Hi NG...

I have a question for you propeller experts.
I´m making a reduction drive for my G-62 and I was thinking about making a 4:1 ratio. According to a prop calulator (no... not ThrustHP...) I found on the net the engine will be able to swing a 33,6x37 4-blade prop at 2000rpm... The program predicts that the max speed will be in excess of 70mph, but I don´t know how accurate the software is.
Does anyone know if a setup like this will work at all??? I´m not sure that a prop spinning at such low rpm will produce any thrust/speed at all... But then again its getting close to the size and rpm used on ultralights/paragliders, and it seems that they fly quite well...
The program I´m using is called "AeroDesign Propeller Selector" and is freeware...

Looking forward to your words of wisdom...

I'm no aerodynamic engineer but,

It's a great idea to use a reduction drive, to maintain the engine at it's design RPM range and to give it more torque to swing a larger and "heavier", aerodynamically speaking, propeller. With a higher pitch , at least in theory, it's possible to reach high speeds, but not 200Mph with a 37" pitch at 2000RPM, that would require at least 5000RPM. you can increase further the pitch, but that would make things worse for the low speed performance. The prop which you said, would be stalled for sure at 25 degrees (which is it's angle of attack at .75 radius, given by the formula Atan(Pitch / (2 * pi * ([email protected]))) in static conditions, so the thrust developed by it in this consdition would be very small, thus reducing a lot the takeoff acceleration and increasing the distance required for it.

For high speed flight, it's better to have a high pitch prop, revving at high RPMs, but there will be a compromise in the takeoff acceleration. Take a look at a Ducted Fan unit and see how high is the pitch on the fan blades, and they swing at 20000 plus RPM. The low speed blade stall is reduced by some amount by ducting the air flow, but airplanes equiped with them still require a long takeoff run.

But, it would be an interesting experiment, although the plane, even if perfectly clean aerodynamically, wouldn't exceed 90MPH with the prop you mentined at that RPM.

Thanks for the reply...

The reduction drive will ultimately be used for a 1:5 scale Hawker Typhoon... Thats the reason for the 33,6in dia prop, as this is the scale size.
I´m well aware of the problem of a stalled prop. My software suggests that up to a speed of 33mph the prop will be partially stalled, and that the prop will stop producing thrust at 75mph.
What I´m most concerned about is that the prop, when being stalled, will not even make the plane move... I guess that it will be like starting your car in 5th gear....uphill...
But on the other hand... When the plane starts accelerating, the thrust will increase steadily during the takeoff, as the prop gets out of stalled condition and should provide good speed/thrust just before takeoff when it´s most needed. If the 33mph the software suggests is accurate, it would not be much of a problem, as I don´t think I´l will be able to keep the plane flying at that speed anyway.
I have also thought about reducing the pitch slightly and run the engine at higher rpm. With the current setup the engine will be spinning at 8K on the ground, but originally being a chainsaw engine I don´t think that it will kill the engine going 10K instead, which is where it will develop the most power.
The complete model will probably weigh somewhere between 33-40lbs.

Even when stalled the prop will produce thrust. It just won't be as much as when it is moving well. Also the stalling of the prop may prevent the engine from coming up to the full rpm at a standstill so you may hear the engine pick up rpms partway through the takeoff roll.

What you are finding here is exactly why the full scale aircraft changed over to variable pitch props as soon as it was practical to do so.

Using such a large prop on a scale model will probably also bring with it the torque and P effects of the full sized aircraft. In other words you will have to test for and watch out for the dreaded low airspeed power on torque roll during touch and go's and quick lift offs.

Maybe I´m a fool, but I would be very pleased to have the model fly as much as possible like the original... And as far as I know the fullsize also has huge amounts of prop torque to deal with... So I guess that it will be the ultimate scale experience.
Based on your replys I think I will make the reduction drive at a 4:1 ratio and make up a 33,6x37 4-blader, and test the combination on an old towplane first. Just to see if it will fly at all.

Thanks

Not a fool by any means. Just up to the challenge of a new experience. If you can find the pilot's notes for some of these early planes like the Tempest and Mustang it may offer some hints on what to expect and how to treat it. Certainly this won't be a model that takes kindly to being yanked off the ground as we usually do.

Good luck with this interesting sounding project.

Yes, the prop when stalled produces thrust, but the load on the engine is higher, so the RPM will be lower and then the thrust even worse. But yes, it will takeoff but will take a long while...

It's an interesting project BTW.