Pusher cowling ideas

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rv6ejguy

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Combine area of water and oil coolers about 105 in.² My Russian friend apparently have some experience with this engines and he thinks I need about 60% of this area for inlet. I don't know if he's right.
With a good duct and Vy at 60 knots, 40 square inches should be fine for the water and oil, assuming a well designed duct. If Vy is slower than that, you'll need more inlet area. With all the obstructions aft of the rad, you may well need more inlet area.
 

Eugene

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With a good duct and Vy at 60 knots, 40 square inches should be fine for the water and oil, assuming a well designed duct. If Vy is slower than that, you'll need more inlet area. With all the obstructions aft of the rad, you may well need more inlet area.
My problem is to get duct long enough to be efficient with BRS box. So 50% or 60% will work hopefully

Screen Shot 2021-08-06 at 17.35.56.png
 
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AdrianS

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OK, a few things to take in here.

First, Eugene you talked about a straight duct being valuable. I do not get that. Look at all of the nice efficient inlets - they start with a small inlet and then monotonically diverge to smoothly case the flow to convert from high speed and low pressure to low speed and high pressure. Any straight section just adds wetted area without pressure recovery and also thickens the boundary layer. If you have no alternative, by all means do it, but if you can be increasing duct area as you go aft to the radiators, you should do it.

Second, this whole notion that "we are flying a slow airplane so drag reduction does not matter much" is not complete. I computed q for sea level air at 100 knots, and it is 34.1 lb/ft^2. For a 70 square inch radiator with full pressure recovery on the whole area, that is about 16.6 pounds. If we did halve the drag by putting in a duct that gives us full pressure recovery, we would decrease drag by about half that, or eight pounds. Eight pounds at 100 knots is 2.46 hp. In a nominal 100 hp engine that cruises at more like 70 hp, that is 3.5% of available power freed up from the radiator to power the plane. Yes, not huge by itself, but definitely real, and you could measure the increase in cruise speed or decrease in fuel consumption from this improvement. So, if it can be achieved, it is worth doing.

Now let's remember the big reason for the cowling. Yes, it can reduce cooling drag, but mostly it is being investigated to greatly reduce the immense amount of flow disruption over the wing. While weight may be the biggest enemy to airplane performance, making the flow go over the wings as if there is no fuselage is also really important to making lift at min drag. That open uncowled engine in the middle of the wing is a forest going through the air and messing up the low pressures over the whole top of the wing. So the big purpose of the cowling here is to allow air to flow cleanly over the wing. If we can also make a fairly efficient air inlet and keep the exit size modes, we can get some more improvement. If we can do so lightly and easily, why would we not?

A nicely shaped duct is not a win/lose type of a thing, you do the best job you can with all the other stuff in the way. In this case, the BRS is up front and below the path for the duct Eugene wants to use. OK, so you can build a really short duct that brings pressure up across the whole HX face and is out of the way, and you dump 5 hp doing it. Or you make a long snout that allows you to only dump 2.5 hp, then figure out how to get it out of the way of the BRS package. There is inbetween too. I have already talked about a hinge on the top and a couple stout springs with a release design can hold the snout on until you pull the handle, and then starts it rotating around the hinge pin. Once it gets over center, the wind takes it from there and the rocket motor and 'chute has an improving exit path through out the rotation of the snout. Rig the handle for the BRS to pull the snout release a little ahead of popping the BRS, and it might just be nicely staged. You could even put a couple of vanes on the outside of the snout to make it want to go up too so the over center point is further down.

People have commented that stuff will go through the prop. So what? BRS is to save your butt, not the airplane, not the engine, and not the insurance company. Besides, you are supposed to cage the engine just before yanking the handle to prevent the prop blades from cutting your rescue 'chute free from the airplane.

Billski
If Eugene were to go with the hinged duct top approach, the simplest (to me) way to keep it "latched down" would be with a couple of magnets. They are light, simple and maintenance free, can be got in whatever strength you like, will hold things closed, and once the lid is lifted 1/4" or less will be completely released.
 

Eugene

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OK, I looked it over and arrived to conclusion that moving BRS forward not going to give me much if I wanted to do it right. So, It will have to be removable intake scope upfront. I'm sure there is a good way to do it. I will think about it when I get there.

