Baffling Baffles and Inlets

Discussion in 'Firewall Forward / Props / Fuel system' started by Monty, Sep 23, 2017.

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  1. Sep 23, 2017 #1

    Monty

    Monty

    Monty

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    Moderator Note: Split this discussion off from the "What did you do on your airplane today" thread. /Note

    Perhaps we should have a baffling baffles thread. I certainly learned a few things while fixing mine.
     
    Last edited by a moderator: Sep 27, 2017
  2. Sep 23, 2017 #2

    Toobuilder

    Toobuilder

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    Re: What did you do on your airplane project today?

    Baffles are one thing, baffle seals are another. I tried to do one piece on every run - sealed pretty well (visually).

    I have not yet addressed the area around the ring carier, but as far as I'm concerned, this is how you do the perimeter:

    [​IMG]

    [​IMG]

    One piece of rubber and even clamping at the fasteners assures minimal leakage. I have a few hundred hours on it now and the edges of the seal show perfect contact with the upper cowl.

    That said, even if I am able to continue the sides down and around the great wall under the chin and back in one piece, logic dictates there is still going to be some leakage. And even if the leakage is minimal, as soon as you throw that many running feet at a solution, the total loss might be worse off than the conventional system.
     
  3. Sep 24, 2017 #3

    proppastie

    proppastie

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    Re: What did you do on your airplane project today?

    Not sure what you mean here, but if you measure the pressure drop and meet specifications I would think you are good to go.....You also could probably blue the seals and find any leaks and fix them. You could attach lots of thermo-couple to the engine surface....the Chinese ones are really cheap on e-bay, but you need a base line..... Of course there will be leakage at 200 mph, but will it matter?

    And I think that looks beautiful.
     
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  4. Sep 24, 2017 #4

    Toobuilder

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    Re: What did you do on your airplane project today?

    What I mean is that for every "perfect" appearing seal, it really isn't. That baffle material, though showing a nice feathered edge is still bleeding a bit of air - every inch of it. If you have a tremendous length of seal as I'm planning, then there is going to be a relatively large cumulative effect of bleed loss. Will the engine cool? No doubt. But is that bleed over causing drag? You bet.

    My main reason for going with the open baffles is to allow easy access to the engine and to avoid the weight of a full plenum. Once I looked at the complexity of building the great wall baffle and the added seal material (which aint light!), the full plenum started to look better. And I have an idea which will allow the top to come off in seconds, so the access will hopefully be resolved too. And I will know that I will achieve a near 100% sealing effectiveness.
     
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  5. Sep 25, 2017 #5

    proppastie

    proppastie

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    Re: What did you do on your airplane project today?

    Certainly being 100 % effective and then lowering the opening sizes to only what is required for cooling, would be lots less drag.......great project, sorry I misunderstood.
     
  6. Sep 25, 2017 #6

    Monty

    Monty

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    Re: What did you do on your airplane project today?

    The area around the ring gear/inlet is the worst part. If I had known I was going to be doing body work on the nose bowl, I would have extended the nose bowl inlet lips and mounted the seals to the bowl for that part. I HIGHLY recommend that method if you are starting from zero.
    KIMG0180.jpg
    This is the mess you wind up with otherwise.

    The inter-cylinder baffles are also something that gets neglected. The baffles I bought from Van's didn't even have any!!

    There were none on the cylinder head in my previous installation. This causes the back right cylinder to run hot around the exhaust....guess which cylinder failed compression at annual..
     
  7. Sep 25, 2017 #7

    Toobuilder

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    Re: What did you do on your airplane project today?

    Doing a high Vi/lo Vo inlet (divergent duct) is the ultimate. That said, it is extremely finicky to proper shape and AoA. Fine for a racer, but for an everyday airplane like mine the low Vi/Vo and a throttled exit is the way to go. a "too large" inlet will spill overboard and create a natural streamline. As long as you are throttling the exit effectively then the overall drag is pretty close to a "perfect" divergent duct.
     
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  8. Sep 25, 2017 #8

    Rockiedog2

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    Re: What did you do on your airplane project today?

    yeah Too. I did all that on my 701.

    :roll:
     
  9. Sep 25, 2017 #9

    Toobuilder

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    Re: What did you do on your airplane project today?

    Monty-

    Is this "starting from zero" enough for you?

    [​IMG]
     
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  10. Sep 25, 2017 #10

    Monty

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    Re: What did you do on your airplane project today?

    Yep....:gig:

    I should have done that....
     
  11. Sep 25, 2017 #11

    proppastie

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    Re: What did you do on your airplane project today?

    I have been told that and it makes sense to a certain extent, but the later "Lopresti Style" cowls are advertised to much increased speeds. The Mooney mod of the guppy cowl picks up 5 to 7 knots. So I guess I do not understand why in the original guppy cowl for example the "spill overboard" was not just as good as the newer styles. The overall dimensions of the cowl top to bottom does not change....does that mean the flat plate drag is the same?
     
