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  #41  
Old 05-09-2014, 07:07 AM
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MarkW MarkW is offline
 
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OK I will bite.
If we reduce the exit, how much is gained by decrease of mass airflow and how much is gained by increase of outlet velocity?
If outlet velocity increase (jet effect) doesn't get us that much would we not be better off reducing mass at the inlet? Reducing the inlet would reduce form drag (barn door effect). Would this not also reduce friction drag that is internal to the cowl?
Reducing the exit simply increases velocity by increasing restriction (static pressure).
Now if the exit velocity helps that much then "never mind". (in my best Gilda Radner voice)
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  #42  
Old 05-09-2014, 08:06 AM
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Quote:
Originally Posted by pierre smith View Post
Dan, I've been wanting to apply these ideas to my -10 for quite a while now. Could I simply leave the Van's inlets and accomplish MOST speed increases with the exit? Perhaps some internal changes like a rounded firewall bottom?
If the 10 cowl responds like the 8 cowl, yes. Ken posted that data.
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  #43  
Old 05-09-2014, 09:44 AM
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I get that the exit geometry is critical to get the velocity close to original freestream, but I'm wondering about the inlets. You have to size the inlets large enough to give sufficient flow during high demand (high power hard climb), and that means that at cruise they are too large and you'll end up either swallowing more air than you need, or building up back pressure in the plenum and spilling that air out over the lip of the inlet. How much drag is represented by that spillage? Or do we even care to quantify it, since we may not be able to do anything about it? Is it reasonable to think you may want to accept some higher in-cowl cooling drag to avoid inlet-spillage which would be (in my mind anyway) a higher per-mass drag, since the momentum loss is essentially 100%? Or is the propwash in this area so disturbing that it really gets hidden in the noise?
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Last edited by airguy : 05-09-2014 at 09:47 AM.
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  #44  
Old 05-09-2014, 09:55 AM
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Quote:
Originally Posted by airguy View Post
I'm wondering about the inlets. You have to size the inlets large enough to give sufficient flow during high demand (high power hard climb), and that means that at cruise they are too large and you'll end up either swallowing more air than you need, or building up back pressure in the plenum and spilling that air out over the lip of the inlet. How much drag is represented by that spillage?
Somewhere in a deep and dark corner of my memory, I seem to remember that the drag from any spillage is way less of an issue than folks would seem to think.-------and in some cases actually help a bit. Remember this is only one factor in the total drag of the cooling system.

But, then my mind is getting older than the rest of me----except possibly my knees
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Last edited by Mike S : 05-09-2014 at 09:57 AM.
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  #45  
Old 05-09-2014, 10:02 AM
jj_jetmech jj_jetmech is offline
 
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Quote:
Originally Posted by DanH View Post
7

Your task is to reason a system design that maximizes both goals.

7
Thanks Dan... Wait a second did you just give me homework? This is fun stuff...

I'm committed to my cowl choice now but you really have me thinking Ill build my own plenum and choose the inlet ring size and exit later as I learn more.

I have a friend on his second 8 he is planning to build his plenum and has already modified his Vans cowl and machined his own inlet rings. Ill have to see what size he chose and why. He doesn't know it yet but I'm planning a collaboration with him in this area..

Would you share pics of your plenum and exit?

Thanks
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  #46  
Old 05-09-2014, 11:47 AM
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Dan and others have posted a lot of data on this subject before. Search "The Shrinking Exit".

Try here too: http://www.vansairforce.com/communit...+effect&page=7 Post 66 has some more links.

It's very clear that throttling the exit is the way to go, the same way we do with liquid cooled setups.
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  #47  
Old 05-09-2014, 12:48 PM
BillL BillL is offline
 
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Quote:
Originally Posted by jj_jetmech View Post
Thanks Dan... Wait a second did you just give me homework? This is fun stuff...

I'm committed to my cowl choice now but you really have me thinking Ill build my own plenum and choose the inlet ring size and exit later as I learn more.

I have a friend on his second 8 he is planning to build his plenum and has already modified his Vans cowl and machined his own inlet rings. Ill have to see what size he chose and why. He doesn't know it yet but I'm planning a collaboration with him in this area..

Would you share pics of your plenum and exit?

