Engine cooling on pushers

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Eugene

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Side benefit from my airframe movement = that BRS went back by 2 inches and down by 2.5 inches. Looking into the future I am brainstorming about engine cowling. Wondering if I should copy Seabird Seeker or icon A5 cooling intake?

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Eugene

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This is apparently most efficient airplane on the planet. And air intake almost directly attached to the fuselage without a space in between.
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wsimpso1

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I like the splitter designs (your first drawing, N381GC, and VH-SUA).

The inlets that go to the next surface down swallow the boundary layer, which is significantly slower flow and forces a bigger opening to get better cooling. Plus the splitter type look better to my eye.

We can not see the outlet on VH-SUA, but the outlets from N381GC look way oversize. You only need to be inlet size enlarged by the ratio Tout/Tin - using absolute temperatures. When you make the outlets a lot bigger than they need to be in a pusher, the outlet flow is slowed way down over that section while being tossed into the prop. Ideal function is for all of the prop to see the same inflow speed, so ideally you want cooling outflow back up to whatever the speed is on the rest of inflow to the prop.
 

Eugene

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I like the splitter designs (your first drawing, N381GC, and VH-SUA).
Thank you! What that means for me is that I need to find a way for whole splitter air intake to be able to fly away with parachute on deployment. In other words needs to be removable.

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If I copy Icon A5 approach, I can simply leave that area open and build around it, like on picture below. Wondering if we are talking about big difference at 100 miles an hour?

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rv6ejguy

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None of the Rotax installations pictured are well designed from a cooling drag perspective as they break almost every rule in the book about momentum recovery. If you want something that works, you can copy aspects of them. If you want it to work with low drag you need something a lot different than these.

Almost certainly the internal duct on N818WM doesn't look as depicted. These are feeding both rads and intercoolers and would first diverge the air aft of the inlets to meet the HXs and then re-converge the air just prior to the variable geometry exits.
 

Eugene

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they break almost every rule in the book
I wonder why people do that? Do they believe that books have been written by stupid people? Just to make money?

My real concern is BRS. Before they give me installation approval letter, guys from BRS requested to remove larger zip ties from rocket and installed only three much smaller ones. Apparently the rocket during deployment doesn't have much push to break away any additional material. That is why I am concerned about it.

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pylon500

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Pusher installations are a little different to tractor ones, and the shape of the inlet is not overly important, about the only consideration is that the air going in is fairly 'clean' aerodynamically.
To this end the separated inlet on the Seabird is probably better than the flush scoop on the Icon.
More important is the exit, and on a pusher life is a little easier, all you have to do is make sure the rear opening is as close to the propellor as can be safe (around an inch clearance is all), this will ensure the prop will act as a fan and extract the cooling air as best as possible.
 

Vigilant1

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A simple annular fan attached to the shaft moves air more efficiently than the pulsed passage of a prop, and the high "solidity" of a fan is better for generating a pressure differential across the opening. This might be especially important for a slow, draggy pusher where we can't count on a lot of dynamic pressure at the inlet during TO/ climb or even in cruise and the engine might be operating at relatively high power levels (a lot of heat).
The rear engine of the Cessna 336 and 337/USAF O-2 uses an annular fan for cooling. The fan has 20 blades with tip angles of 25 degrees, and draws just 2 hp to cool the 210 HP engine (so, less than 1%). As a by-product, an installation like this can clean up the aerodynamics of the rear cowling quite a bit.

(I was surprised that I couldn't easily find a picture of a Skymaster with the rear cowling off, clearly showing the fan. It is NOT rare to have the cowlings/panels open on these planes. :) )
Seosan South Korea September 13 2020 Stock Photo (Edit Now) 1826587778
 
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Eugene

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More important is the exit, and on a pusher life is a little easier, all you have to do is make sure the rear opening is as close to the propellor as can be safe (around an inch clearance is all), this will ensure the prop will act as a fan and extract the cooling air as best as possible.
I really like this logic. Thank you!
 

BBerson

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I believe fan was chosen because of low speed taxiing on the water.
Low speed taxi is at idle. The pusher suffers from low pressure in an extended climb. Needs about 100 mph for 4" water pressure to cool an air cooled engine. Hard to climb at 60 and get 4" pressure below 100 mph. Might be why Skyboy is not cowled.
 

wsimpso1

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My real concern is BRS. Before they give me installation approval letter, guys from BRS requested to remove larger zip ties from rocket and installed only three much smaller ones. Apparently the rocket during deployment doesn't have much push to break away any additional material. That is why I am concerned about it.
It is common in many ballistic parachute applications to fire through a port with a cover that is supposed to be broken free. Look at the Cirrus install and the retrofits on a variety of certificated ships.

When the rocket first fires, there is no speed on the package, just the rocket motor pushing. Even a modest extra force could stop the parade. Put a panel a few inches above the ejecta and now the ejecta has momentum and kinetic energy that can pop a fragile line of adhesive or blow through a deliberately brittle panel. Arrange it so that only a narrow amount of that frangible line is being broken at one moment during the progression, and then the join unzips with pretty low forces. BRS must have some guidance on the topic for you.

Billski
 

Eugene

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Might be why Skyboy is not cowled.
To design and build engine cowling was additional effort and money that nobody wanted to spend at the time.

But most importantly nobody really believed that and this particular aircraft engine cowling will make any difference at all. At one time I was talking to Searay designer and he insisted that engine cowling on his airplane doesn't make a bit of a difference. Only looks!!! Some Russians ,that I talked to, did tell me same thing as well.

Bill is offering to us different explanation of how open engine is actually dividing one uniform wing into two separate parts with four wing tips. Efficiency of that wing is deteriorated to the point that needed in cruise to have 4 to 6° angle of attack at 85 MPH. Someday I will find out if this theoretical explanation is correct or not.
 

Pops

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A simple annular fan attached to the shaft moves air more efficiently than the pulsed passage of a prop, and the high "solidity" of a fan is better for generating a pressure differential across the opening. This might be especially important for a slow, draggy pusher where we can't count on a lot of dynamic pressure at the inlet during TO/ climb or even in cruise and the engine might be operating at relatively high power levels (a lot of heat).
The rear engine of the Cessna 336 and 337/USAF O-2 uses an annular fan for cooling. The fan has 20 blades with tip angles of 25 degrees, and draws just 2 hp to cool the 210 HP engine (so, less than 1%). As a by-product, an installation like this can clean up the aerodynamics of the rear cowling quite a bit.

(I was surprised that I couldn't easily find a picture of a Skymaster with the rear cowling off, clearly showing the fan. It is NOT rare to have the cowlings/panels open on these planes. :) )
Seosan South Korea September 13 2020 Stock Photo (Edit Now) 1826587778
True-- Helped a friend maintain his 1966 337. I got to fly it some, the airplane is OK , but the rear engine lives a rough life, and the Cont 360 is not my favorite engine.
 

Eugene

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I believe at one time I was reading about this kind of explanation. I am trying to show this on picture below. Apparently air scoop that visually represents a big bump to the airflow not going to act like a big bump at all. Apparently if you control amount of air with outlet on your cooling system this packet in front of intake will be filled with turbulent air and only a little bit of that air will go into the radiator. The message was, that you don’t have to worry about filling the packet, it would be filled naturally. Wondering if this is correct or not.
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Aesquire

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If you look at the inlets on the fat teardrop that promises a lot... There are scuppers to cut away the slow turbulent air next to the fuselage, much like the P-51, and every jet that's decent.
 
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