Pusher cowling ideas

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

Eugene

Well-Known Member
Lifetime Supporter
Joined
May 26, 2017
Messages
1,787
Location
Merrill, Wisconsin, USA
So, obviously cylinders on 912 engine sticking out too far. Peter Garrison was explaining at one time that cross-section of every part on aircraft should look like airfoil if possible. What would be wrong with building bump-outs for cylinder heads and finishing up as airfoil in front of the propeller maybe even with a little opening for cooling air to escape?

I need advice from somebody who "can see air" because I can't.

tempImageg9T8OB.pngtempImageU07lRu.pngtempImagefyrgh4.png
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,979
Location
US
I need advice from somebody who "can see air" because I can't.
I can't, either.
Thoughts:
- With your engine now enclosed and your induction air being taken from within that area of warm air, you will lose some engine performance (and reduce the risk of carb icing somewhat.)
- Since the new nacelle will be open at the back anyway to make room for your muffler and exhaust, that seems like the best place to have the air exit. It will help cool the muffler, it is a place that looks like it will have low pressure, the air will already be turbulent there, and it is easy. I'd just make one smooth lump over each side to hold the valve covers, no vents on them. It might even be better to make the whole nacelle a little wider and skip the separate bumps on the side. Puting air exits on the bumps will increase drag in a place that might otherwise have pretty good flow.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,979
Location
US
Eugene, you may also need to pay some attention to other areas that could be harmed by engine heat when you reduce the amount of air swirling around the engine with this nacelle. For example, if your wing fabric is close to your exhaust headers, it surely gets a lot of radiant heat from them. Maybe that was fine when gobs of air at ambient temps was swirling everywhere to take heat away, but the fabric could be damaged with less airflow (and all of it warmer).
 

Eugene

Well-Known Member
Lifetime Supporter
Joined
May 26, 2017
Messages
1,787
Location
Merrill, Wisconsin, USA
I can't, either.
Thoughts:
- With your engine now enclosed and your induction air being taken from within that area of warm air, you will lose some engine performance (and reduce the risk of carb icing somewhat.)
- Since the new nacelle will be open at the back anyway to make room for your muffler and exhaust, that seems like the best place to have the air exit. It will help cool the muffler, it is a place that looks like it will have low pressure, the air will already be turbulent there, and it is easy. I'd just make one smooth lump over each side to hold the valve covers, no vents on them. It might even be better to make the whole nacelle a little wider and skip the separate bumps on the side. Puting air exits on the bumps will increase drag in a place that might otherwise have pretty good flow.
This is what I think. And I'm sure it's incorrect. But sounds good to me at this point.

Engine cowling will be stationary part, but engine itself would be moving part by nature because has rubber mounts. I need to provide enough clearance from muffler to the cowling itself. And that clearance will naturally serve as my exit duct for cooling air. Muffler is simply positioned to close to the propeller to do anything else. Sort of like on picture below from Russian airplane.

Not sure if my sketch of bigger call cowling with no bumps out look good. Looks too fat to me. But what do I know.

Screen Shot 2021-07-12 at 20.36.09.pngtempImagebj88YW.png
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,979
Location
US
This is what I think. And I'm sure it's incorrect. But sounds good to me at this point.

Engine cowling will be stationary part, but engine itself would be moving part by nature because has rubber mounts. I need to provide enough clearance from muffler to the cowling itself. And that clearance will naturally serve as my exit duct for cooling air. Muffler is simply positioned to close to the propeller to do anything else. Sort of like on picture below from Russian airplane.

Not sure if my sketch of bigger call cowling with no bumps out look good. Looks too fat to me. But what do I know.

