Hi everyone. I am wanting to build a hexcopter and need some assistance with motor and propeller selection. Saving money is always a goal, but I'd rather buy the right equipment the first time. That being said, I don't have an unlimited budget.
I would typically be taking off from 7500-9000 ft and I'd like to be able to easily lift about 5-9 pounds of photographic equipment. I'm finding it difficult to find recommendations on props and motors to handle this. I've found one example for heavy lift, but no references to altitude.
Any recommendations would be greatly appreciated.
Thank you,
Dave
Replies
I also need help for upgrading parts or if i'm doing it wrong or right for aerial video productions/photo, here is my set up:
http://diydrones.com/forum/topics/multicopter-not-lifting-too-heavy...
I would like to heavy lift and stable in high altitude.
Hi. Helicopters use something like 16:1 energy vs. airplane. So, time aloft may be a consideration. Given that most energy is consumed trading for altitude, size and mass to haul some 9 pounds of camera gear to altitude becomes significant. Next is the question of speed, how fast does this heli have to climb? The faster the climb, the more energy burned up. For maximum power and or time aloft vs. weight, a gas motor wins.
Problem: what budget do you have to accomplish this goal? Knowing this greatly facilitates resolving your options. Perhaps you may consider a blimp, a small one anyway but the tradeoff is speed. One could fly up your platform on a rocket, private plane or zepplin depending on what you want to do.
Incidently, my platform -- not a chopper -- See: Botmite.com can climb to 20,000 feet and uses electric motor but a lot of electric power is spent in the process. To see the camera mount, click camera from main page.
DSCF1294_10.JPG
Thank you for your thorough reply.
Consult Principles of helicopter aerodynamics By J. Gordon Leishman
My understanding, based entirely on math, not experience here, is that a good portion of the effect of altitude density in rotor aircraft is balanced by the proportional loss in parasitic drag. Moreover, most literature discusses full scale helicopters, with variable pitch rotors, and gas or turbine engines.
As regards the power plant, I am not aware of any reason, other than temperature, that the motor would be affected with altitude. As an electrical system, air density is not a factor, so the lost engine power is not a significant factor. I haven't run the calculations to fully understand the effects of scale on altitude, since the Reynolds numbers surely play some part in the calculations. However, I suspect it is not that significant to "just getting it working."
As a first general guess, I would suggest designing your copter with these three alterations in mind:
* Support for 20-22% more overpower, as you can expect around that loss at 9000 ft
* Ensure that your props are selected to handle slightly higher RPMs, maybe avoid Slow Fly props (unless testing shows they can handle the inertial forces without breaking - its true the parasitic forces of drag will offset, but will the prop bust as a result of the forces of acceleration while moving at significantly higher than its design RPM?)
* Nudge the prop angle of incident up slightly over that of your flatland friends. I'm not certain which props are available, but a survey of similar props at flybrushless.com should give some idea. I don't think you need to increase this by much, and it is really intended to keep from having to spin those props faster than they would otherwise need to turn.
So generally speaking, I think you'd want to design to a 1.41 lift to weight ratio at hover throttle, and 2.75 l/w ratio at full throttle.
Find a prop/motor combination that will deliver this performance at your target build weight, and it should be fine.
But if you're not comfortable with everything I've said, I would strongly recommend building an ArduCopter reference design first - I think the original ArduCopter/jDrones design would be best for you, since the frame is lighter, and you are already behind the power curve because you need higher lift to weight ratios than everyone else. There are hundreds of mistakes you can make with the "standard build" and the confidence you'll gain in terms of experience and expectations for how things should function will pay dividends. Adding two very complex challenges, that of high altitude launch, and of heavy lift, to your design when you do not already have the experience with a basic build, will put your project at a much higher risk of failure.