Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Convoluted speed theory

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Old 07-09-2007, 08:24 PM
  #1  
vicman
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Default Convoluted speed theory

I would like to have some input and comments on my latest cowel idea.
This is on a HOB .25 Texan. My thinking is that I don't need the huge scoop for cooling as long as I have increased volume going past the important bits. If I open the area in front of the engine and also have the majority of the exit area focused on the same important bits I should get adaquate, possibly even better cooling than if I had the whole thing opened. The speedy guys in Germany and CL have fully coweled engines and I am thinking this may work here too.

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Old 07-09-2007, 08:51 PM
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Mike Connor
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Default RE: Convoluted speed theory

It looks good and cooling should be more then adequate but I am not sure aerodynamically. you have a lot of flat area closed in on the front that would normally be opened on a radial engine. It seems the guys over in aerodynamics and the speed nuts do not always agree but posting over in the "aerodynamics" forum may get interesting results. If you are looking for looks and cooling then you are probably OK. If you want the 12% speed increase from a properly cowled engine then we will wait for the experts.
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Old 07-09-2007, 08:58 PM
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Default RE: Convoluted speed theory

My thought is your forward opening should be no bigger than the fin area on the head and cyl. And should be ducted or directed right at the motor, look at Andy Lennon's book on design. Also the flat area on your firewall will cause an area of high pressure reducing the amount of flow through the cowl. You want all of the airflow through the cowl to be focused on the engine filling the cowl with air does nothing for cooling any the air that does not go directly past the motor does nothing for cooling at all.
Ask Da rock or Bmatthews in the aerodynamics forum about it.
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Old 07-09-2007, 10:39 PM
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Electric Delta
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Default RE: Convoluted speed theory

Vicman,
here is a picture of the front end of my AT-6 during construction. I set the firewall back around 3/8", so that the cylinder head would be in the middle of the side of the cowl, per the plans (the TT .25 GP is longer than the originally foreseen .15-.19 engines.). My cowl is open in front, per the plans, with just a little extra clearance in front of the carb. I didn't like the flat firewall plate and sharp corners where the cooling air is supposed to exit, so I framed the firewall with balsa leading edge stock facing forward, and made epoxy/cotton-flock fillets between the firewall and these pieces. The idea was to reduce drag by getting the incoming air to attach and flow smoothly following a curve, rather than force a sudden change of direction going around a sharp corner. I have seen the mostly-closed-cowl-idea used successfully as well by some of the other AT-6 flyers, but have my doubts that it is any cleaner aerodynamically than what I have done. Plus, engine cooling might quickly become marginal, especially at those tweaked needle settings we tend to race at. Midairs notwithstanding [&:], I have had some racing success with my AT-6, and submit that mine is at least as fast as anyone else's racing this format. I've been outflown much more often than I've been outrun .

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Old 07-10-2007, 01:03 AM
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Kmot
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Default RE: Convoluted speed theory

Put a big spinner on it to streamline the airflow over the flat part of the cowling. [sm=thumbup.gif]
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Old 07-10-2007, 06:34 AM
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Ed Smith
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Default RE: Convoluted speed theory

The attached pictures show what is commonly called a "Pressure Cowl" The cowl is for an FAI pylon Racer. Engine revs at around 28k+ with a full tuned pipe. The inner shroud is a tight fit around the engine with about 1/8 clearance arouind the pipe. The incoming air is ducted and forced through the cooling fins and over the head and pipe, not around them. This setup is essential for effective and controlled cooling.

There is nothing new about this. Control Line Team Racers and speed fliers have been using this for years. WW 1 aircraft had shrouds around the engines as did the original VW Bug.

Your engine will not over heat as it is basically out in the open but your cowling is pure drag. You would do better to open it up and let the air flow through it.

Ed S
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Old 07-10-2007, 07:11 AM
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vicman
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Default RE: Convoluted speed theory

Thanks guys, I am not looking for the uber set-up just something a little diffrent that won't really hurt anything.
ED, I remember seeing the coping around the firewall on your model. R.I.P.
More of these engines go sour on a run than I care to see so my thinking is to focus on cooling the engine rather than have whole nose scooping air. I am also thinking I will have less drag with the closed area vs. a scoop.
Please continue with the comments. I can always cut some more out.

Tom, look closely you can see the large spinner size.
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Old 07-10-2007, 07:36 AM
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Electric Delta
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Default RE: Convoluted speed theory

Here is another shot of my installation from straight on. You can see I tried to keep the rear of the cowl as open as possible, all the way around, as well. I even used streamlined stand-offs to hold the cowl [:-]. I seem to remember hearing or reading a rule of thumb, that the exit area of a cowl should be larger than the inlet area by 50%, if possible, for good cooling. Otherwise a lot of the hot air gets trapped / flows out of the cowl more slowly.

