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Post  gkamysz Sun Jan 07, 2024 12:58 pm

Wing boundary layer suction to engine intake? I was wandering the net and stumbled in to this. Of course, I've seen various similar topics in research papers, but this is the first time I've seen it on a model.

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Post  rsv1cox Sun Jan 07, 2024 2:45 pm

Interesting. It's like an effective way to turbo charge.
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Post  balogh Mon Jan 08, 2024 12:28 am

Bob,

While this suction of the air from the airfoil top may in principle help the lift of the wing, I doubt this works as a turbo charger from the engine's perspective.

I think it is like choking the venturi with your finger, and given the static pressure of air at the airfoil top being less than the air pressure in the fuel tank,  helps fuel suction by  increasing depression in the venturi, but will allow engine performance increase - if any-  with SPI engines only, where the loss if air intake by depression in the venturi is compensated by additional  air intake through SPI under the piston skirt?
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Post  GallopingGhostler Mon Jan 08, 2024 1:30 am

Article is 73 years old. That was during the infancy of glow engine flight, some ignition engines were still in use. I imagine they were looking for ways to increase an engine's marginal performance. Cox IMO changed the game as Leroy Cox's higher performing half A engines at lower prices helped to dominate small powered F/F.

Later free flight competition used engines with improved consistent performance.

With all the improvements through development, things seemed to have morphed now into cook book (proven designs and techniques).

Any, my Two Cents Tired w/ Coffee Read Clean my glasses lol!
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Post  balogh Mon Jan 08, 2024 2:50 am

George,

just for my own education, I did a simple calculation to check what impact the air sucked away by the engine from the boundary layer on the top of the airfoil may be.

Assumptions: (sorry for the SI units instead of imperial ones, I know you prefer the latter)

Engine swept volume 1.5 ccm
Engine speed 15k rpm = 250/sec
Wingspan 1m = 100cm
Laminar layer thickness of air over main spar where air suction pinholes for the engine are located: 2mm = 0.2cm
Flight Speed of airplane: 10m/s = 1000 cm/s

With the wingspan, thickness of laminar layer and flight speed, the horizontal airflow through the laminar layer where the foil is tapped for the engine air intake is 1000 cm/s (speed) * 0.2cm (layer thickness) * 100cm (layer length=wingspan) = 20,000 ccm/s..
The engine air suction is 1.5 ccm (swept volume) * 250/s (suction cycles per second) = 375 ccm/s

The % of air sucked away by the engine (375ccm/s) from the airflow through laminar layer (20,000 ccm/s) is 375/ 20,000 = ~ 1.9%....I do not believe this 1.9% will have a major impact in enhancing the lift by the airfoil, so even if my calculation is very approximative, I am not surprised that such experiments 73 years ago did not conclude in a disruptively new concept of flight aerodynamics.

On the other hand the engine (not yet with SPI 73 years ago?) was probably starving air intake and must have seen some performance loss...
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Post  rsv1cox Mon Jan 08, 2024 7:13 am

balogh wrote:Bob,

While this suction of the air from the airfoil top may in principle help the lift of the wing, I doubt this works as a turbo charger from the engine's perspective.

I think it is like choking the venturi with your finger, and given the static pressure of air at the airfoil top being less than the air pressure in the fuel tank,  helps fuel suction by  increasing depression in the venturi, but will allow engine performance increase - if any-  with SPI engines only, where the loss if air intake by depression in the venturi is compensated by additional  air intake through SPI under the piston skirt?

Thinking about it, it's more like a pull up effect than a push?
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Post  balogh Mon Jan 08, 2024 7:29 am

Bob,

regarding the wing uplift, the diversion of some of the airflow from the top of the airfoil in principle adds to the lift effect of the airfoil by reducing the static pressure of air on the top versus the overpressure at the bottom of the airfoil.

The air removal from the airfoil top spar area, if done by the engine suction, is very marginal, and while some associated uplift exists in principle, I doubt that it has a really measurable effect .

As for the engine power, because the air inflow into the engine is reduced by the depression at the airfoil top, and by the throttling effect of the plastic tube extension, the engine will run in relative air starvation, and the need for a certain stoichiometric air-to fuel ratio dictates that the needle valve needs to be closed and fuel intake reduced...so the engine will certainly not outperform a normal operation case.

Just my humble opinion...
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Post  rdw777 Mon Jan 08, 2024 7:36 am

Looks to me like the forward movement of the wing would conflict with the engine drawing air into its intake……Like blowing across the top of a straw to raise water in a drinking glass….An inefficient venturi…Interesting thought though….
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Post  balogh Mon Jan 08, 2024 7:44 am

Exactly Robert, in a free air stream the total pressure - which is the sum of static and dynamic pressures - remains constant (as per the Bernoulli law) , thus the faster the air moves, the lower its static pressure and more the depression inside the air-stream..your analogous example with the straw is perfect, the old style, non-pressurized perfume dispensers worked the same way, blowing air across the free end of the straw will cause the fluid at the straw bottom to rise.....and as we all know, the Pitot tube measures the airplane velocity by the same principle...
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Post  rdw777 Mon Jan 08, 2024 8:18 am

Nice Very Happy …One thing we have to give the designer credit for is potentially applying to a model and testing it out….. Models (not only model airplanes) have been a wonderful tool to test ideas with much cost or risk…Love that part in our own little corners…

I appreciate the calc’s you did on volumes and flows that show the real story Andras…Beyond me,….. But I keep few back pocket formulas for airframes, props, and electrical , etc….Great tools for predicting how things may go Very Happy
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Post  GallopingGhostler Mon Jan 08, 2024 10:35 am

András, thanks for your research. I took a fluid dynamics course 47 years ago, still have the book somewhere, it speaks of the Bernouli effect, had equations and discussion as an inttroduction into airfoils, how a flat plate reacts (like our inexpensive glider wings), and etc. (Fluid dynamics was a favorite subject of mine, even took a graduate fluid dynamics course for credit as an undergraduate.)

Air accellerates over the curvature of the upper wing surface, dropping the air pressure, which causes lift.

I cannot see the merits of putting an engine's intake along the boundary layer of the upper wing. For one, his long tubing to that location creates suction loss to the engine due to pipe losses in his tubing. His experiment sounds more like a high school science project.

It is over such a small area, my gut level feeling tells me that any improvements, as you point out are very small. Some experiments were done say running a bunch of evenly spaced holes along a wing with an airpump pulling air through them creating a vacuum, to improve lift.

Regardless, goes against the KISS principle (Keep It Simple, Stupid).

We use bladder systems and modified venturis to provide greater fuel flow to increase combustion strength in an engine for greater power and speed.

Reducing venturi inlet air pressure akin to flying at higher elevations goes against wisdom.

Article provided a teen or young man with impetus for higher learning, to think outside the box. (They didn't have Internet.) Books and magazine content filled one's mind through reading. Such articles had more value in creating a young scientist, who'd move on to greater things as he aged.

For me now, this is my impetus. Very Happy

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