1st PRA Open source prerotator project is done and ready for more testing.

All_In

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We have two prerotartor projects in the works. This first project completed by San Diego University senior engineering students used low tech car starter but they have created a two-speed ball transmission for low and high gear. All of the Solid work drawings, CNC code will be provided to either create your own or PRA will explore having them made for you. In either case, you will want to buy the parts from PRA as we plan on buying volume and holding it in inventory reducing the cost to members.

They are now creating a 2nd unit that we will install on an RAF our test platform with 30' blades for long term testing. You can expect 180 to 250 RRPMs as we are changing the 2nd gear ratio for even more speed.

If weight is a problem do not rush into building this model. Another college San Diego State University is building a lightweight one using a more powerful state-of-the-art motor with no transmission.
Also, another SDSU PRA team is building a 'G' impact fuel cut-off switch we hope will reduce the fire danger.

Stay tuned for full testing of the 1st car starter transmission version and then 2 new PRA Open source project coming soon.
 

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Kolibri

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John, what a great idea for a project; thank you for posting about this.

I'd toyed with the idea of electrically prerotating my RAF, and ran some numbers.
Prerotating 8.5"x30' blades to 250 rpm would require about 8 hp. (Cube's law is a bitch.)
Most starter motors provide less than half that.
You may have to go to an 6000+W brushless motor.

Looking forward to news of your progress, and I wish you all every success!

Regards,
Kolibri
 

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John, what a great idea for a project; thank you for posting about this.

I'd toyed with the idea of electrically prerotating my RAF, and ran some numbers.
Prerotating 8.5"x30' blades to 250 rpm would require about 8 hp. (Cube's law is a bitch.)
Most starter motors provide less than half that.
You may have to go to an 6000+W brushless motor.

Looking forward to news of your progress, and I wish you all every success!

Regards,
Kolibri
Hi Kolibri
"Prerotating 8.5"x30' blades to 250 rpm would require about 8 hp. (Cube's law is a bitch.) "
You do not happen to still have the formula you used? If you do I'd like to send it to the students.

I just discovered an 18HP motor that is being used on a ride on lawnmower. So, I'm thinking next year we create two new open source projects for two JUMP TAKE-OFF birds using 18HP motor????
 

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PS I thought it squared the drag not cubed it?
 

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PSS: I need to call Dick Degraw to get permission to give his name to a new media guy. So I will ask if he might help the students next year with the jump takeoff birds and learn what problems we can expect and how to overcome them. More when I learn more...
 

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18 HP is not enough HP for jump takeoff according to Dick so I asked what about doubling the motors and putting on on each side of the gear and he said that will work and he will help us. He suggested we use a Robison rotor head and controls. These projects are so much fun.
 

Kolibri

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Hi Kolibri
"Prerotating 8.5"x30' blades to 250 rpm would require about 8 hp. (Cube's law is a bitch.) "
You do not happen to still have the formula you used? If you do I'd like to send it to the students.
I'll dig it out and PM you. It's simple, but a bit involved.
I got it from the aerodynamics engineer who kindly confirmed that air pressure increases underneath gyro blades in ground effect. :)

What kind of rotor RPM do you expect to require for jump takeoff? I'd agree that 18hp would be insufficient.

Regards,
Kolibri
 

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I'll dig it out and PM you. It's simple, but a bit involved.
I got it from the aerodynamics engineer who kindly confirmed that air pressure increases underneath gyro blades in ground effect. :)

What kind of rotor RPM do you expect to require for jump takeoff? I'd agree that 18hp would be insufficient.

Regards,
Kolibri
I could find a formula but I do not know how to calculate the drag or even the pitch of the different blades. So that would be most helpful to our project.

Yes, Dick said we will need 30 for a two place. So simple = two 18HP motors on one gear but they are expensive.
Next year seniors are going to love this new open source project.
Thank you for helping our community!!!
 

Kolibri

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Hi John, you're welcome, glad to help; see my PMs.

A formula which did not work was:

P = 1.2 N^3 D^4 C / 10^13

P is horsepower
N is rrpm
D is diameter in feet
C is chord inches
I ran that past the engineer, and he replied, "Wow. No idea who makes these equations up - never seen this one before. But, yes, power will go with the cube of rpm. It is actually a function of tip speed so Omega*R, with Omega being in radians/sec, and blade area so number of blades* chord* radius, and air density, and also the airfoil drag coefficient. But where the 10^13 comes from in this equation then I have no idea."
 

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I just checked out your PM. You even did the math for the students showing them the steps.
That earns my highest accolade... U-ROCK!
 
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