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  #31  
Old 04-18-2018, 10:41 AM
Tom Martin Tom Martin is offline
 
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If I may rephrase; a vacuum could be created with the passage of the faster moving airstream, relative to the slower air mass exiting from the cowling which could be at a lower pressure then the passing airstream.
When I did my differential air pressure tests I did not test the relative pressure exiting the cowling relative to the airstream around the bottom of the fuselage. I will refrain, in the future, from using speculation.
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Last edited by Tom Martin : 04-18-2018 at 10:55 AM.
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  #32  
Old 04-20-2018, 06:57 PM
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Onewinglo Onewinglo is offline
 
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Quote:
Originally Posted by Tom Martin View Post
Over the last 20 years or so I have spent a lot of time on cooling exits, always trying for a balance between cooling and speed.

Lessons learned

1. Not all engines, even those with the same designations, cool the same
2. Until you get the cowling inlets flowing properly, gradually smoothing the inlet area of the plenum and plugging all holes, there is not much use playing around with the outlet
3. Spinner gap seals help the system work properly, and yes the cooling system is just that, from inlets to outlet, a system.
4. the goal at the outlet is to get the air exiting the cowling to align with the outside air.

Number 4 is where I see a problem with the sub cowling outlet in this thread.
the outlet actually forces the air down at an angle to the relative airflow surrounding the aircraft. This will add drag and likely negate any benefits of the part. I would suggest cutting the "bump" out of the part and glassing in the outlet sides so that they are parallel to the airflow. This will get your outlet air travelling in the same direction as the relative airstream. If this works you can then reduce the outlet size to get the outlet required for hot day climbs, or to add a cowl flap of some kind so that you can manually adjust cooling in the air.
You will know if you have things right if the oil from the breather runs straight down the belly and remains attached to the fuselage the entire way.
I have found that extending the bottom of the cowling exit at least two inches aft of the firewall greatly aids in getting the outlet air flowing in the proper direction.
The exit air is pushed out of the cowling due to the differential pressures within the cowling. I have no proof, but I feel that if you get your outlet air flow in the right direction, the outside air may even help to pull the air out.

I have a cowl flap on my aircraft but seldom use it now that have the system flowing properly. If you live in the deep south then you will likely need some adjustment if you are looking for maximum speed/cooling.
Tom,
Thanks for sharing your experience on this issue. I'm sure you are correct about the exit air should be aligned with the slipstream. After tuft testing and pondering everyone's suggestions the concept is getting much clearer. Our current plans are to fly for a while and evaluate a modification later.
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  #33  
Old 12-28-2018, 07:52 PM
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Onewinglo Onewinglo is offline
 
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Default Revised RV-8 Sub-Cowl

I tufted our RV-8 and flew with the modified sub cowl a few days ago. The point of the sub cowl is to reduce the exit opening to raise CHT and oil temperature, which it has. (Our XP360 was running a bit too cool). The below video indicates the exit flow is not perfect but better than the first attempt.
https://vimeo.com/308621727

Here is a video of the first attempt. Note the large lip on the trailing edge of the opening which caused flow separation.
https://vimeo.com/308619285
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  #34  
Old 12-29-2018, 02:06 AM
rv8ch rv8ch is offline
 
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Default really good flow

Quote:
I tufted our RV-8 and flew with the modified sub cowl a few days ago. The point of the sub cowl is to reduce the exit opening to raise CHT and oil temperature, which it has. (Our XP360 was running a bit too cool). The below video indicates the exit flow is not perfect but better than the first attempt.
https://vimeo.com/308621727
That looks like some really good flow. Any feeling for impact on speed?
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  #35  
Old 12-29-2018, 08:14 AM
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Onewinglo Onewinglo is offline
 
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Default Speed Increase? - Maybe

Mickey,
Nothing conclusive about speed yet. I've only made two runs at altitude due to weather. One flight due North and one South.
Normally our -8 yields 185 mph true, at 2500 rpm, at 8000 DA.
Flight with modified sub cowl yielded 187 mph true, at 2500 rpm, at 8000' DA.
More importantly our temps increaseed as desired. Oil temp was 172* and CHTs 295 - 320*. I forgot to record the OAT but it was a cool Louisiana day, 60* F at the airport.
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  #36  
Old 12-29-2018, 08:34 AM
rv7charlie rv7charlie is offline
 
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I feel a bit silly chiming in, with actual aero engineers participating, but...

No one's mentioned the down-turned exhaust pipes' effect on flow out/around the opening. Is my failing memory deceiving, or hasn't there been testing that shows that and exhaust column exiting at an angle to the free-stream causes a significant increase in drag? (IIRC, the penalty for parallel exit flow is increased cockpit noise.)

Likely a minor detail, but what about filling in the sides of the afterbody? How is the air behaving in those 'corners'? Any chance that it flows back into the corners, then tries to follow the curve down and then horizontal toward the pipes, at an angle to the freestream?

Charlie
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  #37  
Old 12-29-2018, 04:04 PM
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Onewinglo Onewinglo is offline
 
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Default Cut the pipes?