  12. Sep 25, 2017 #12

    Monty

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    Re: What did you do on your airplane project today?

    It boils down to whether you are using external diffusion or internal. Internal is the lowest drag under the right circumstances. External diffusion can cause the boundary layer to go turbulent prematurely. Not that big of a deal on the front of the cowl. The prop has already stirred that up, and the typical airplane shape is not laminar flow anyway. It doesn't really "spill overboard". The air "backs up" and the flow essentially forms its own fairing. The diffusion is happening in front of the inlet, and the speed through the inlet is much less than Vo.

    My guess about the Lopresti speed mods: they just handle the cooling air better, and reduce the overall amount needed, or the fix some horrendous stuff the factory did. A true internal diffusion inlet requires quite a bit of length work, and a diffusion angle no greater than 7 degrees. I don't see there being room for that in the typical aircraft cowl. They may use "dump" diffusion into a plenum. That is the least efficient way to do things, but it will work.
     
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  13. Sep 25, 2017 #13

    mcrae0104

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    Re: What did you do on your airplane project today?

    There's probably a lot more difference than just the Area[SUB]inlet[/SUB]/Area[SUB]outlet[/SUB] change going on with the different Mooney cowls. For one thing, the guppy cowl is taking in a lot of dirty flow right off the spinner. Also there is probably considerably more separation from the inlet to the top side plenum, meaning less-than-ideal pressure recovery (i.e. more drag).
     
  14. Sep 25, 2017 #14

    Toobuilder

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    Re: What did you do on your airplane project today?

    Thats a good question and I dont have a ready answer. That said, theres probably a practical limit to "too big" for a natural streamline. And by all accounts, the early Mooney inlets were huge and irregular shaped. The area right behind the prop was wide open and subject to a localized low pressure area caused by the spinner and blade root, so its likely there was significant reverse flow there. The smaller Lopresti inlets were more outboard, and the current ones are circular. In fact, the current Ovation cooling scheme was my original blueprint for cooling - low Vi/Vo inlets and a chin baffle.
     
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  15. Sep 25, 2017 #15

    Monty

    Monty

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    Re: What did you do on your airplane project today?

    The key to making external diffusion work is to have a cowl with a big round lip, that guides the flow over the rest of the body. You can't have a sharp transition, or the flow will separate and cause a lot of drag. Think of a radial engine inlet. HUGE! But the cowl lips are large and rounded to allow the stagnation point to move around without causing flow separation.
     
  16. Sep 25, 2017 #16

    Toobuilder

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    Re: What did you do on your airplane project today?

    Shouldn't have much trouble with flow separation on the inlet lip. The contour is about the size of a nickel and the inlets are way forward and outboard as far as practical.

    [​IMG]

    The problem as I see it is maintaining the inlet's 29 square inch section area all the way into the plenum as it squeezes by the prop governor. The right side will be easy to bell mouth this out into the plenum, but the left side is going to pinch. I can stretch the duct taller to compensate, but I need to be cautious of getting any sharp transitions which might trip the flow. Hopefully the velocity will remain low enough that it wont matter. At any rate, you can see in post 81 that I have a red box inked out on the front baffle... that's the minimum area I need to maintain to keep parity with the inlet ring. That is going to be one strange looking duct when its all said and done!
     
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  17. Sep 25, 2017 #17

    Toobuilder

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    Re: What did you do on your airplane project today?

    If you have baffles sourced from Van, are you aware that they intersect the head right at the zero fin depth area on #2 and #3/5, effectively starving the lower fins of any meaningful airflow? There is a VERY effective fix for this that I use on all my Lycomings now. Read about it here.
     
    Last edited: Sep 25, 2017
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  18. Sep 26, 2017 #18

    Monty

    Monty

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    Re: What did you do on your airplane project today?

    You scared me that I had missed something. Fortunately not a problem on my O290. I did have to shorten the baffles on #1 because it was completely blocking off airflow on the bottom near where the cht sender would go. I'm using the spark plug type, so no sender there.

    KIMG0198.jpg
     
  19. Sep 26, 2017 #19

    Monty

    Monty

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    Re: What did you do on your airplane project today?

    Think about different angles of attack. I'm not sure a nickel is big enough. The prop wash causes the angle of attack to change a lot. Tuft testing is the only sure fire way to check.

    The original inlets don't look too bad as far as the radius goes. It may be an optical illusion, but the new inlets look like a bigger capture area.
     
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  20. Sep 26, 2017 #20

    Toobuilder

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    Re: What did you do on your airplane project today?

    Yes, the new inlets are about 25% more area. The std Rocket inlets are a bit undersized for some of the mission profiles I fly. Significant testing will ensue!
     
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