BTW, it's not R.I.P., just temporarily MIA, she'll fly again. I already have the fuselage repaired, just need to build a complete new wing.
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Old 07-10-2007, 07:47 AM
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Electric Delta
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Default RE: Convoluted speed theory

Here's some shots of how some of the other guys have done their cowls.
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Old 07-10-2007, 07:53 AM
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Default RE: Convoluted speed theory

I couldn't resist providing one last shot of what NOT to do to your AT-6 cowl. Doing this results in an extremely high drag rise and loss of forward speed .
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Old 07-10-2007, 08:14 AM
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Default RE: Convoluted speed theory

That is pleanty of cooling air inlet. Looks good to me.

The main aero drag component involved is frontal area. With the AT-6 design, you will have the same frontal area regardless if the cowl is entirely cut out or just cut enough to allow in engine air. (front area includes the entire front profile of the aircraft)

The second component is (over simplified) induced/turbulent drag. That comes from air having to travel over or around or through features. When air has to change direction, it takes energy to do that.

Also, all air in must be capable of exiting. I believe most of you reading this understand that cooling air exit area has to be larger than air inlet area (fill in your own equation here, I prefer 2x).

Only allowing what is requied for cooling air, and leaving remaining flat cowl blocking the air is better than a huge cowl inlet and turbulient air both hitting a flat firewall and trying to flow through a very disturbed path... then trying to exit the cowl through openings less than the open inlet area of the cowl.

How much better???? Well on the AT-6 it might be in 'the fuzz'. But with that AT-6 particular model, all it takes to find out is a consistant engine, a consistant pair of thumbs, a stop watch...... and two cowls

Avoiding turbulent drag is the reason many race cars use air dams in front, and the designers try to keep those as close to the ground as possible. You would imagine that adding the air dam is an increase in frontal cross section area - and it is. However the benifit is worth the trade-off. It is better to have a flat plate forceing the air to flow over or around the vehicle (somewhat predictable and managable flow path), rather than having that air go under the vehicle where all the unpredictable mess/drag/turbulence can occur.

You will see many scale aircraft (corsair, P-47) where actually using the dummy engines will improve engine cooling buy only allowing cooling air in where the engine/cylinder/muffler are located... only one flow path in. Some have noticed this also tends to decrease aircraft drag too.
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Old 07-10-2007, 09:38 AM
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Default RE: Convoluted speed theory

Everything bob27s says makes sense. Since I need to make a new cowl for my AT-6 anyway, I'll give the mostly-closed-front concept a try, making the inlet area in front of the engine 50-75% of the outlet area. I still like my idea with the firewall treatment, so I won't change that. I'll patch up my old cowl well enough to try for comparison too, and it might be interesting to try a spinner vs. the spinner nut I have been using. Those are easy things to swap around and try at the field. WaveOsScope will tell all!
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Old 07-10-2007, 01:23 PM
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Default RE: Convoluted speed theory

the benifit is worth the trade-off. It is better to have a flat plate forceing the air to flow over or around the vehicle (somewhat predictable and managable flow path), rather than having that air go under the vehicle where all the unpredictable mess/drag/turbulence can occur.
My thoughts precisely! In my circle track days that was a major upgrade. We would form sheetmetal panels around the important parts to increase pressure/ala efficient volume and direct the heat out the back under the car. We also would panel in the underside of the engine compartment to help in this area. More directing less scooping.

Mike, I still need to make an access area for tech to verify the carby and I think I am going to fly it the way it is. On the test bench the engine would come into a rich setting after about two minutes and pick up 3-500 rpm. I have plenty of practice time for the next race.
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Old 07-10-2007, 02:14 PM
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Default RE: Convoluted speed theory

I guess my thinking up to now was that exit area = cooling. With a larger front opening, you have a positive pressure situation vs. ambient inside the cowl, so no more air enters the cowl than is exiting, or in other words the excess oncoming air would just spill around the outside of the cowl anyway, just as it would with the front of the cowl mostly closed. I do see the point of guiding the air around with the cowl fron mostly closed though, vs. leaving it to chance what path the air takes.

Vic, when we inspect engines, the cowl must be removed completely, so there is no need to increase access to the carby with a bigger hole in the cowl.
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Old 07-10-2007, 02:35 PM
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Default RE: Convoluted speed theory

The mods to your fuse/firewall you will make the air move out of the cowl faster, this as important as getting the air in. Keep in mind only the air which moves over the motor provides cooling.
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Old 07-10-2007, 02:47 PM
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Default RE: Convoluted speed theory

If you have more entry area that exit area you will slow down the air inside the cowl and greatly reduce the cooling efficiency. Like has been said. You only need a large enough opening to allow enough air to cool the engine and it should be directly in front of the portion of engine you want to cool. The exit can really be anywhere in the back of the cowl as long as it is at least 2 times the size of the inlet.

