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Post  fredvon4 Fri Mar 10, 2017 12:11 pm

Some many decades ago I saw a Full sized GeeBee with giant radial engine and a propeller that barely was larger diameter than the engine and cowl

I asked, and was told, that a propeller PULLS and does NOT push thus it cares not what is behind it

Until that old gent reminded me..I had the notion that just the exposed prop tips were doing all the work....

So in my mind for the abrupt fire wall... I tend to think it just creates aircraft drag--- but has little to no effect on how the engine will PULL the craft through the air
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Post  balogh Fri Mar 10, 2017 12:38 pm

fredvon4 wrote:Some many decades ago I saw a Full sized GeeBee with giant radial engine and a propeller that barely was larger diameter than the engine and cowl

I asked, and was told, that a propeller PULLS and does NOT push thus it cares not what is behind it

Until that old gent reminded me..I had the notion that just the exposed prop tips were doing all the work....

So in my mind for the abrupt fire wall... I tend to think it just creates aircraft drag--- but has little to no effect on how the engine will PULL the craft through the air

Yes, Fred, the wide and blunt nose creates more drag...but the useful thrust of the engine is also impaired by a wide, abrupt nose...imagine mounting the engine on a large vertical wall, that would also not pull it forward too much, even if there were no friction under the wall. The reason it would still pull it a little bit is because air is compressible.

The forces acting on the nose by the airflow from the prop will partly offset the pull of the engine as the 2 forces are internal in the same dynamic system and act against each other. The smaller the prop area coverage of the engine nose, the less pull the engine can effectively generate, I guess.

Sorry if I explained here something that you also certainly know, I guess Brad explained the same in his post in this thread on Jan 17th 2016
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Post  fredvon4 Fri Mar 10, 2017 2:59 pm

I took my feeble brain a very long time moving from model airplanes to rockets to wrap my head around just what Thrust is/was

And even harder to conceptualize that AIR is a fluid medium that flows

If you google my full name you might find a fluid dynamics professor who wrote about the Gortler vortices in boundry layers
https://www.researchgate.net/profile/J_Floryan/publication/23601664_Stability_of_Gortler_Vortices_in_Boundary_Layers/links/00463530fa665965bd000000.pdf

But in the end I was taught we can pull or push something to make it move
The needed energy to move an object (more to the point efficiently) is well beyond the scope of these musings...

A perfect Propeller -at 100% efficiency with a (diameter does not matter((except to the engine))) 6 inch pitch, will always move forward exactly 6 inches for every revolution-- no matter how close or far from a rear ward object...Assuming zero drag... and of course any drag is actually what makes a propeller less than 100% efficient

... We can create close to, but never true 100% efficient propellers because AIR is a compressible medium...and gravity, and drag, and angle of attack ----but these limitations do not change the science and physics of what is happening....


Take the GeeBee I described earlier... If that plane had a propeller the Exact same diameter as the Cowl/ radial engine, it would still pull forward and fly
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Post  balogh Fri Mar 10, 2017 3:13 pm

Thanks Fred I guess we talk about the same thing though my English vocabulary in fluid mechanics - and in general- is certainly not flawless. lol!
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Post  FlyingCox Sat Mar 11, 2017 8:52 am

I tend to think of the prop in it's true aspect. It's simply a wing which is spinning. In the classic explanation of an airfoil a low pressure is created above or in this case in front allowing high pressure below/behind to push up or in the case of a prop forward obiously you'd have a tough time getting your boat to move by blowing into your own sail so there will be some losses due to a large blunt firewall. I'm not trying to insult anyone so I hope my comment is received that way. Just trying to lay out a different prospective if it helps anyone.
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Post  gcb Sat Mar 11, 2017 1:58 pm

fredvon4 wrote:Take the GeeBee I described earlier... If that plane had a propeller the Exact same diameter as the Cowl/ radial engine, it would still pull forward and fly

If you take a closer look at the GB cowl you will find that there is lots of room to flow air THROUGH that area to cool the engine...it is not a solid wall.

George
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Post  FlyingCox Sat Mar 11, 2017 3:32 pm

Also very true of course the parasitic drag of the engine has to be still quite taxing. Speaking of which I wonder if the the air being heated as it's passing through the cowl causes the advantage of the meredith effect which would further reduce the effect of aerodynamic resistance? That would be a really interesting design feature.
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