Here is question for today. Cowling walls must to be vertical against the wing? Just like I have my wooden blocks on this pictures?

Still trying to understand how airflow supposed to go into propeller best way possible around the muffler. Maybe it will get obvious at some point when I actually fill this whole thing with the foam. We will see

IMG_6134.jpegIMG_6146.jpegIMG_6144.jpegIMG_6143.jpegIMG_6142.jpegIMG_6141.jpeg
 

Eugene

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With a good duct and Vy at 60 knots, 40 square inches should be fine for the water and oil, assuming a well designed duct. If Vy is slower than that, you'll need more inlet area. With all the obstructions aft of the rad, you may well need more inlet area.

Here is my attempt to create drawing of well design duct in real dimensions on paper. 3.5 x 12" opening will give me my 40% of 105 in.² total area for water and oil coolers together. Obviously I am doing many things wrong here, but I don't know what they are at this point. Please help me!!!

Note: - Radiator needs to stay where it is and can't go up. You can see it on my pictures, because of mounting brackets where they are.

IMG_5834.jpegIMG_6149.jpegIMG_6151.jpeg

IMG_1732.jpegIMG_1733.jpegIMG_1734.jpegIMG_1735.JPGIMG_1739.jpegIMG_1740.jpeg
 
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Vigilant1

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Eugene,
I'm probably not understanding the reasons behind your present design.
It looks like the height of the opening (3 1/2") doesn't change until expansion (to match the 5" radiator face height) begins aft of the BRS "hump." Also, that splitter/scoop under your inlet will be draggy--where will the air under there go?

Why not just start your scoop/opening immediately at the top rear edge of the BRS housing? Put a rubber strip there so there's no draggy gap between the BRS lid and your scoop's bottom surface. There probably won't be any significant boundary layer anyway at the top of that short BRS lid, IMO you don't need a splitter/standoff. The bottom of the duct then smoothly drops down just 3/4" to the lower edge of the radiator face. The top lip of the duct is aligned vertically with the back of the BRS lid, and the opening starts 3/4" lower than the radiator face, the duct smoothly diverges up to meet the top of the radiator face.
Advantages:
1) A lot less wetted area and skin friction drag inside the duct and on the outside of the nacelle (it is 7"shorter)
2) Less weight
3) No interference with the BRS, no doubt that it will work as designed.

Disadvantages:
1) A 3/4" expansion (top and bottom) within 3" duct length = 14 degrees of divergence, so possible turbulence in the duct. Okay, is the air likely to be moving straight anyway given the presence of that BRS hump? If desired, small guide vanes could be put into the duct to even out pressure (and airflow) across the radiator face. I'm guessing that some manometer/pressure probes put in there will show that guide vanes aren't needed.

My description may be confusing, I can provide an ugly sketch if it would help.
I can't say if mine is "better" than your idea, but it is different!

Mark
 
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Eugene

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Eugene,
I'm probably not understanding the reasons behind your present design.
It looks like the height of the opening (3 1/2") doesn't change until expansion (to match the 5" radiator face height) begins aft of the BRS "hump." Also, that splitter/scoop under your inlet will be draggy--where will the air under there go?

Why not just start your scoop/opening immediately at the top rear edge of the BRS housing? Put a rubber strip there so there's no draggy gap between the BRS lid and your scoop's bottom surface. There probably won't be any significant boundary layer anyway at the top of that short BRS lid, IMO you don't need a splitter/standoff. The bottom of the duct then smoothly drops down just 3/4" to the lower edge of the radiator face. The top lip of the duct is aligned vertically with the back of the BRS lid, and the opening starts 3/4" lower than the radiator face, the duct smoothly diverges up to meet the top of the radiator face.
Advantages:
1) A lot less wetted area and skin friction drag inside the duct and on the outside of the nacelle (it is 7"shorter)
2) Less weight
3) No interference with the BRS, no doubt that it will work as designed.

Disadvantages:
1) A 3/4" expansion (top and bottom) within 3" duct length = 14 degrees of divergence, so possible turbulence in the duct. Okay, is the air likely to be moving straight anyway given the presence of that BRS hump? If desired, small guide vanes could be put into the duct to even out pressure (and airflow) across the radiator face. I'm guessing that some manometer/pressure probes put in there will show that guide vanes aren't needed.

