Autorotation Question For My Invention: CopterBox

[dropmaster]

Background:
I'm the Director of Engineering of DropMaster, Inc. My patented invention, CopterBox, can be seen at:

http://www.dropmaster.com

Also please see the 'Why CopterBox' link on the home page:

http://mywebpages.comcast.net/dropmaster/why_copterbox.htm

There is a video, some articles and a white paper.
This information is provided for background purposes only and not for selling, as we only sell to government agencies due to liability concerns.

The Army (Natick Labs) has come to us asking for a 500 lb payload version, which I'm calling MonsterBox (the current CopterBox is designed for 60 lbs but will actually handle 100 lbs) and I have some ideas about how to increase lift. CopterBox has patented, faceted, cardboard rotor blades. MonsterBox will likely have extruded foam rotor blades with a more efficient airfoil. My main question is this:
CopterBox has always had 3 rotor blades. We can now add more, but have never really flown with 4 or 6 blades. I'm wanting MonsterBox to have 6 blades and I'm looking at innovative ways to increase lift. My primary interest is the effectiveness of (3) blades having -6 degree pitch like Copterbox (to initiate rotation) but the other alternating (3) blades having 0 degrees of pitch, which will at first not have any forward thrust but hopefully will drive the blades forward once maximum RPM has been achieved and the velocity triangles have had a chance to catch up. The (3) blades with 0 degrees of pitch will provide more lift (and drag) than the -6 degree pitch blades. There can be some vertical separation of alternating blades in case there is some adverse cascading effect. Is this approach a valid one to consider for significantly increasing lift? Another approach is to reverse rotor pitch prior to ground contact, but I'm trying to keep this simple and low-cost.
Does anyone have any input or know of someone who can help?

Thanks!

Chase

http://www.dropmaster.com

 

[GregLocock]

I may be completely wrong here, but generally, in engneering, systems operate in a near linear fashion. As such, your idea of mixing 0 and -6 degree incidence blades will, in all likelihood behave in the same way as if they were all at -3 degrees. In fact my guess is that they will actually be slightly worse, as there will be second order interactions where the two sets of blades are fighting each other.

Having said that I have zero experience with auto gyros, or auto rotation in general.

Neat product, I'm astonsihed that it is cheaper than a parachute, well done.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

WRT variable rotor pitch for soft landings, I'm thinking that if there were a weight hanging under the box, some linkage between the weight and the box could increase pitch when the weight hit the ground.

<ignoring the complexity of said linkage for now... >

If the weight also had a significant cross section in a horizontal plane, e.g. saucer- shaped, it could have enough aero drag in free fall to _partially_ stroke the same mechanism. So increased vertical velocity would cause the weight to increase pitch, and hence, lift, and slow the descent rate.

... Both problems solved. I'll bill you later.




Mike Halloran
Pembroke Pines, FL, USA

 

[dropmaster]

Thanks for the input. Since we have been working on CopterBox for about 11 years now, we have considered pretty much everything, including having a weight hang below to trigger a blade pitch reversal. Our conclusion that since the entire box is spinning at about 450 RPM, there would be no way to keep the weight from flinging about to one side and pulling the whole system off balance. I came up with the idea of an inflatable cone that would inflate from rotational ram air and the pitch reversal system would be pneumatic and act when the cone hit the ground. Unfortunately, CopterBox's welded wire landing skid sees much abuse before exiting the aircraft, so anything inflatable is a non-starter. I have considered artillery shell proximity fuzes, but I think they're pretty expensive for what we want to do. I have found another radar proximity sensor for car alarms for convertibles. However, since the project has been funded almost entirely out of our own pockets, playing around with these ideas will have to wait until a customer comes along who is willing to fund it. We received a 98% rating from Natick on our SBIR Phase I and were next in line for Phase II funding when the money ran out - 2 years in a row. Getting someone to care about this and fund it has been next to impossible. However, CopterBox is in use in combat in Afghanistan and for earthquake relief in Pakistan.

It's possible that the effective rotor pitch would be -3 degrees, but it will take (funded) testing to prove it. I have 1/10th scale paper models that I need to fold up and test. I think if 3 blades were at -6 degrees and 3 were at -3 degrees, there would definitely be a fight. I'm hoping for minimal negative effect until max RPM is achieved.

CopterBox is cheaper than parachutes not because of its price, but because of its lack of logistical cost. It's all on the website.

Thanks! Any other input?

Chase

http://www.dropmaster.com

 

[vattiat]

Sorry, I can't stop myself from replying to this despite having no helicopter experience...but...

Why not control the blade pitch through blade angular velocity through some simple, centripetal force mechanism?

 

[dropmaster]

Possibly, but that doesn't sound simple and inexpensive. CopterBox is disposable. This is why I'm looking at a no-moving-parts (rotor pitch-wise, anyway) solution.

Thanks!

Chase

http://www.dropmaster.com

 

[CompositeGeek]

Isn't the rate of fall a function of weight and pitch in auto-rotation? I would expect the "terminal velocity" or "fall speed" to be a function of both.

If that is true, then perhaps you can add a pitch angle adjustment feautre/setting to your wire frame rotor hub such that by looking at a matrix you can set an optimum pitch per unit. I can imagine a couple ways to set up an feature so that during assembly you can adjust pitch at zero, two, four, and six degrees.

I spent 10 years building helicopters, my intuition says it could work. Unfortunately I am not a designer and it would not be the first time a designer told me I was full of it for thinking too simplistically.

Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

 

[MikeHalloran]

Well, if you abandon the idea of the blade axes being exactly radial, you are then allowed to explore interactions between centrifugal forces and aero forces and flexure of the blades. I.e., a fancy complicated linkage and active control system may not be necessary.



Mike Halloran
Pembroke Pines, FL, USA