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  #1  
Old 06-24-2020, 06:00 PM
sailvi767 sailvi767 is offline
 
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Default Inlet sizing

We are in the process of swapping a 260 HP 540 C4B5 for a Barrett 300Hp engine. The Barrett has a RSA10 servo vice the current RSA5. The inlet is the Jim Winings mod that is a round 3?. Will we be able to flow enough air with that inlet? Getting a lot of conflicting opinions. Rule of thumb says a round 3? is only good for 220 HP.
George
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  #2  
Old 06-24-2020, 07:21 PM
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Mike S Mike S is offline
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Have you read this thread?

http://www.vansairforce.com/communit...d.php?t=183743
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  #3  
Old 06-24-2020, 09:47 PM
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DanH DanH is offline
 
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It's another one of those things for which there is no perfect size.

First plan A, external diffusion.

Assume 2700 RPM, 540 cubic inches, a VE of 0.95, and zero forward airspeed. Velocity through the intake ring would be approximately...

3" = 81 knots
4" = 45 knots

We really don't care about zero airspeed values, but it does illustrate the effect of intake area given constant flow. Less area means higher velocity, and that detail comes into play given forward airspeed, because for external diffusion, manifold pressure rise due to dynamic pressure is based on TAS in excess of that static value.

So, assume a 540 at 2600 RPM, same 0.95 VE, 230 TAS, 8500 ft, standard day. Further assuming no loss due to separation or other errors, available pressure before the filter or throttle body would be...

3" = 22.66" Hg
4" = 23.09" Hg
5" = 23.32" Hg

Recall I said "no perfect size"? Larger and larger simply offers diminishing returns. The difference between 3" and 4" rings is 0.43" Hg, while the difference between 4" and 5" is 0.23", and so on. You can push intake area up to get the last little bit of MP, but you have to fair the larger ring with an even larger external shape. You'll likely wind up balancing some additional power against some additional drag.

The engine is an air pump, so assuming same compression, timing, etc, the difference between 260 and 300 HP would be VE, volumetric efficiency. I used 0.95 above. Let's assume a hotrod motor does 100%. The result is a tiny loss of calculated manifold pressure (0.03"), as the static intake velocity is about 2 knots higher.

Plan B is internal diffusion, which requires a well shaped diverging duct inside the cowl. Velocity through the intake ring can be as high as freestream velocity, i.e. TAS. The slowing and expansion resulting in conversion of available dynamic pressure to increased static pressure takes place inside the duct, not out in front of it. Again, no perfect size. At 230 knots, theory says the intake area can be down around 2.54 sq inches, a ring diameter of 1.8". However, given the same RPM it will cost HP at a lower airspeed.

You can see those tiny little pitot inlets on some F1 racers at Reno. Plenty of internal cowl length when you have an ultra long propshaft extension. I used external diffusion on my -8 because it's a 4-cyl with no spool...barely enough length for a decent filtered airbox.
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  #4  
Old 06-25-2020, 05:21 AM
sailvi767 sailvi767 is offline
 
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I really should have studied more in college! It does sound like the current 3? intake might be ok. First engine run might be tomorrow with a test flight this weekend. Will report back on how it seems to do.
George
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  #5  
Old 06-25-2020, 07:09 AM
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Toobuilder Toobuilder is offline
 
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To look at it differently, a 540 at 100% VE only flows 421 CFM at 2700 RPM - pretty modest. Pull that back to the more realistic 85% and you're down to 358 CFM. Can't find any hard flow data on a 3" bellmouth inlet, but considering the RSA 10 servo has a throttle shaft and thick butterfly directly in the airstream, I don't think the open 3 inch inlet is the LIMFAC. And then there is the significant Q provided by the forward speed in cruise - makes the size even less of a concern. That is, unless you intend to do a zero airspeed, "hover" maneuver at an airshow!

For the record, I have a 3" inlet on my Rocket, but also have a well crafted internal diffuser FAB. First Flight #2 (new engine/prop) is tomorrow, so I'll be happy to provide results before too long.
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Last edited by Toobuilder : 06-25-2020 at 07:12 AM.
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  #6  
Old 06-29-2020, 08:12 PM
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Toobuilder Toobuilder is offline
 
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Some real dirty data for you today:

With the engine off, the MP showed 26.9 (2630 MSL field elevation, 82 degrees OAT)

On TO roll (about 35 knots), MP showed 26.1 (full throttle)

A pass down the runway at 200 KIAS showed 28 inches MP.

This is a full airbox, 100% filtered, drawing through a 3 inch inlet ring.
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  #7  
Old 06-29-2020, 08:50 PM
scsmith scsmith is offline
 
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To expand on the benefits of external diffusion that Dan discussed, it is worth noting that typical high-bypass fan engine inlets are oversized by about 30% at cruise speed.

Take-off thrust is crucially important for jets because it drives take-off field length, and this has a big influence on inlet design. But very often, engines and inlets optimize out nicely in a balanced design such that both the take-off condition and the "top of climb" cruise condition are both equally critical. As Dan pointed out, take-off wants to have a big inlet to gulp air. Cruise condition can have a much smaller inlet, but for well-designed inlets, making them too big and spilling up to 30% of their air just does not cause any external spillage drag, so there is no down-side to oversizing.

That last statement deserves emphasis on the 'well-designed'. The key is to have a generous lip radius on the inlet so that it is tolerant of big changes to the local onset flow angle without flow separation. Look to jet engine inlets on modern transports to get a rough idea of how to shape your intake.
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  #8  
Old 06-30-2020, 06:07 AM
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Quote:
Originally Posted by Toobuilder View Post
Some real dirty data for you today:

With the engine off, the MP showed 26.9 (2630 MSL field elevation, 82 degrees OAT)

On TO roll (about 35 knots), MP showed 26.1 (full throttle)

A pass down the runway at 200 KIAS showed 28 inches MP.

This is a full airbox, 100% filtered, drawing through a 3 inch inlet ring.
Dirty perhaps, but if I assume 2400 RPM, 0.9 VE, and the values above, theory predicts 28.01"

So what was the actual RPM?
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  #9  
Old 06-30-2020, 08:17 AM
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Toobuilder Toobuilder is offline
 
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Takeoff was 2700; low approach was 2600.
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
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RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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  #10  
Old 06-30-2020, 08:47 AM
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rocketbob rocketbob is offline
 
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I don't see the point of hand wringing over a 3" inlet when you have intake runners that have a 1.75" ID after the airflow makes two 90 degree bends from the servo thru the sump.
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