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Benefit of a Clean Plane


GaryP1007

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According to Bob Kromer, former Mooney president and test pilot, having clean leading edges and back a few feet really is good for a few knots as it keeps the laminar flow, well, laminar. Anything that increases the micro-turbulence that can cause separation of the airflow will create drag. 

Edited by Jeff_S
clarification and grammar correction
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Like Byron, I’ve seen notable reduction in indicated airspeed in rain.  However, when I had the plane painted, from chalky white and porous paint to new gloss, I was disappointed not to see any notable increase in speed.  I would think this is analogous to wax.  

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3 hours ago, Jeff_S said:

According to Bob Kromer, former Mooney president and test pilot, having clean leading edges and back a few feet really is good for a few knots as it keeps the laminar flow, well, laminar. Anything that reduces the micro-turbulence that can cause separate of the airflow will create drag. 

Bob says clean and waxed above and below leading edge to wing spar is good for 4 knots.

I did a before and after flight and found 3 “free” knots.

 

-Seth

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A couple of ways to look at this...

From a post regarding aerodynamics and how thick the ‘boundary layer’ is with Prof. Erik in another thread....

 

 

Hmmmm...

Boundary layers measured in inches...! :)

Air is a strange fluid.

The stuff has been unchanged for millennia, but it’s secrets are still not very well known...

Removal of the boundary layer is what makes a convection oven interesting.  Cooking a big turkey faster by forcibly moving the stagnant boundary air around the bird...

Boundary layers of slow moving high viscosity fluids are probably measured in microns...

This could keep the door open for @Piloto piloto’s lubrication theorem...  WD40 or RainX used for minimizing air boundary layers?

The strength of the boundary layer is incredible.  Essentially, it is responsible for the friction known as parasite drag.

Imagine for a moment... a wing being even more efficient by cutting through the air with less drag... like a hotter knife cutting through butter at a higher rate...

A simplified version of parasite drag is done by comparing flat plates of cross sectional areas...  adding an inch above and  below the plane’s known dimensions accounts for the boundary layer tax...

Since the classic CSA comparison already takes into account the boundary layers, removing the boundary layer would be akin to cutting an inch off the dimensions in all directions!  A small, but effective way, that Al Mooney used to make his plane faster and more efficient, less draggy, than the others.... but in this case, the cabin footprint doesn’t get smaller, just its drag gets smaller... :)

Thanks Erik, for the applied transport phenomena reminders!  :)

Having a cup of coffee with the prof... makes you feel smarter?

Best regards,

-a-

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Another thought of rain slowing the plane down...

Rain is a good thang, in some areas, but not so much in aviation...

 

What slows the plane down is impacting molecules... air and water.....

Some impacts are ideal (aka elastic), and energy isn’t removed during the collision.

 

Erik was describing the boundary layers of the wing’s surface.

Byron is describing the same phenomena but that of water.

 

If a plane is carrying gallons of water on its surface, those gallons need to be accelerated to the flying speed of the airplane.

 

I know it takes a fare amount of energy to move a gallon of milk around the kitchen.  It takes a whole lot more to throw it 150 knots through the air!

 

In Byron’s example, mixed with Erik’s boundary layer discussion...  we visually see that there is a big boundary layer of water attached to the wing.  This giant layer has both mass and momentum that need to be conserved...

in other words... impacting heavy droplets is going to slow you down and probably remove some paint off the sharper edges of the wing...

So, include these three items in your discussions on friction....

  • Mass
  • Momentum
  • Plastic impact (deformation) vs. ideal or elastic impact...

Fun stuff you think about as a Private pilot flying a real airplane.  PP thoughts only...

Best regards,

-a-

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A third aspect...

Power generated and measured by MP kind of gets affected by the amount of water molecules entering the engine.

There is a fair amount of thermodynamic energy sapped from the engine by heating water into steam and dumping it out the exhaust.

 

Efficiency of the prop also gets affected by impact with water drops, and the boundary layer of water remaining behind...

 

PP though only, not an aviation engineer...

Best regards,

-a-

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The fourth aspect is called...

The golfball dimple effect.

A dirty plane is like a golfball with dimples.

Done properly, the uneven surface, plays to the boundary layer challenge...

The boundary layer still exists, but is broken up some by surface geometry....

 

Find a way to eliminate the boundary layer of air, and you may find a way to improve the dynamics of friction and drag affecting your plane...? :)

 

 

PP though only, not an aviation engineer...

Best regards,

-a-

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Competition sailplane pilots plot ( or the mfd does)various polar graphs plotting l/d for various wing conditions.Glider wings are laminar thruout there cord but suffer greatly from being wet or dirty.Mfd have actually plotted "buggy" Polars...i.e. Leading edge bugged up and it's amazing the performance drop off.Also with wet wings...not as bad but still showing a decrease in still air gliding distance.So there is a real benefit to a clean wing

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