Pusher cowling ideas

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Eugene

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And their situation is much worse than that. For a given "flat plate" drag area, doubling the airspeed results in 4 times the drag (force). And, making that force requires much more power at higher airspeeds.
Thank you. Maybe I will move BRS forward by only few inches to get some small benefits. One of them is moving CG forward. To have 3 inches between the rocket and radiator will give me opportunity to have small but nice looking logical cooling intake. It definitely started to feel like it is too much effort and risk for questionable benefits. I will think about it. But thank you for waking me up!


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Eugene

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So, do we simply make short diffuser and stop right in front of the rocket ( plan A)?. Or we will make a diffuser longer, all the way to the BRS box and notched opening for the rocket (plan B)?

I was for sure thinking that there would be no more questions today. I was wrong again…


F52AE01D-25BC-47FD-B61D-83152EF043AA.jpeg
 

wsimpso1

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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
 

Vigilant1

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I don't have answers (sorry), just two more questions:
1) Do you know how much opening area you'll want? Your heat exchanger isn't very tall, (approx 5"? Less?) so you could probably achieve reasonable reductions in area with a fairly short duct even while keeping the angles reasonable.
2) Does the rocket need to stay below the radiator, or could you possibly mount the rocket body (vertically) inside the duct with the top of it flush with the top of your nacelle? Obviously, this is a no-go if the heat from the radiator (in flight or post shutdown) will degrade the rocket. Otherwise, it would let you extend the duct all the way to the BRS hump. Blocking of the radiator by the rocket body wouldn't seem to pose significant cooling issues, especially with a tapered fairing or guide vanes around the rocket to reduce turbulence. Yes, a little more surface area in the duct. Anyway, just a thought. Obviously, I have no particular expertise in this area.
 

wsimpso1

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So, do we simply make short diffuser and stop right in front of the rocket ( plan A)?. Or we will make a diffuser longer, all the way to the BRS box and notched opening for the rocket (plan B)?

I was for sure thinking that there would be no more questions today. I was wrong again…


View attachment 113916
Even with that short inlet diffuser (B), I would still roll the lips in about 7 degrees on all sides and cut down some inlet area. If B is 6" long, that would actually reduce inlet area from 70 in^2 to about 40 in^2, which is still worth doing. Make the snout 12" long and you can get the inlet below 25% of radiator area. The price for that drag reduction is how you get past the snout. Maybe you form the snout from 3/32" aircraft plywood and just let the rocket blow it to pieces, or maybe you configure it to release and rotate out of the way, or maybe you attach it with little explosive bolts... Strike that last idea. It works, but it gets kind of scary and fussy.

Billski
 

Eugene

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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.


Billski
I am not hundred percent sure that I understanding all the statements correctly. I don't remember talking about straight ducts being valuable. If I show straight ducts with straight lines on all my sketches, that is only for simplicity of making it this way. In real life everything will be beautiful!!! Without straight lines
 

Eugene

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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.

Billski
I have no idea how I missed that!!! That is such a cool idea!!! Why didn't I think about it by myself??? Now instead of sleeping I will be laying in bed and designing this whole thing. Thank you!!!

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WonderousMountain

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Plan B is good by me.

You are wanting to slow the air down as much as possible.
However turbulance is bad, so we end up not doing it. Also,
what I like is that the piece can be made solid permanent w/o
elements of motion & apparatiks.

Compared to the other guys it's starting to look fetching.
 

Eugene

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I don't have answers (sorry), just two more questions:
1) Do you know how much opening area you'll want? Your heat exchanger isn't very tall, (approx 5"? Less?) so you could probably achieve reasonable reductions in area with a fairly short duct even while keeping the angles reasonable.
2) Does the rocket need to stay below the radiator, or could you possibly mount the rocket body (vertically) inside the duct with the top of it flush with the top of your nacelle? Obviously, this is a no-go if the heat from the radiator (in flight or post shutdown) will degrade the rocket. Otherwise, it would let you extend the duct all the way to the BRS hump. Blocking of the radiator by the rocket body wouldn't seem to pose significant cooling issues, especially with a tapered fairing or guide vanes around the rocket to reduce turbulence. Yes, a little more surface area in the duct. Anyway, just a thought. Obviously, I have no particular expertise in this area.
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.