View attachment 113037View attachment 113038
Hmmm. I have concerns the air won't smoothly make that turn and that after the widest point it will be turbulent (especially as it is coming together in three dimensions).
There are rules of thumb for this stuff. A scale model might also give some ideas.
The bumps did look cooler!
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,979
Location
US
Those bumps can be finished as two little wings maybe with minimum drag after all?
It would be hard to figure out the optimum shape (cartop models with tufts? Measure relative drag with a scale or spring and camera?). I suspect a generally smooth cowling will be better than it was, and that "with 'wings" or 'without wings" won't make a difference you can measure or feel. So, doing something that looks good will pay off every time you see it.
 

plncraze

Well-Known Member
HBA Supporter
Joined
May 11, 2006
Messages
2,130
Look at the Piaggio Avanti pictures you have. That should give some guidance. You can find how other homebuilders have protected their cowlingss from heat. Try not to dump turbulent air into the prop. That is difficult to impossible.
 
Last edited:

Appowner

Well-Known Member
Joined
Mar 30, 2021
Messages
72
I too vote for the bumps however, do you intend to baffle things to guide the air? Typical is to basically split the space around the engine top and bottom. And the air flow comes in typically on top and passes down through the cylinders and heads and exits out below. That said you may want to adjust those bumps so their exit is below the engine center line.

Just a thought, I could be way off.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,979
Location
US
I too vote for the bumps however, do you intend to baffle things to guide the air? Typical is to basically split the space around the engine top and bottom. And the air flow comes in typically on top and passes down through the cylinders and heads and exits out below. That said you may want to adjust those bumps so their exit is below the engine center line.
Eugene's engine is liquid cooled, so there's not much need to guide a lot of air through the heads and cylinders (as we'd have to do with a Lycoming, VW, etc). The vast majority of the heat will get into the cooling air via the radiators (for oil and coolant) at the front of his nacelle.
 

AdrianS

Well-Known Member
Joined
Jul 5, 2014
Messages
755
Location
Australia
This is what I think. And I'm sure it's incorrect. But sounds good to me at this point.

Engine cowling will be stationary part, but engine itself would be moving part by nature because has rubber mounts. I need to provide enough clearance from muffler to the cowling itself. And that clearance will naturally serve as my exit duct for cooling air. Muffler is simply positioned to close to the propeller to do anything else. Sort of like on picture below from Russian airplane.

Not sure if my sketch of bigger call cowling with no bumps out look good. Looks too fat to me. But what do I know.

View attachment 113037View attachment 113038
I can't see flow into the prop staying attached in that drawing.
 

Topaz

Super Moderator
Staff member
Log Member
Joined
Jul 29, 2005
Messages
14,752
Location
Orange County, California
I can't see flow into the prop staying attached in that drawing.
It would be hit-or-miss. Prop-stopped, I agree, I don't see flow staying attached. Under power, it's entirely possible it might (details will be important), because the inflow field from the prop will help. This is the basis of the "wake field propeller," or the fact that a Cessna Skymaster is faster in the rear engine than the (identical) front engine.
 

Map

Well-Known Member
Joined
Dec 29, 2020
Messages
101
Location
California
Pushers are more difficult to cool because you don't have the prop wash to help with getting the air through the cowling. This is particularly bad on the ground. So you need a pretty big opening in the back, preferably up high because hot air goes up on its own. All other considerations are secondary.
Don't put the oil cooler directly behind the water cooler, because the oil would always be very hot (gets heated from by the air that just came through the water cooler).

The cylinders on a Rotax are aircooled and are much hotter than the water or oil. They need good airflow around them.
 

Eugene

Well-Known Member
Lifetime Supporter
Joined
May 26, 2017
Messages
1,787
Location
Merrill, Wisconsin, USA
Ok, here is some info:

Rotax 912 (100 HP) cooling requirements -

Water - 28 KW (95500 BTU) = 67%

Oil - 8 KW (27300 BTU) = 19%

Air - 6 KW (20500 BTU) = 14%

Seems to me this engine doesn't care how it's installed as long is there enough airflow through water and oil radiators.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,979
Location
US
Ok, here is some info:

Rotax 912 (100 HP) cooling requirements -

Water - 28 KW (95500 BTU) = 67%

Oil - 8 KW (27300 BTU) = 19%

Air - 6 KW (20500 BTU) = 14%

Seems to me this engine doesn't care how it's installed as long is there enough airflow through water and oil radiators.
I got things wrong earlier, forgot about the need to flow air over the fins on the cylinders. They get hot.
 
Top