Thanks
With all these questions you have about cowls and cooling, there are many excellent posts by DanH and others in this discussion. One that you should read is by Alan Judy. Catch it now and capture the remaining photos before the links expire.

http://www.vansairforce.com/communit...t=Cowling+mods

I saw Alans' 6 some years ago when he had the cowl off and my son and I had a long conversation with him. He keeps a little black book of all performance comparisons. I did not know the significance of what I had seen until later.
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  #48  
Old 05-11-2014, 11:31 AM
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Inlet size: Consider these four simple bodies with identical cooling passages and exits.



A and B are low Vi/Vo inlets (inlet velocity divided by freestream velocity = ratio). C and D are high Vi/Vo inlets.

In terms of cooling, all four inlets perform the same. They all flow the same mass and slow it to the same internal velocity just prior to the heat exchanger body, trading dynamic pressure for static pressure. (Iimagine cylinder fins or a radiator core at the end of the streamlines). A and B slow the flow externally, so velocity through the inlet is slow. C and D do it internally; flow through the inlet is near to or even higher than freestream. The high velocity means some loss due to internal drag, as compared to the near-frictionless external diffusion. With all else equal, C and D would exhibit more cooling drag, i.e more momentum loss.

A and B require less physical distance between the plane of the inlet opening and the plane of the exchanger. The cowl can be shorter. The shape of the duct inside the entrance is non-critical. If entrance velocity is slow enough, it doesn't even need a duct.

C and D require more physical distance between the inlet and the exchanger, as it needs an appropriate diverging duct for best performance. The internal duct cannot diverge too quickly without risking flow separation. If the flow separates into turbulence, the energy in the dynamic flow is wasted, and the resulting static pressure is reduced. In our application, the duct length requirement dictates an extended propeller hub.

Now let's consider external drag. Body A is a mess. The body shape did not allow the flow to remain attached at the inlet lip. It's much the same separation problem a high Vi/Vo inlet would have internally if the duct diverged too quickly. The result here is turbulent flow over much of the body. Drag is high.

Good lip shape means flow remains attached to the outside of Body B, at a cost of some greater frontal area. (Take a good look at at the side profile of your standard Vans inlet.)

Body C is our F1 Reno Racer example. The engine is packaged very tightly, as is the sole occupant, so overall frontal area is at an absolute minimum. There is no downside to a prop extension, so the design choice is to stretch out the cowl, use the high Vi/Vo inlet, and keep the cowl skinny.

Body D is a high Vi/Vo inlet, with prop extension and ductwork, grafted on a body where overall frontal area was driven by the size and shape of the engine and/or passenger compartment. External drag will be similar to B, or a little better. The difference would not be due to the inlets or the external shape immediately adjacent to the inlets, but rather the less blunt portions of the cowl inboard of the inlets.

I'm just a student working toward intern; always to happy to hear from the pros.

Quote:
Originally Posted by airguy View Post
...building up back pressure in the plenum and spilling that air out over the lip of the inlet. How much drag is represented by that spillage? Or do we even care to quantify it, since we may not be able to do anything about it?
That would be body A above. It's been quantified (NACA-TR-1038, Fig 12), and the answer to to pay attention to external cowl shape in the vicinity of the inlet, not the actual size of the inlet. Short version: being well shaped, bodies B, C, and D above would all maintain a reasonable level of attached flow when operating above roughly 0.3 Vi/Vo, and as you describe, the adverse external pressure outside the lip is further reduced as inlet flow is increased (i.e. exit area is increased, allowing more velocity through the fixed inlet).
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Last edited by DanH : 05-11-2014 at 01:14 PM.
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  #49  
Old 05-11-2014, 01:40 PM
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What a great set of illustrations. Very similar to some basic fluid dynamics texts. For the laymen just reading along, also keep in mind that this isn't just a vertical section, but also applies all the way around each and every section to the horizontal. Boy, then the ideal gets a whole lot more complicated. See how I said that without muddying the waters with real math .
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  #50  
Old 05-11-2014, 08:20 PM
glenn654 glenn654 is offline
 
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Seeing Dan's illustrations gave me an idea which may be either ridiculous or not. We have discussed exhaust augmentation of the cooling flow but has there been any trial of ram air augmentation.

What I envision is a high vi/vo tube running from an external inlet on the lower cowl and passing straight through to the cowl exit. The tube would have inlets along its length to pick up air from the inside of the cowl as it accelerates and gives a velocity boost to the cooling flow as it exits the cowl

What do you think?

Glenn Wilkinson
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