And rocket is already vertical and sticking out little bit above radiator

IMG_5790.jpeg
 

Vigilant1

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Another simple approach would be to put the rocket near one corner of the BRS box and just start the duct offset from that (I.e you leave the left side of the inlet relatively straight, the right side opening is moved 3" leftward to make room for the rocket outside the duct). That helps achieve your desired inlet area reduction. Use guide vanes inside the duct as needed to reduce turbulence and to help equalize pressure/flow across the radiator face. This would eliminate latches, hinges, frangible bits, etc.
 
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Eugene

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Another simple approach would be to put the rocket near one corner of the BRS box and just start the duct offset from that (I.e you leave the left side of the inlet relatively straight, the right side opening is moved 3" leftward to make room for the rocket outside the duct). That helps achieve your desired inlet area reduction. Use guide vanes inside the duct as needed to reduce turbulence and to help equalize pressure/flow across the radiator face. This would eliminate latches, hinges, frangible bits, etc.
I will get real life dimensions in the next few days and try to put this idea on paper. Otherwise I don't have a good enough imagination to get the whole picture. But one of these days I will find a compromise that everybody happy with including myself. Maybe
 

wsimpso1

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Submerged inlets are great where:
  • One only needs air through it part of the time;
  • One is trying to keep high velocities;
  • One is not trying to make pressure.
Now if you need to make some pressure from the inlet to push air through a radiator, then you need an expanding diffuser. Inlet area is smaller than the radiator, the wall shapes are smoothly expanding, and one might need some guide vanes to “wet” the whole radiator with raised pressure Air.
 

Extreme Engineering

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Why not add half of the inlet duct to the top of the BRS upper cover? As long as it is fairly light, it shouldn’t be a problem. There would have to be a seam from front half (BRS) to back half (engine) with a rubber seal but that is easily done.
With this type of design you get the best of all worlds
 

Eugene

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Submerged inlets are great where:
  • One only needs air through it part of the time;
  • One is trying to keep high velocities;
  • One is not trying to make pressure.
Now if you need to make some pressure from the inlet to push air through a radiator, then you need an expanding diffuser. Inlet area is smaller than the radiator, the wall shapes are smoothly expanding, and one might need some guide vanes to “wet” the whole radiator with raised pressure Air.
As far as my wife concerned, I stayed home today purposely, so I can catch up on business paperwork.

So I made some progress

A9935213-ACF3-4E3D-846A-12503ACCE4B9.jpeg
 

Eugene

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Why not add half of the inlet duct to the top of the BRS upper cover? As long as it is fairly light, it shouldn’t be a problem. There would have to be a seam from front half (BRS) to back half (engine) with a rubber seal but that is easily done.
With this type of design you get the best of all worlds
I believe Peter Garrison was suggested to me the same thing couple years ago. I remember how I got very skeptical about it as far as attaching anything to BRS. It is relatively flimsy cover simply silicone in place to have minimum resistance during deployment
 

blane.c

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I believe Peter Garrison was suggested to me the same thing couple years ago. I remember how I got very skeptical about it as far as attaching anything to BRS. It is relatively flimsy cover simply silicone in place to have minimum resistance during deployment
I have spent considerable time removing something that was temporarily siliconed in place and other times something siliconed just fell off.
 

Tiger Tim

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Since the radiator outlet looks like it’s going to be an aerodynamic mess, does this inlet business even really matter much? Would Eugene be money ahead to move the radiator UP and aft and enclose it in a sort of upside-down P-51 scoop on top of his new cowl? That way everything stays behind the BRS and the inlet/outlet design for the radiator (and oil cooler?) can be tightly controlled.
 

Vigilant1

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Since the radiator outlet looks like it’s going to be an aerodynamic mess, does this inlet business even really matter much? Would Eugene be money ahead to move the radiator UP and aft and enclose it in a sort of upside-down P-51 scoop on top of his new cowl? That way everything stays behind the BRS and the inlet/outlet design for the radiator (and oil cooler?) can be tightly controlled.
That would allow a very nice optimized stem-to-stern pressure recovery duct system for the coolant and oil heat exchangers. OTOH, the cylinders, exhaust and muffler can't be buttoned up tight inside of a tight nacelle, so now we'd have to have another (though smaller) inlet required to do that (and pressure recovery from that will be just as hard as with the present setup, though flow will be less). And the increased size/weight of the nacelle, the maintenance access issues (still need to service that engine, the carbs are near the top) and the potential increased blockage/turbulence of the prop infeed air would need to be addressed if the nacelle aft area is increased. I suspect sticking with the present layout is best overall, even if there's not much pressure recovery possible at the aft end.
 
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