air toward the rear of the plane as much as possible?
Wouldn't the best solution be to have your exit facing the rear of the craft at the tail even if you need to add a bit of weight to duct it?

rmh

speed cowls:
the compromise is to get enough exit flow with out adding excessive drag-
ideally you have a cavity which just clears the cylinder
the inlet size and shape must provide flow around the cylinder -all around with no hot spot left at the rear of the cylinder
So---- the streamlining of the "exit becomes the real task
unless pressure is low enough - you get no real flow thru the cowl
The cowl shapes on old models such as th DeBolt Speedwagons - is pretty good.
a rather short cowl with a tall exit.
Box shaped cowls do nothing to accomplish the real task: make the air hug the engine and have a constant flow
On gasoline industrial engines such as leaf blowers weed cutters - same engineering is used by forcing a flow from the inlet thru the fins .

iron eagel

Dick,
The picture of you baffel setup on your twin clearly illustrates the method of keeping the airflow over and around the engine cylinder. Andy Lennon also stressed this consideration in his book about model airplane desing in the section about cowls. Another point he made is one regarding allowing for an area for the heated air to expand before you send it back out of the aircraft, he mentioned the possibility of using this thermal expansion to creat thrust. But as you said is we need to get the exit flow to work without adding excessive drag, wouldn't a exit at the rear of the airframe serve this purpose?
I am concerned that the hot air exit can cause drag over the rest of the airfram by causing a seperation bubble to occur near the cooling exit and increase the drag caused by the airflow seperating from the wing.

Then on top of that is another consideration, with a high speed aircraft the volume of air moving through the cooling system becomes a consideration. I have notice that many of the exotic speed designs have very small inlets, my understanding is that limits the volume of air moving through the cooling system. The reason for that is as I understand it much akin to the "windchill" aspect that we hear often in weather reports, where the speed of the air enhances the cooling effect.
But to go even one step further when designing your cooling system, shouldn't we take the type of airframe into consideration when we are venting the hot air out?

rmh

The inlet is a strange thing to analyze -
It appears to be a "ram air " inlet - it isn't.
actually -the air behind the prop - at the base of the prop -is somewhat stagnant (?)
What I mean is that it is swirling and moving outward at that point on the prop--so no air is being rammed into the cowl.
The exit air (lowering of pressure in the cowl) is the controlling factor.
a very small inlet does the job as long as the exit air pressure drop is sufficient

you can theorize about this ----but in real world application - -you have to cut n try
the why is because the engine heat will vary -(obviously ) as power output increases
Higher speed will produce more effective low pressure so that can appear to be a self controlling (closed loop setup) - but it ain't.
You have to sneak up on the setup.
the why is that the power -while increasing -tries to increase engine temp - which is not wanted -- so--you have to have the setup matched for the wortst case (most power) scenario.
On the big twin --lots of guys got this wrong -they figured air rammed into the cowl - which is not so -
Here is another pic of that setup which has a very small inlet and a HUGE low pressure setup. on a .50 glow enginbe --this is all far easier as alky fuels cool much better than gasoline fuels. ( fuel to air ratio) The large bowl shaped cowl has as it's exit a semi circle with a lip on the LE to cause lowered pressure. The engine ran very cool under all conditions

iron eagel

"The inlet is a strange thing to analyze -
It appears to be a "ram air " inlet - it isn't.
actually -the air behind the prop - at the base of the prop -is somewhat stagnant (?)
What I mean is that it is swirling and moving outward at that point on the prop--so no air is being rammed into the cowl.
The exit air (lowering of pressure in the cowl) is the controlling factor.
a very small inlet does the job as long as the exit air pressure drop is sufficient"

Dick I really don't understand that...

whereas in FS we were told:
Basic principles of air cooling

"The majority of aircraft piston engine cooling is done by air. The aircraft engine cowling is equipped with air inlets that allow the air to be directed into the engine compartment. On a standard design aircraft (with engine in front) the propeller and the relative wind drive air into the inlets. In cruise, the relative wind does most of the work of pushing air through the inlet. When the aircraft is flying the air entering the inlets is denser due to the ram action of flight. This ambient air creates a high pressure area inside the engine cowling than the static air around the aircraft."

rmh

I clipped out this part of the quote-because --in order for the air to flow thru the fins the pressure in the cowl must be lower not higher than ambient.
A cowl full of high pressure air is a burnt up engine waiting to happen
Cowl flaps on an old radial create a powerful low pressure at the rear of the engines -this is what creates the flow. NOT high pressure blowing air thru the engine. IF there is positive pressure at the inlet it does help but in many cases there is little or no pressure at the base of the prop.