Pusher cowling ideas

[blane.c]

OK, so that's not going to be a problem? That's a good news! Softpack if I remember correctly have much smaller dimensions. Maybe I can hide it somewhere else instead of right in front of radiator? I don't know, just brainstorming at this point.

View attachment 113796

Move BRS up, air intake will be under chute. Must the BRS sit level? It would fair in better if it was at a angle (if moved up) which also may have some benefit of "shooting" it away from the propeller possibly giving the propeller an extra instant or two to completely stop?

 

[wsimpso1]

Also wondering what would be recommended distance from intake diffuser to the radiator itself.

Look to Ross's photos of his and Russell's design for best practice. You will be limited by your package at both ends.

 

[Eugene]

Move BRS up, air intake will be under chute. Must the BRS sit level? It would fair in better if it was at a angle (if moved up) which also may have some benefit of "shooting" it away from the propeller possibly giving the propeller an extra instant or two to completely stop?

I will sit down and think about my possibilities tonight after work. Thank you. Don't have a clear picture of what you're talking about. But if you see this possibilities, I should be able to reproduce it on paper

 

[Eugene]

Look to Ross's photos of his and Russell's design for best practice. You will be limited by your package at both ends.

Now I know where I'm getting confused. Ross is the guy with subaru RV-6 yellow. Who is Russell?

 

[wsimpso1]

They did talked to me and helped to install my unit after purchase. They send me recommendations based on their previous experience working with Skyboys. I had to convince them that my way is better and they agreed. Picture below is what I was sending to them. They want me to connect one strap to the front connection with only one 6mm bolt.

View attachment 113800

They are the experts. Remember, this gadget is to save your butt, not your airplane. If it is repairable after the landing, that is a bonus, not the expected outcome.

I suspect that the single bolt forward is only needed to keep the device in place in normal flight and for launch/breakaway of the lid, and is not carrying much if any load once the canopy is established and your velocity vector is all z axis.

 

[Eugene]

Talked to my Russian friend and he tell me not to worry about it. Front scoop can easily fly away with a parachute and should be no problem to make it happen. I was told to concentrate on general shape and make it right aerodynamically.


So far two people much smarter than I am tell me that it is very possible. One of them is Peter Garrison.

At work today I was thinking about different ways this can be done. And picture below is what I think is the best. This front scoop can slide vertically like injection seat.

 

[Kiwi303]

Why not try running it around the BRS in two nozzles like a fighter jet twin intake? Look at the SAAB Gripen, one engine, two intakes, they merge just before the engine.

So if you lift the BRS, then the air intakes can go around it so the BRS top surface is the top surface of the plane, unobstructed, with the air ducted each side of the BRS cannister.

I made a (very) crude MSPaint drawing which I hope makes sense. looking down the red is the BRS, the blue is the water/oil exchangers and the green is internal fairing/blocking to keep air heading where it should.

 

[Voidhawk9]

Absolutely no fillets on engine cowling, but straight vertical walls against the wing. I wonder why?

The nacelle on the Avanti expands in cross section past the trailing edge, so there isn't an adverse pressure gradient from the nacelle to deal with, thus no fillet required.

 

[Eugene]

Why not try running it around the BRS in two nozzles like a fighter jet twin intake? Look at the SAAB Gripen, one engine, two intakes, they merge just before the engine.

So if you lift the BRS, then the air intakes can go around it so the BRS top surface is the top surface of the plane, unobstructed, with the air ducted each side of the BRS cannister.

I made a (very) crude MSPaint drawing which I hope makes sense. looking down the red is the BRS, the blue is the water/oil exchangers and the green is internal fairing/blocking to keep air heading where it should.

View attachment 113869

If you do it this way, whole structure will get pretty wide. Also, I was reading about Russell Sherwood and Ross Farnham and how they cooling Subaru engines. And I made a conclusion that air intake should be as straight as possible. And because of that I like my picture much better.

 

[wsimpso1]

Talked to my Russian friend and he tell me not to worry about it. Front scoop can easily fly away with a parachute and should be no problem to make it happen. I was told to concentrate on general shape and make it right aerodynamically.


So far two people much smarter than I am tell me that it is very possible. One of them is Peter Garrison.

At work today I was thinking about different ways this can be done. And picture below is what I think is the best. This front scoop can slide vertically like injection seat.

