SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old Jan 12, 2003, 10:13 PM
Registered User
Pittsburgh, PA
Joined Dec 2002
6 Posts
Realistic design?

I'm part of a group of people hoping to build a flying-saucer like machine. The idea is that it would look something like this (another view). We haven't seen any other similar designs - the closest use 2 stacked counter-rotating fans.

Hopefully by the end of the week we will have access to software that will let us really analyze the design, but does anyone see any immediate problems with the design? (Why aren't there any other designs like this that we've found?).

The videos use the DivX 4 codec.
CTho9305 is offline Find More Posts by CTho9305
Reply With Quote
Sign up now
to remove ads between posts
Old Jan 12, 2003, 10:46 PM
Team 30 Micro EDF
Mike Taylor's Avatar
Camarillo, California
Joined Apr 2002
4,516 Posts
There are some other similar designs. The navy tried contrtra-rotating props on a number of VTOLs, and the army had the platform you stood on back in the 50's. These tend to have very complex, critical and fragile trtansmission systems. It will be quite a challenge. Good luck.
Mike Taylor is offline Find More Posts by Mike Taylor
Reply With Quote
Old Jan 12, 2003, 10:50 PM
Registered User
Pittsburgh, PA
Joined Dec 2002
6 Posts
Quote:
Originally posted by Mike Taylor
There are some other similar designs. The navy tried contrtra-rotating props on a number of VTOLs, and the army had the platform you stood on back in the 50's. These tend to have very complex, critical and fragile trtansmission systems. It will be quite a challenge. Good luck.
So they had the same outer/inner blade design? No major aerodynaic problems? Thats good, because we have a few idea on how to solve drive/pwoer problems
CTho9305 is offline Find More Posts by CTho9305
Reply With Quote
Old Jan 13, 2003, 02:28 AM
Team 30 Micro EDF
Mike Taylor's Avatar
Camarillo, California
Joined Apr 2002
4,516 Posts
Well, actually they were coaxial blades with one shaft turning one within the other.

Two questions with this design come to mind.

One, how would you balance the differing torques and gyroscopic forces from the two different sized, weighted and speeded fans, and

Two, how do you propose to handle the collective pitch control on the ring mounted fan. That's a big swash plate or ring.

Torque is always easiest to manage if the two forces are equal and opposite. Some sort of linked swash plates with two helicopter rotors spinning on opposite directions would seem much easier to do.

I would also think you need some sort of contrtolable vanes under the fans to control yaw. Trying to balance and vary the torque to control yaw is tricky at best.
Mike Taylor is offline Find More Posts by Mike Taylor
Reply With Quote
Old Jan 13, 2003, 02:45 AM
Team 30 Micro EDF
Mike Taylor's Avatar
Camarillo, California
Joined Apr 2002
4,516 Posts
And then there is this...

http://www.visionpacific.com/humming...HomePage2.html
Mike Taylor is offline Find More Posts by Mike Taylor
Reply With Quote
Old Jan 13, 2003, 09:08 AM
Registered User
Pittsburgh, PA
Joined Dec 2002
6 Posts
I'm an electrical engineering student, so this isn't my strongest field, but I think we are hoping we won't need variable pitch blades. To handle yaw, the idea was to slow down one of the blades, making the torque unequal and rotating the craft.

My understanding is that if we put the same force on both fans, it should not spin, even during the startup period. And once the blades are spun up, while their speed is steady, mass should be irrelevant.

To get around variable pitch blades, our current idea is to use fins on the bottom to direct or block thrust.
CTho9305 is offline Find More Posts by CTho9305
Reply With Quote
Old Jan 13, 2003, 12:49 PM
Team 30 Micro EDF
Mike Taylor's Avatar
Camarillo, California
Joined Apr 2002
4,516 Posts
" My understanding is that if we put the same force on both fans, it should not spin, even during the startup period. And once the blades are spun up, while their speed is steady, mass should be irrelevant."

This would be more nearly true with two rotors of the same size & pitch mounted on the same shaft centerline. When you have two different sized fans, there will be some different gyroscopic forces as well as aerodynamic forces operating on the two fans.

"To handle yaw, the idea was to slow down one of the blades, making the torque unequal and rotating the craft."

Slowing/speeding one blade to change the torque would also change the thrust, gyro forces, and have the same inherent problem that fixed pitch helicopters have - slow response to the throttle inputs (and subsequent over-control as a result) since there is so much mass to spin up.

"our current idea is to use fins on the bottom to direct or block thrust"

That is a good idea. If you made the shutters act as 'stators' to counteract the torque of each fan and add control inputs to them, you would be on the right track. Look at the way the V-2 rocket had guidance fins located in the rocket's exhaust stream to provide low-speed control. You could use the same vectored thrust to help control the craft.

The other worrisome point is how much spinning mass that outter hub will represent, how fast it would need to rotate, and how you would make it stiff enough to not distort, and light enough to fly.

The tip speed off a rotor can be very high, and even a Piccolo's tip speed is high enough to cut you up if you get into it (100 mph?). The speed the bearing surfaces is moving is still quite low, since the bearings are moving around a 2 1/2 mm circle. At half span, the bearings there would be moving 50, a 100 or more times that speed.

I think the differencec on the masses and moments of the two rotors would yeild very different forces.

But then I'm older than you and less adventuresome than I used to be. Radical designs were a lot easier to build and fly when it was easier to get the bottom of the barranca or top of a tree and back again.

Good luck on your project. I'll be interested in your progress.
Mike Taylor is offline Find More Posts by Mike Taylor
Reply With Quote
Old Jan 28, 2003, 07:39 PM
Registered User
Pittsburgh, PA
Joined Dec 2002
6 Posts
We're ready to build a testbench for the drive system (probably this weekend). We still don't have access to the aerodynamics software though.

We will be testing this magnetic drive system (see the PDF for a long description). Hopefully it works
CTho9305 is offline Find More Posts by CTho9305
Reply With Quote
Reply


Thread Tools