shorter engine longer prop?

[wolf.]

In restoring my VE I will install a BMW Motorcycle 2 cylinder engine (which is being sucessfully used since 20 years, up to 125hp out of 178lbs installed). The engine is about 8 inches shorter than a O-200. If I have my prop station 8 inch advanced I can install a longer prop (the engine being geared a longer prop would make sense)

Now my question, does a longer prop make sense on the VE, if yes how much longer can it be??

 

wolf.

[TMann]

A shorter engine? I wonder how that will effect your CG.

[Lynn Erickson]

In restoring my VE I will install a BMW Motorcycle 2 cylinder engine (which is being sucessfully used since 20 years, up to 125hp out of 178lbs installed). The engine is about 8 inches shorter than a O-200. If I have my prop station 8 inch advanced I can install a longer prop (the engine being geared a longer prop would make sense)

Now my question, does a longer prop make sense on the VE, if yes how much longer can it be??

 

wolf.

What would you gain by a longer prop? the prop still has to drive the same aircraft at the same speed with the same HP. if the prop is longer then it may need to be thinner or different blade shape. what does being geared have to do with it. the faster vari ezes are running 3000 + RPM and they use shorter props

[wolf.]

What would you gain by a longer prop? the prop still has to drive the same aircraft at the same speed with the same HP. if the prop is longer then it may need to be thinner or different blade shape. what does being geared have to do with it. the faster vari ezes are running 3000 + RPM and they use shorter props

the faster VE are using direct drive engines and you have to run them high for max power, 3000+ needs a short prop to remain efficient. (Actually I was still not able to find out if the short prop recomanded by Burt is due to the high engine RPM of the O-200 or the chance of Propstrike)

 

But a geared engine can turn the prop slower and a bigger prop with higher torque is more efficient (especially if it is a CS) (propulsion is impulse (v x m) so more air at lower speed is more impuls, that's why jet engines are inefficient at lower airspeeds)

 

SInce the VE suffers from high weight and back CG any engine which is lighter and shorter will be of benefit, my question remains -8 inch in prop station how much bigger can the prop be???

 

wolf.

[ZUCZZ]

Suppose one must look at the efficency of a longer prop ... at what point would the tip reach max effective velocity?

 

There are some prop programs that can answer this, sorry I have no links to these.

[jdubner]

There are some prop programs that can answer this, sorry I have no links to these.

You could play with Ed Dokus' "PropDesign" program which I ported to MS-Windows many years ago.

 

http://mail2600.com/PropDesign/index.html

 

It's somewhat simplistic but at least the price is right (free!).

 

--

Joe

[wolf.]

Guys I need the prop size which the VE airframe geometry would allow if I would retract the prop station by 8" (Don't want to take the VE of my friend and and tilt and measure (rather cumbersome) once I know the max I will talk with the engine designer (since I am opting for a light geared engine I will install one of the new light CS props anyway)

 

wolf.

[Lynn Erickson]

Guys I need the prop size which the VE airframe geometry would allow if I would retract the prop station by 8" (Don't want to take the VE of my friend and and tilt and measure (rather cumbersome) once I know the max I will talk with the engine designer (since I am opting for a light geared engine I will install one of the new light CS props anyway)

 

wolf.

why would anyone know this? there aren't any vari ezes built this way. if your engine is lighter why would you not put the engine back farther and the prop where it is designed to go? with the prop forward it may have more interference drag caused by turbulence of the wings and fuselage.

[Edge 513]

SInce the VE suffers from high weight and back CG any engine which is lighter and shorter will be of benefit, my question remains -8 inch in prop station how much bigger can the prop be???

 

wolf.

I wasnt aware of this 'suffering'' but here is the reason.