My description may be confusing, I can provide an ugly sketch if it would help.
I can't say if mine is "better" than your idea, but it is different!

Mark
Thank you, this gives me something to think about, I will go and create another picture to make sure I understand you correctly.
 

Vigilant1

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Your version looks nicer, though I worry about the BRS...
I'll learn from the forthcoming comments.

Mark
 

Eugene

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I am curious why the upper lip couldn't start farther back around the numeral 3 on the graph? I also wonder if the 7 degree angle couldn't be in relation to the top of the BRS and not to level?
I don't know answers to those questions. I only know that you're not supposed to increase 7-8° because you're going to have flow separation. So, if you need to shrink your duct and 7° is your boss you have no choice but to go some distance to get area that you want.
 

blane.c

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I don't know answers to those questions. I only know that you're not supposed to increase 7-8° because you're going to have flow separation. So, if you need to shrink your duct and 7° is your boss you have no choice but to go some distance to get area that you want.
If the air is following the top of the BRS then you can make it 7 degrees in relation to that? And then shorter?
 

blane.c

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Six (6) things.
(1) You are in a box mentally and need to get out of the box.
(2) Symmetry is not your friend, at least not necessarily.
(3) The BRS doesn't care were it is located and doesn't need to be centered on the fuselage aka it can be off center.
(4) The radiator doesn't care were it is installed either it can also be juxtapositioned.
(5) The "duct" and the "cowl" are not the same thing. You can design the duct together with the radiator to work as combined elements and then cowl that.
(6) The radiator should be lined up to the airstream presented to it, the duct may not (probably never would and never will) flow perfectly straight back in perfectly straight back lines. The air reaching the duct inlet is likely not going to go in from a perfectly straight line entry point either. So the front of the duct needs to be aligned to the airstream presented to it and after that you can use the duct to change the direction some but also you can orient the radiator so the duct doesn't change the airflow unnecessarily.

So for example move the BRS forward and to the port side as far as possible and establish the front of the duct to the starboard side, it can be more squarish at the opening and gradually become more rectangular as it gets closer to the radiator (within reason and ain't saying it is easy). The radiator can be moved to center itself on the rear of the duct and the radiator doesn't need to be perpendicular and square to the airframe, it needs all that in relation to the duct (the airflow). After you are satisfied with that establish the cowl unsymmetrically. And if it doesn't work well maybe at least you are out of the "box".
 
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Eugene

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And if it doesn't work well maybe at least you are out of the "box".
My "box" is flexible. My problem is that I don't know what I'm doing. But willing to learn. That is why am asking this question. I admit not always easy for me to create mental picture based on description from you guys. But I will keep on trying. Thank you!

BRS sitting between the wings with quarter inch clearance on each side. It can move forward little bit, but I am not convinced about benefits. It blends so beautifully right now with fuselage.

I understand that we don't need to be symmetrical, but it needs to be perpendicular to the airflow. I was simply trying to accomplish this easiest way possible and working with what I have presently. Anything can be changed and I did already many changes, I'm not afraid to do that, but need convincing arguments to start moving that direction.

Yes I do understand that correct ducting is completely different task from correct external housing. Apples and oranges.

Again, I will keep creating more pictures (it's so easy to do) and one of these days, one of these pictures will be complete success. I hope...
 

blane.c

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Well you could remote locate the radiator, and it could be a different radiator, considering what you have spent so far the price of a different radiator seems insignificant. If you were just cowling the engine on top it would be pretty simple for you and then just find a place to hang the air-scoop for the radiator somewhere else.
 

Eugene

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Well you could remote locate the radiator, and it could be a different radiator, considering what you have spent so far the price of a different radiator seems insignificant. If you were just cowling the engine on top it would be pretty simple for you and then just find a place to hang the air-scoop for the radiator somewhere else.
Yes, anything can be done, it's not a question about money or time. I simply need good enough reason to do it. Need to convince myself that benefits will worth the effort.
 

blane.c

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Well it is like trying to watch someone pound a square peg into a round hole, so maybe move the peg over a different hole and try for a while.
 

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