I would appreciate some feedback on a very small hovering airframe using coaxial fixed blade rotors (no cyclic or collective) powered by brushless motors. Props will be in the range of 10-12" diameter, with total aircraft height of less than 8", and total weight, including electronics and battery less than 9-oz (250-gm). I plan to control lateral movement either with steerable aerodynamic surfaces or weight shifting once hovering issues are resolved.

I've built a couple of prototypes using the HiMax CR2816-1100 brushless coaxial motors ( http://www.nesail.com/detail.php?productID=5429 ) with 12" and 13" APC props. These motors are much heavier than what I plan to use, but they were available, and I've since located some smaller coaxial pairs which should be here soon. In any case, I can get airborne, but have not achieved stable hover. I think the main problem at the moment is top-to-bottom weight balance, but it seems that others here have worked with this type of design, so I welcome any comments / suggestions.

Here are some photos of the prototypes. The first one was quite unstable, but the second one showed some promise. My thought is to nest the bottom motor in the airframe to lower the center of gravity. Also, it would be nice to find a cleaner way to stow the electronics.

wowo this looks so intersting, i wish i was near and could see how it is going cuz i am working on the same thing

if i was in your shoes the first thing is to get stability and one way is maybe by puting the weight CG up on the props and hav a shroud around te props with intake lips. ..kinda like the HILLER, I am no expert but maybe it is worth a try


sky-walker

When I tried something similar, I found the vertical CG was important for stabiilty. When it's above the majority of the fin area, the fins will try to 'weathercock' behind the CG, leading to the model tipping over when it's moving sideways.

With a high CG, some stability is possible, but as I was only using a single prop, it wobbled around in a circle due to gyroscopic precession.

I don't think a stable hover is possible with no cyclic control or flybar and only static aerodynamic surfaces.

The lower motor is now nested in the airframe. Stowing the electronics/battery and balancing the payload weight is now the main challenge - with the components taped loosely into place, this version seemed to be more stable, but I won't know until I can properly distribute the weight.

Robin - I looked at flybar designs and purchased a Blade CX2 coaxial heli to study, but I don't like the vibration that's introduced by that type of design. There's no question that active control surfaces will ultimately be required, but I want to see how far I can get with static surfaces. In any case, the counter-rotating blades allow very simple control of rotation through the vertical axis.

Sky-walker - with the latest version, I had a similar vision of adding a shroud around the blades. I would guess that the benefit is to direct more airflow across the wings, which probably leads to better control. Before trying that, I'll probably first go back to smaller blades - these are 13"x6.5" + 12"x8" on a 15" wings, and it will be interesting to compare this with my 10"x5" + 9"x7" set.

Howard

Update ...

Neither lowering the props toward the airframe or changing to smaller props seemed to improve matters. My sense is that large props and a low CG will prove to be a better combination, e.g.



(from http://www.grc.nasa.gov/WWW/K-12/TRC...elicopter.html )

I'll move back to my earlier configuration, and do more work on balancing the load.

Here's version 4 - still top-heavy, but definitely an improvement relative to previous versions. I actually managed a few seconds of relatively stable hover. The smaller coaxial motors should help a lot ... hopefully they will arrive soon.

I am also pursuing the vtol hover, without electronic gyros, it is possible. Do a search on the hiller flying platform to stable for it's own good.

Interesting ... there's a nice historical article about the Hiller Flying Platform here - http://www.howtoadvice.com/Sky-High - and the videos are impressive. However, there's a noteworthy quote in the article -

Stand-up flight is in its toddling stage today. For one thing, no present flying platform -- jet board, Hiller tube or Heli-Vector -- will stay right side up in free flight without correction from its human rider. It needs balancing, like a bicycle, which you provide without conscious attention.

Take a baseball bat and balance it on one finger with the heavy end up.
Then try it with the heavy end down. Corrections are necessary in both cases. One is easy one very tough.

Originally Posted by hg1

Interesting ... there's a nice historical article about the Hiller Flying Platform here - http://www.howtoadvice.com/Sky-High - and the videos are impressive. However, there's a noteworthy quote in the article -

hello hg!

i look forward your finished project and I will give as much help as I can!

well.. I have made a lot of research around the mid 50s HIller platform and about its unquie stability and I can tell you that after what i have read that the human NO WAY can control the stability simply by the human reaction which would be too slow and result in flipping teh platform=the platform had a natural static stability from a "gyroscopic procession" which I honestly dont get exactly but the stability is a result of two main things:


1. the intake lip around the shroud increased teh stability, so evry time the tube tilted a little the intake on that side would be much faster and do more thrust=and right the tube up again

2. the pilot standing up on the platform made a better balance due to a HIGH CG!!(which i still thing sounds strange but true)

sooo what I have read is going against my experinces a little. cuz as you know having a low CG should have a "pendulum balancing effect" but no, this is not teh case here strangely..

you can chsck this my post about hiller platform
http://www.rcgroups.com/forums/showthread.php?t=701480

Originally Posted by Stef

Take a baseball bat and balance it on one finger with the heavy end up.
Then try it with the heavy end down. Corrections are necessary in both cases. One is easy one very tough.