Blessings, Terry
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Old 07-10-2007, 02:50 PM
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Default RE: Convoluted speed theory

Actually the exit area only need be 1.4 times the entry area to work well, according to Andy Lennon.
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Old 07-10-2007, 06:12 PM
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Default RE: Convoluted speed theory

Vic, your plane looks good.....get one of those $45 How Fast things and do some R&D for us! The comparison to how 1/4 mile racers block the air openings on the front of the car is good, but don't forget that those guys are the same ones who would try to get a jacked up '55 to do a 9 second pass with the aerodynamics of a barn door.
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Old 07-11-2007, 09:20 AM
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Default RE: Convoluted speed theory

(friggin message time out again !!)

Ok... this is sort of on topic.
Back when I was an engineering student, I reviewed a case of an inventor back in the 1920's who set up a sedan to drive backwards. His wind tunnel data and practicle tests indicated the car had less drag configured like that, and demonstrated better fuel milage.

This is somewhat related to what I noted, so presented as interest. Driving either direction, the vehicle presents the same "Frontal cross section area". So drag comes down to surface features and air flow energy.

Driving "backawards", the blunt end of the vehicle creates a stagnation point - a sort of virtual leading edge. Yes it creates a drag force, because it requires energy to cause airflow to change direction. The stagnation point forces the airflow around the vehicle fairly evenly - where it stays pretty much attached along the flat surfaces. The engine section actually works as a boat-tail to help re-attach airflow as it exits at the "trailing edge". Driving the "forward" direction, oncoming air is immediately met with numerous surfaces, stag points, edges, corners - creating multiple flow paths. Every change in direction requires energy (drag). The radiator inlet is a killer too. As air exits the vehicle, the flow is turbulent since it essentially drops off a sharp edge.

There was a video/film associated with that experiement. Maybe someone can find it out on youtube or somewhere.

Clearly, todays vehicles are designed to be most efficient driving "forward".

Bob
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Old 07-11-2007, 09:22 AM
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Default RE: Convoluted speed theory

CP, I am really thinking about getting that thing. Maybe a few of them. It will be a week or so before I can let you guys know how it works (stupid work travel thing). The idea came to me as I was running the engine in on the test stand and how after a minute the motor really came into song on it's own. Seemingly after the engine got a bit warm. Nice big plume of smoke coming out the pipe so I was confident I wasn't getting too hot. When I went back to idle and let it set on idle for a minute or two I could touch the muffler without it being too hot so I figure it is set pretty close to hoyle.

Mike, what rpm is your engine running?
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Old 07-11-2007, 09:55 PM
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Default RE: Convoluted speed theory

What kind of nose gear will it have?[sm=lol.gif]
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Old 07-11-2007, 11:21 PM
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Default RE: Convoluted speed theory

If I fly it backwards like Bob suggests it will be fixed with the vertical actuator... non-retractable Einstien.
I have three F-20's eagerly awaiting your retractable nosegear point of brilliance sir!

The intellectual content of this post is soley for the retired Army aviator. [8D]
Feel the love bro

I am feeling pretty good about my decision at the moment. If for some odd reason I am handed my rudder when I put it up with my fellow racers I am only a dremel tool away from a normal cowel.

Mike, I was going to pop a hole in the top to allow for tech.
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Old 07-11-2007, 11:41 PM
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Default RE: Convoluted speed theory

around 13,000-13,500 on the ground. WaveOsScope puts the level flight speed in the low 80 mph range.
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Old 07-12-2007, 07:18 AM
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Cyclic Hardover
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Default RE: Convoluted speed theory


ORIGINAL: vicman

If I fly it backwards like Bob suggests it will be fixed with the vertical actuator... non-retractable Einstien.
I have three F-20's eagerly awaiting your retractable nosegear point of brilliance sir!

The intellectual content of this post is soley for the retired Army aviator. [8D]
Feel the love bro

I am feeling pretty good about my decision at the moment. If for some odd reason I am handed my rudder when I put it up with my fellow racers I am only a dremel tool away from a normal cowel.

Mike, I was going to pop a hole in the top to allow for tech.

Well, my***** is off to NH for a week tonight on the old Redeye
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Old 07-12-2007, 07:55 PM
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Default RE: Convoluted speed theory

Thanks Mike,
That is about where it was on the stand.
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