engine cowl
 

for drag and cooling issues, should I close up part of the area behind the cylinder head? If so, how much of a opening is needed for cooling and should the opening be lower or even with the cylinder head?
Any thoughts from the experts? :D
Thanks

RE: engine cowl
 

Mike,

Using Andy Lennons' method for cooling a cowled engine the inlet for cooling should be equal to the finned area of the cyl and head, the outlet should be 140% of the inlet size. I would look at some of the speed fliers airplanes to get an idea of what the do for their designs, if I recall you are working on a very fast airplane, and there methods may serve you better. Although I think that the method that Andy describes in his book might work better and be more aerodynamic, the weight involved with enclosing an engine entirely may be a problem for you in your application.

One bit of advice any square angles should be avoided this will cause drag, round everything as much as is possible inside and out to get the cleanest flow pattern, in the engine compartment as well as the exterior. I would also "Round Out" the front of your cowl as well going to a teardrop rear configuration using a NACA type of scoop for the exhaust. I would also look at enclosing the fuel lines and needle valves as well, the bulk of the cowl to cover these will be made up for by the cleaner flow over your fuse and wings.

And I am no expert by any means I use the swag method a lot!

Regards
Paul

RE: engine cowl
 

If your goal is to have the least drag for the fastest possible airplane, you need to look at some of the CL speed airplanes.

The lack of streamlining behind the engine in your mockup is going to be very draggy. And with the front profile, that rear area isn't going to be doing much cooling. Your flat front isn't really doing a lot for you either. There is a lot of just plain flat plate up there. And your cool air opening really isn't large enough.

Speed guys learned long ago to completely enclose the engine and then control the intake area and heat exhaust areas to perfect the cooling. They discovered they could cause the engines to run too cool that way. Or too hot. But either way, the complete coverage was the way to go fast.

They also found that they could reduce their intake size with higher speeds. Of course, they didn't have to taxi out for takeoff. So they often had less cool air opening than the area equal to the fins and head. But it almost always was an opening that went from the carb intake to the top of the head. The width was what was usually tuned in size. And the exhaust area would be what was quoted from Andy's book.

RE: engine cowl
 

Thanks for your reply Paul. I wasn't going to have a engine cowl to test my new airframe but thought I could use the practice making one and I was right. My attempt to use tri stock to allow sanding round corners was only partially successful. My next attempt will use contest balsa for rounding and glass for strength. I understand avoiding 90* angles. The German speed cup planes use very rounded cowls but it seems the front opening is less then the finned area unless the head is not included since it is not covered. The head fins are above the cowl on mine. I am testing the airframe with a TT 46 Pro and will move to a speed engine and new cowl if things work well.

RE: engine cowl
 

da Rock,
Are you saying my cowl doesnt look like the attached? :D The opening on this cowl looks very small to me.
It seems I could build the plane in the the time it will take to get the cowl right.

RE: engine cowl
 

Mike,
DA Rock made some important points about what you want to do and how to do it.

It took me 3 months to get the cowl I wanted just right for my 21 century cub design that I am working on. One of the biggest issues I had was getting the exhaust portion of the cooling system nailed down. It required coming up with a way to duct the hot air out of the engine compartment through the fuselage and venting it in a low pressure area without adding to the drag or frontal area of the aircraft. All told it was about six months for the whole system to be developed, I still have not finished up on the plane, as flying and other projects have cropped up. All told this plane has been three years in the making but the end result will be worth it.
As far as your question about the small intake I think that is due to the high speed, and the fact that they leave the head exposed. I am just guessing about that but it seems to be a logical explanation.
I would suggest that you read Andy Lennons' book on designing and building RC aircraft. He has an entire chapter devoted to cowls alone, if it was that important to him I would say it is an important design consideration to all of us. In it he mentions how the British Hurricane use the heat produced in cooling to add to the thrust of the aircraft. His setup for cowled engines are set up using the principals of a jet or rocket motor to use the heat produced as additional thrust. While in a model the added thrust may not be all that much but it does make sense to use everything you can to enhance performance.

I will see if I can scan that chapter in for you tomorrow night so you can read it.

Regards
Paul

RE: engine cowl
 

Mike, the opening needs to be partly based on how fast the model will be flying as well.