Charlie, I agree the pipes in the slipstream likely cause drag. I sent Clint at Vetterman an email asking about cutting the tips at an angle parallel with the sub-cowl opening.
I want the exhaust pointed away from the belly to reduce noise, but the tips may be a bit long as they are. I look forward to hearing Clint's opinion.
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  #38  
Old 01-01-2019, 12:18 PM
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gmcjetpilot gmcjetpilot is offline
 
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Quote:
Originally Posted by rv7charlie View Post
I feel a bit silly chiming in, with actual aero engineers participating, but...

No one's mentioned the down-turned exhaust pipes' effect on flow out/around the opening. Is my failing memory deceiving, or hasn't there been testing that shows that and exhaust column exiting at an angle to the free-stream causes a significant increase in drag? (IIRC, the penalty for parallel exit flow is increased cockpit noise.)

Likely a minor detail, but what about filling in the sides of the afterbody? How is the air behaving in those 'corners'? Any chance that it flows back into the corners, then tries to follow the curve down and then horizontal toward the pipes, at an angle to the freestream? Charlie
Great question and you are right. The drag you talk of is called "interference drag". It is low speed air mixing with high speed air at angles. The mixing is drag. However as was said there are practical issues. If you have the pipe exiting parallel and near to the belly it can cause not only noise but structural damage.

1) The exhaust will cause noise a thumping into the cabin. People find it very objectionable.

2) Sonic and pressure pulse will vibrate and fatigue the structure, causing cracks and loose rivets.

3) On the Pro side, 4 into 1 with 2/125 collector exiting parallel ti free air-stream can produce thrust.

4) The down turn pipe minimizes or negates 1 through 3 (good and bad).

You can minimize 1 and 2 by dampening the belly with sound deadening liner in the cabin and placing a heavy plate on the belly over the skin. All this adds weight. Nothing is free.

Note Augmentor Tube - The augmentor tube uses the velocity of the exhaust gases to produce a low pressure on one side of the engine that helps pull cooling air through it. There is a large tube (augmentor) with curved bell mouth on inner cowl side. The exhaust pipe dumps into the augmentor tube entrance with cowl cooling air... The Piper Apache twin I owned had this. It clearly speeds up the cowl exit air, which induces better cooling. However I read it provides some thrust as well? The Pipe augmentor tube was buried in the nacelle and fairly aft facing. I rationalize air coming out as fast or faster than air stream and parallel to it, the better. The RV does not have room to develop an augnemtor tube, or does it?
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Last edited by gmcjetpilot : 01-01-2019 at 12:32 PM.
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  #39  
Old 01-02-2019, 02:14 PM
Marc Bourget Marc Bourget is offline
 
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Proper design of an Augmenter Tube - one factor is sufficient length that an exhaust pulse is still in the tube when the next cylinder's pulse enters the Augmenter Tube, denying a chance for reverse flow.

# of cylinders and operating RPM are relevant factors to length

FWIW
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  #40  
Old 01-02-2019, 07:41 PM
scsmith scsmith is offline
 
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Default exit pressure must match local exterior flow at exit

From Tom Martin:
" I have no proof, but I feel that if you get your outlet air flow in the right direction, the outside air may even help to pull the air out."

The fundamental rule of subsonic exit flow is that the internal flow pressure at the exit must match the external flow pressure at the exit.

When you open a cowl flap or have a diverging exit angle, you are creating an area of lower pressure in the external flow, which yes indeed helps extract more flow through the internal flow path, because you are exiting into lower pressure. The penalty for this is the increase in frontal area needed to create the accelerated external flow, and the likelihood that exiting internal flow may be at a lower velocity than the external flow.

When you have an exit in an area where the external geometry is tapering or sloping back towards smaller cross sections as you go downstream, the other flow is slowing down, pressure increasing, and you are exhausting into an area of elevated pressure compared to the free stream. The internal flow at the exit is therefore at a higher pressure, lower velocity than it could have been. In small doses, this region of decelerating flow, with rising pressure, can be good - the pressure recovery on the aft-facing slope provides some thrust. But if the pressure gradient is too adverse (pressure rising too fast) the boundary layer will separate and you will have a separated flow region trapped in the area where the body is getting smaller, causing pressure drag - which is a way of saying that you are not getting the pressure recovery that you would have gotten had the flow stayed attached.

The general shape of the lower cowl is causing a flow acceleration (cross section area increasing as you go downstream) that peaks somewhere just ahead of the firewall, causing high velocity and low pressure. From that point, the flow starts decelerating to return to nearly free-stream velocity along the underside of the fuselage where the flow is in roughly the free-stream direction and no longer curving. This is one reason why the area near the flrewall is a particularly good place to have the cooling flow exit.

Note also that although the exit pressures must match, the exit velocities in general will not, because the internal flow has gone through other passages that have caused a loss of total pressure (pitot pressure) compared to the free stream flow. This is partially offset by an increase in temperature in the case of cooling flows. In a well-designed cooling flow process, it is possible to have the exiting cooling flow actually have higher velocity than the external flow at the exit, and can produce thrust.
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