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Cowl Flaps


Yetti

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So the F has straight cowl flaps and it looks like the J has a bump for the exhaust pipe.    The cowl flap is a pretty simple piece of sheet metal.     Would it be worth making a new one that included the bump for the exhaust pipe?

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2 hours ago, Yetti said:

So the F has straight cowl flaps and it looks like the J has a bump for the exhaust pipe.    The cowl flap is a pretty simple piece of sheet metal.     Would it be worth making a new one that included the bump for the exhaust pipe?

The entire cowl and engine airflow is different between Fs and Js, so I wouldn't assume it would be an improvement unless you changed all of that.    What problem would be solved by adding the bump?

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18 minutes ago, EricJ said:

The entire cowl and engine airflow is different between Fs and Js, so I wouldn't assume it would be an improvement unless you changed all of that.    What problem would be solved by adding the bump?

External skin should see improved airflow with less things sticking out.

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6 hours ago, carusoam said:

If copying a good design helps...

Go O, no cowl flaps, and great CHT control...

Best regards,

-a-

https://www.airmodsflightcenter.com/advert/mooney-n904hs

So that would be the question are the bumps more aerodynamic than the flat open flaps.   Air volume wise I can see the bumps on an O moving as much air as as an open flap.   But since you can close the flap and only one exhaust pipe an F with closed flaps would be better at least on the port side since there is no exhaust pipe.

Does the O have the Always open flap in the center? It does not look like it in the picture.  But then there are screw heads holding the "no flaps" with bumps in place on the O.    Are the O no flap flaps aluminum or fiberglass?

Not sure if it happened, in the Howard Hughs movie they made one of his race planes.  He flew it and then told them to rebuild it with flush rivets for more speed.   Should we start talking about countersinking the screws? 

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Oh thank NASA I don't have to build a wind tunnel and my guess about cowl flaps is correct. " Exit location showed a large effect on drag.   By adding a diffuse to the inlet,pressure recoveries of up to 95% were demonstrated. That improvement alone can eliminate most of the need fora cowl flap and thereby save up to 3 % of total aircraft drag."

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980214918.pdf

Next topic cooling drag

 

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8 hours ago, Yetti said:

It looks like they put a Nacel vent thingy on the middle cowl.  I would guess this is used to create a vacuum in the low pressure area of the cowl?   Or is this ducted to the accessory case area?

On the R model? It’s the filtered air intake for the engine. 

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O Cooling Airflow...

  • in through the nostrils...
  • down through the cylinder fins...
  • out around the exhaust pipe bumps...
  • No additional airflow control beyond good silicone seals...
  • one mod... fairy/pixie hole in front of cylinder #5... co pilot side nostril opening...

O minimized drag...

  • Smoothly curved fiberglass construction, carbon fiber in the more modern versions...
  • nostrils sized for airflow control... not too much excess air to handle...
  • all fasteners are quarter turn and flush machine screws...
  • A proper spinner is tightly fitting and smooth lines with the cowl opening... (not a TopProp spinner yet)

Is there room for improvement...?

There is plenty of excess cooling in the winter months at altitude... altering the cooling exit to close down the amount of air flowing through the cowl would hold some promise...

Would the added complexity net any speed gains? Would it be worth it?

Want to go a few knots faster in an O..? Raise the step... test the theory, by removing it... then visit with Takair regarding building a retract step...

PP thoughts only, the O is plenty fast for me... sort of... maybe...

Best regards,

-a-

 

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Notice the cowl openings in the picture.....

Sitting on the ground the airplane is sort of in a climb attitude thus opening is normal to the airflow for better cooling in climb.  Transitioning to cruise there is a less effective opening because the angle of the opening is more down.  So adjustable cowl is not needed.  This is the modern cowl design also used by Cirrus, Diamond and others.

Also notice the how deep the 'tube' is in the opening.  This keeps the airflow smooth as it goes into the cowl (as the air slows down) resulting in better pressure recovery and more mass flow.

As for the side duct..don't know but maybe a cooling duct for fuel pump or magnetos.  NACA ducts are not about scooping but smooth airflow and pressure recovery for intakes.

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3 hours ago, MikeOH said:

I see barely 1 knot when I close the cowl flaps on my '70 M20F.  Frankly, I just leave them open most of the time and don't have to monitor CHTs as closely.

What do you other M20F guys see for speed delta?

I see a few knots. In the summer I pretty much do what you do (...keep them open). I have seen an impact in the oil Temp if I close them at the wrong moment. In the winter I usually follow the POH and close the cowl flaps when at altitude. 

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I see barely 1 knot when I close the cowl flaps on my '70 M20F.  Frankly, I just leave them open most of the time and don't have to monitor CHTs as closely.
What do you other M20F guys see for speed delta?


Do your cowl flaps have a speed restriction? Mine do. 150 MPH (75F).

35cf5a8cf65437e40fb6a0d0d461a7c5.jpg


Sent from my iPhone using Tapatalk Pro
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