Also if you look at typical speed model cowls they are actually tightly fitted around the fins except for the air inlet at the front and a narrow outlet at the rear. The idea being for the cowl to force the air to travel all the way around the cyinder and keep it forced to flow between the fins until it gets to the very back and only then can it flow back and out through the exit slot.

Also most of the heat, other than the exhaust gas, exits our engines through the head. This is why the head fins are generally exposed where their longitudinal fins can see a lot of airflow.

RE: engine cowl
 

Paul, That would be great. I will make another attempt at a cowl and would love to read it.

Bruce, Thanks for your input. I had a feeling I would learn a lot from you guys.

RE: engine cowl
 

What part?

Yes, your T shaped opening is T shaped. :eek:

But a cowl is all the stuff from front to back and bottom to top. I'm going to take your :D to suggest humor and answer the question about the opening size.

The picture of the speed plane is just that, a picture of an airplane that's evolved (important point: evolved) to fly in a somewhat narrow speed range. BALLS2DAWALL speed range, but still fairly narrow and very predictable. So the builder had a real good handle on how fast the airflow was coming into that T shaped opening. And he also had the luxury to whittle or fill. And that little opening had a really important secret. It fed a closed tube that tightly surrounded the engine AND MOST IMPORTANTLY that ended at an opening that was in negative pressure. The T you see in the picture was actually only half the reason the engine got cooling air, and probably not the most important reason. The streamlined cowl behind the engine and it's opening was doing a TON of the work to cool the engine.

BTW, the cowl you see in the picture is one of the easiest scratch built structured you can ever hope to do. Wrap the engine with a couple layers of masking tape to give the appropriately tight clearance and two sides of wood "pin" right up on either side. Cut a hole in the top plate of wood to fit the engine head and cap those two sides with that plate. You're almost done. Block sanding some, hand sand a little more, and you are.

I'm not trying to insult you but I gotta suggest that what you've got probably isn't going to streamline that airplane. And from what I've seen, it could easily cause your engine to run hotter than it would without it. It just might have more area than the engine has and actually is shielding a good part of the engine. And then actually keeping air away from the back of the engine. Sorry to be blunt, but I don't know any easier way to say it.

BTW, what is it going on?

RE: engine cowl
 

Yes humor, I needed a little after seeing my first attempt at a cowl. :D I am here because I need suggestions to do this right so blunt is fine and I am not insulted. I will build the cowl around the engine and not the airplane next time. Could you elaborate on "...and two sides of wood "pin" right up on either side... " please? I was thinking about using contest balsa to round the cowl. What are you suggesting?
Thanks for your reply

RE: engine cowl
 

Mike,
Sorry for the delay in getting that chapter from the book scanned in, my scanner for some reason is being a bit of a pia. I bring the book into work next week and scan it in there, I can also make it as a .pdf that will make the file size a lot smaller. I think what Da was saying that the actual distance you should have for the spacing from the engine to the inside of the cowl is about 1/32 of an inch max, the thickness of a t pin. You want the air to flow around the engine, not through the cowl, and as he mentioned how it leaves the airplane as important as how it get into it. Also the shape must blend the airflow back around the fuse so the back of the cowl is as important as the front.

RE: engine cowl
 

Paul,
I appreciate what you are doing and PDF or any format if fine. While I don't anticipate competing in the German Speed Cup 2008, I would like my cowl to help more then it hurts performance. I have been looking at a lot of pictures and look forward to the article.

RE: engine cowl
 

Mike,
I got the stuff scanned in however after I did the ftp my isp somehow dumped the file, perhaps I went over my space (it came out over 100mb) . I'll put it on a cd and bring it home tomorrow so I can ship it to you it is six pages at around 25-30 mb each.

Regards
Paul

RE: engine cowl
 

As long as you worry about inlet -NOT the oulet path - you are missing the boat .
No air is forced in the cowl---
none
zero
nada
A DeBolt speedmodel from 1950 will show you what you really need
as will a quick look at 1930/ 1940's inline engine o rpresent full scale racers .
as long as there is a good setup for the high speed air flowing over the fuselage, to extract air aft the engine - it will cool.