That is a very interesting example. I guess the principle is that with the CG far away from the adjusting force a bigger leverage arm is the result and thus there is more torque acting on the CG giving more and better control?

But does this apply a VTOL hovering in free air? After all, the baseball batsī handle is more or less "fixed" in the persons hand, making instability due to "over-correction" a non-issue.

I am beginning to suspect that there is no such thing as a "naturally stable" hovering layout. There are only better and worse ways to apply the controlling/balancing forces.

I believe the stability you are observing with the Hiller-style platform is mostly due to "in ground effect" hover - see

http://cybercom.net/~copters/aero/ground_effect.html

Note that this effect is observed when the blades are fairly close to the ground, as shown here -



As the blades get closer to the ground, there's more air pressure which results in more lift which forces the blades back to a horizontal path. However, as the blade height increases, the ground effect is reduced.

Unfortunately, ground effects don't help with my application - I need to hover well above ground level and carry a downward-looking camera. However, it's useful to consider all possible design ideas in hopes that there is a "magic bullet" in achieving hover stability.

Originally Posted by hg1

I believe the stability you are observing with the Hiller-style platform is mostly due to "in ground effect" hover - see

http://cybercom.net/~copters/aero/ground_effect.html

Note that this effect is observed when the blades are fairly close to the ground, as shown here -



As the blades get closer to the ground, there's more air pressure which results in more lift which forces the blades back to a horizontal path. However, as the blade height increases, the ground effect is reduced.

Unfortunately, ground effects don't help with my application - I need to hover well above ground level and carry a downward-looking camera. However, it's useful to consider all possible design ideas in hopes that there is a "magic bullet" in achieving hover stability.

well i dont know if it is only the ground effect, cuz I have seen a video of teh hiller platform hovering very very very stable over 30 meters above the ground,

Originally Posted by Sky-walker

well i dont know if it is only the ground effect, cuz I have seen a video of teh hiller platform hovering very very very stable over 30 meters above the ground,

Right, but to fly "out of ground effect", they had to add gyro stabilization.

After test-flying the Model 1031-A, Hiller found that the vehicle could not produce enough thrust to climb out of ground-effect (about 1.5m or 5 feet, the diameter of the platform). Larger rotor blades were the only solution. This change also necessitated a larger duct and platform and the new device was designated the Model 1031-A-1. By this time, the Department of the Army was dissatisfied with De Lackner's progress and Army officials began to take control of Hiller's Navy contract. Army Air Mobility Division and the Office of Naval Research soon shared control of the project. The new platform, now sporting 2.1 m (7 ft) rotors, first flew on November 20, 1957. It could operate successfully out of ground effect, but testing also revealed a new problem. The increased size of the revised platform also lowered the center of gravity so much that kinesthetic control was greatly impaired. Hiller engineers attempted to correct this by further elevating the pilot 's perch above the duct but the total weight of the platform was now so great that precise, stable weight-shift control was not possible.

Hiller solved this new problem by introducing a gyro-stabilization system that used aerodynamic servos similar to those used on the Hiller UH-12 helicopter. Engineers linked the new system to the yaw control vanes and it significantly improved hover stability. An Army infantryman dramatically illustrated these improvements when he aimed and fired his rifle with both hands while the Model 1031-A-1 hovered in free flight. Forward flight remained problematic except in the calmest conditions. The platform experienced erratic oscillations that the gyrostabilizer servos could not dampen. Engineers tried various duct configurations, but those that showed the greatest increase in stability also produced the least amount of leading edge thrust.

from http://www.nasm.si.edu/research/aero...hiller_vz1.htm

Originally Posted by hg1

Right, but to fly "out of ground effect", they had to add gyro stabilization.

from http://www.nasm.si.edu/research/aero...hiller_vz1.htm

yeah thats right! mate but anyway it is stilll a little strange design in my own view, so the main stability is coming from teh pilot reactions plus teh ground effect or shroud shape and ground effect,,


I think you should if you want and I would also lovely see a mini copy of teh hiller, so take your time and shape a good duct and inverse teh motors so teh props are facing down,,

sometimes the testing is the only way to go mate!