View attachment 113863

Hmm.

I would never be so glib as to think that a fiberglass diffuser could be easily brushed aside by that little rocket engine. I have done a bunch of gunsmithing and been a real live firearms R&D engineer, and understand just how little has to go wrong to suck off the available energy, stopping the whole thing short of proper function. Maybe the rocket has big excess power, but the diffuser might take a lot of energy to blow it out of the way.

Being as the rocket is under the flange, this might work. Critical issue is will the diffuser slide on the tracks? The inertial forces will be high - F=ma, where m is nozzle mass and a is acceleration the rocket is putting on the diffuser. You get some help by the rocket being in the plane of the sliding joint, but it is not centered, and so will tend to cock the nozzle and jam it. Calcs are easy. Force from the rocket on the diffuser time the offset gives moment. Divide moment by distance between tracks and you get forces in the plane of the paper. Multiply them by coefficient of friction and see if that is bigger or smaller than the lifting force. Bigger means it locks and does not slide. Smaller means it slide off.

There is another force involved in getting it go - somehow you have to keep the diffuser from sliding off in normal flight. Whatever that means is - and there are many - you have to also overcome that. I personally like the idea of four frangible clips or bolts. Maybe plastic bolts (McMaster-Carr has a bunch of sizes) that will break at a predictable force. This scheme has to be strong enough to reliably prevent inadvertant ejection in flight, but able to open predictably when the diffuser is hit by the rocket motor. I even like the idea of a band clamp around the diffuser flange with a couple frangible bolts. Only have to break one and give it a nudge - away it goes.

When faced with similar issues for release of external stores, emergency egress systems, and rocket staging, engineers all over the world came up with captive springs systems, explosive bolts, cartridge actuated motors. Maybe a hinge at the top and a couple stout springs at the bottom with an escapement mechanism (look up triggers for cross bows and hand grenades) that is pulled along with the BRS handle. Lots of canopies were ejected during WWII by springs or blank shot shells.

Some other thoughts. You can usually run with an inlet 1/4 the area of the radiator and have plenty of cooling. Let's put that in perspective - that is half the width and half the height of the radiator. Ross' and Russell's experience is you can even go smaller. So your inlet can be pretty small. If you go for splitting the diffuser and keeping the same inlet area, you now have more wall area that makes drag on the air going through the duct, and makes for more air in boundary layer. To be as effective, the inlet area will have to be bigger with it split than as a single. Maybe not a big thing, but it is also more complicated to make work, etc. If I were inclined to go side by side, I would go asymmetric - shove the BRS box to one side and the diffuser to the other side.

My WAEG is still to put the diffuser above with a well configured breakaway design that will not fail.

Billski

 

[WonderousMountain]

Okay, so IDK everything involved here either,
but would it be easier to center the rocket, &
use a split diffuser? Inquisitive minds want to know.

 

[Eugene]

Okay, so IDK everything involved here either,
but would it be easier to center the rocket, &
use a split diffuser? Inquisitive minds want to know.

Or, shoot a rocket forward and let diffuser fly away with it?

 

[blane.c]

If you do not like moving the BRS up, move it farther forward.

 

[blane.c]

OK done bsing when wsimpso1 chimes in things are obvious to those paying attention, move the BRS.

 

[wsimpso1]

I liked your comment because it is a valid option. There are several good options...

 

[Eugene]

If you do not like moving the BRS up, move it farther forward.

I would like to understand what are we going to achieve by moving BRS forward? I am looking at my pictures and I don't see much room at all. Sure I can find probably 2 or 3 inches. Ideally I would like to see 12 inches clearance or more between the rocket and the radiator. That's not going to happen.

And yes rock it doesn't have to be exactly there. This unit was sold as one package to be mounted externally. I'm sure I can rearrange that. Rocket can be in the center and can be sitting lower. In fact Skyboy designer recommended that the rocket should be positioned at 45° angle, instead of straight up. I don't exactly remember his explanation. And I could've misunderstood as well.

 

[Eugene]

My WAEG is still to put the diffuser above with a well configured breakaway design that will not fail.