RE: engine cowl
 

Paul,
I do appreciate this. Looking back it might have been easier for me to buy the book but it is too late to turn back now. I have searched for information regarding aircraft cowls on the Internet with little luck in finding details. There was more info it this thread then I could find out there other then a few pictures. This info should be very useful for gaining speed and not burning up an engine.
Thanks

RE: engine cowl
 

Dick,
The inlets on that picture are very small, similar to the small size used by the German speed nuts. The majority of pictures I have run into show an inlet much smaller then the fined area. Even some of the 1920 radial planes had a modest size inlet. As far as outlets they have gone all the way around a radial engine, on the bottom of the cowl of a full size Cessna or on top of the wing on many speed models. It seems if you are looking for low pressure you would find it on top of the wing.
Thanks for your reply

RE: engine cowl
 

Mike,
No problem....

I will get that file out tomorrow evening for you. If you are serious about doing design I would suggest that you get Andy's book, it is a great resource and has many tips and examples well worth the $20.

RE: engine cowl
 

Mike,
Two details to consider.

The generally accepted rule for our models is to size your exhaust about 1.4 times larger than the intake. It's supported by people like Andy Lennon who wrote the popular book, "R/C Model Aircraft Design" and by lots of experienced modelers who know it works.

Also, there is a group of modelers who've proven intake and exhaust sizing for a few engine sizes. They're the guys who've been flying R/C Pylon. The event demands about the same engine performance you're probably wanting. Pictures of their airplanes ought to be fairly easy to find. Those ought to give you an idea what intake size can work at their speeds with racing engines running nitro fuel.

Good luck.

RE: engine cowl
 

remember -- it ain't the size which matters - it's knowing where to put it ----
frankly the size thing on cooling is such a vague thing - for getting the job done right - I never use it.
the attached photo shows another setup which worked .
essentially the new upper inlets flow , allowed a shorter path around rear of cylinder
as you might guess, the rear of the cyl is the "hot spot"
On the second setup --the scale inlets get blanked off.
on last picture --the inles getblanked - note the duct aft the muffler - it creates a low pressure in the cowl abofe th muffler - this is again -to reduce heat at rear of cylinder .
lots of techniques --
moving the prop as far forward as possible from the cowl, is another good idea.
it is a full scale practice - not in any model airplane books --yet

RE: engine cowl
 

The 1.4 times rule is a rule of thumb. Of course it only works when the inlet area is the right size for this rule to work. A larger inlet might work well with a smaller outlet, and a smaller inlet might work with a larger outlet. The speed of the aircraft matters also. For example with full scale planes fast planes will have small inlets with large outlets and cowl flaps for low speed. This works well but the problem with a small inlet is getting the fresh air to diffuse over the entire engine and not pass most of it over a small area, or short circut the engine altogether. Using a large plenum area over the engine with the tight baffles usually works to prevent this. Because of this the lower speed planes like C 172's will usually have larger inlet areas of the smaller planes.

Because our models have less extremes in their flight envelope, that is there are few really fast planes. The rule of thumb should work well with the right inlet area.

RE: engine cowl
 

--in the right place-

RE: engine cowl
 

Yes, right size in the right place.

RE: engine cowl
 

Lots of good info here and it seems the right size in the right place is the key. However, form all the pictures, there is more then one way to get the job done. Here is a picture of my latest attempt and it may get mods depending on what Andy lennon's cowl chapter says or what you guys say.

I used contest balsa and wrapped it around the front half of the cyl. There is a medium size opening in the front with a height of the cooling fins. The cyl head is partially exposed and the tapered outlet to the rear is at least 1.4 times the fin area. This cowl design when finished will be tested on a inexpensive engine before going on a speed engine.

RE: engine cowl
 

With sport models the size intake isn't a big deal if it's large enough.

With speed models the size does matter for two reasons. Those reasons take on the same importance as your desire for the ultimate top speed is important.

One of the two is that if the intake is too small your engine is going to run hot. Who'd'a thunk it.
The other is if it's too large your engine might actually run too cool for best performance and the airplane won't be as fast as it could be because of the too big hole.

RE: engine cowl
 

If the speckled grey stuff is your cowl you need to consider streamlining the area around the carb. Streamlining is not just putting a slick surface on the shapes you've got to enclose. What you've got down around the carb isn't really going to do much of anything in the way of streamlining that area of the airplane. You've still got basically a large flat area almost perpendicular to the airflow. Look back at that yellow speed plane's entire front end.