Billski

Here is picture of Skyboy with factory installed ballistic recovery systems in Czech Republic in 2004

 

[Vigilant1]

Is it worth putting the whole inlet geometry problem into perspective?
Some factors:
1) This is a 100 knot airplane, dynamic pressure under standard conditions at that speed will be about 0.24 psi (1640 Pa). These Rotax heat exchangers have a net area of about 70 SQ inches, so even if we cut the inlet area by 50% with a near optimum inlet, the drag reduction would be about 8 lbs compared to just having the radiator face at/near the front of the duct.
2) In actuality, the difference won't be that much, because there will be at least some pressure recovery on the back of the duct.
3) Also, if we have (virtually) no inlet, and because the coolant HX and oil HX "stack" will provide resistance to airflow, any air that doesn't go through the exchanger will spill over the top/sides of the duct. With a well designed opening (well rounded edges, etc) this occurs with relatively little drag even compared to a solid "nose cone". This is one reason that modulating the flow of air through a coolant duct is usually done at the exit rather than at the inlet. Drag due to a "can't take any more" duct or a flat surface isn't very high at all IF turbulence is decreased with well designed 'lips" on the duct.

Bottom line: In this case, I suspect a "zero length' diffuser will generate >much< less than the "worst case" of 8 lbs of drag difference noted above. I suspect, at any rate, that the biggest contribution of the nacelle will come from cleaning up the airflow around and behind the engine.

All this aside, if an inlet over the existing BRS is still desired, I'd forget any idea of a sliding snout removal, etc. There are just too many ways for that to fail. I'd go with a single frangible sheet at the top of the duct as far away from the rocket as possible (to let it gain some kinetic energy before striking the surface). I'd test candidate panels with a rig that dropped a dull slug of the same mass as the rocket into the sample sheet at the expected V of the rocket at that point. And do it at different temperatures to assure it cracks as well at 100F as at 20F.

If a frangible sheet, pre-scored snout, sliding snout, etc departs the aircraft in normal flight, there's a good chance it will destroy the prop, at a minimum. Again, my choice would be to just have a minimal inlet right at the radiator face that won't interfere with the already engineered BRS.

 

[Eugene]

Is it worth putting the whole inlet geometry problem into perspective?
Some factors:
1) This is a 100 knot airplane, dynamic pressure under standard conditions at that speed will be about 0.24 psi (1640 Pa). These Rotax heat exchangers have a net area of about 70 SQ inches, so even if we cut the inlet area by 50% with a near optimum inlet, the drag reduction would be about 8 lbs compared to just having the radiator face at/near the front of the duct.
2) In actuality, the difference won't be that much, because there will be at least some pressure recovery on the back of the duct.
3) Also, if we have (virtually) no inlet, and because the coolant HX and oil HX "stack" will provide resistance to airflow, any air that doesn't go through the exchanger will spill over the top/sides of the duct. With a well designed opening (well rounded edges, etc) this occurs with relatively little drag even compared to a solid "nose cone". This is one reason that modulating the flow of air through a coolant duct is usually done at the exit rather than at the inlet.

Bottom line: In this case, I suspect a "zero length' diffuser will generate >much< less than the "worst case" of 8 lbs of drag difference noted above. I suspect, at any rate, that the biggest contribution of the nacelle will come from cleaning up the airflow around and behind the engine.

All this aside, if an inlet over the existing BRS is still desired, I'd forget any idea of a sliding snout removal, etc. There are just too many ways for that to fail. I'd go with a single frangible sheet at the top of the duct as far away from the rocket at possible (to let it gain some kinetic energy before striking the surface). I'd test candidate panels with a rig that dropped a dull slug of the same mass as the rocket into the sample sheet at the expected V of the rocket at that point. And do it at different temperatures to assure it cracks as well at 100F as at 20F.

If a frangible sheet, pre-scored snout, sliding snout, etc departs the aircraft in normal flight, there's a good chance it will destroy the prop, at a minimum. Again, my choice would be to just have a minimal inlet right at the radiator face that won't interfere with the already engineered BRS.

Thank you, this is what I call plain English. Ross and Russell going 2-3 times faster and have 2-3 times more power. Was wondering what kind of benefits I will be getting going at my speed. If can be done without compromising BRS. Sure. But I don't have a clear vision in my head at this point.

 

[Vigilant1]

Thank you, this is what I call plain English. Ross and Russell going 2-3 times faster and have 2-3 times more power.

Drag reduction becomes very important at higher airspeeds. As we know, for a given amount of drag, doubling the airspeed results in 4 times the drag (force). And, making that force requires much more power at higher airspeeds.