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Old Sep 18, 2006, 03:06 PM
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RC Tiltrotor Aircraft - motor recommendations

Hi, I am a third year computer engineering student currently enrolled in a project course with 5 other group members. We have chosen to build a tiltrotor aircraft in order to demonstrate computer-controlled stabilization techniques.

This is what I'm talking about: http://www.kvii.com/uploadedImages/k...ies/Osprey.jpg

Basically, it's a helicopter/jet hybrid. It can take off like a helicopter, and then angle the props forward such that it flys like a jet.

The largest concern we have is keeping the weight down enough to be able to fly, while also providing enough battery power to maintain flight for a minimum of 10 minutes.

I was hoping I could get some recommendations for the most energy-efficient motors and props. We are looking for motors that are ideally light, low power and can provide lots of lift without going over our budget (<$70 a piece is about what we could afford to spend on each motor). This plane, because it is intended to demonstrate automatic stabilization, would mostly be operating helicopter-style so high flight speeds in jet flying mode is not a large concern. We need maximum thrust with minimum power consumption, basically. These motors will be used with RC airplane props, not helicopter-style blades, as they are not designed for forward flight (which we require).

You may be wondering what size of aircraft we want to build, and the answer is that the size of the motors we choose will determine that, so as long as the motor is <$70 it doesn't really matter to us how large it is, we will scale the rest of the aircraft to meet the needs of the motors.

Also, what are the benefits of using props with 3 or 4 blades? Do they provide more lift while only taking slightly more power? Do they only work well with certain types of motors?

Thanks for any help and please keep in mind we are completely new to RC aircraft, so I don't understand all of your jargon!
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Old Sep 18, 2006, 09:45 PM
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Mojo, if you intend to make an osprey style tilt rotor, you need to use helicopter heads / rotorblade menchanisms. I messed around with this idea a lot a few years back for a similar commercial UAV project and found out the hard way it's the only real way to go.

There are huge control issues to deal with in an osprey style machine - lets deal with the simplest - Roll of the aircraft needs to be achieved via varying the thrust levels from each propeller. If you just slow one down and speed one up, this'll do the trick, but the time it takes to do this is VERY long with the throttle compared to collective pitch (keeping rpm the same and changing pitch of propeller blades instead) so you'd have a long feedback loop. Another thing that'll happen is the torque levels from each propeller will change, resulting in a yaw of the machine, so you need some form of trust vectoring like a helicopter to give forward / back thrust on the propellers to allow for yaw control (in opposition) and forward / back control (when operating together). You can't just rotate the whole motor pod easily either, as the precession (sp?) of the spinning prop with fight you and cause all sorts of cross torque issues that have to be programmed out. Since you are not planning to go into fast forward flight, I'll leave that bag of nightmare issues alone .

Perhaps you should look at something like the old experimental POGO instead? Made from depron sheet with huge control surfaces, you would need only a single outrunner motor and the torque from the prop can be cancelled out by oversize elevon deflection like a 3D aircraft does with hovering. Using the wing and upper and lower fin rudders in combination, you'd have all the control in the world with fast response and hence short feedback loops. It's also a known quantity that a normal pilot can probably hover and fly him/herself to set up.

Thats my 38.3 cents worth anyway.
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Old Sep 19, 2006, 01:05 PM
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Mojo

Googleplex is correct - if you are going to do it like the Ospry with only two rotors you will need a helicopter head and somebody who understands it on your team.

ON the other hand - you can use four fixed pitch propellers and two wings - canard not biplane. There was a plane on these lines in the 50's too as a transport.(XC-142)(X-19, X-22) Now it is all in the computer where your expertise lies.

So now you are talking "park flyer" motor and ESC (Electronic speed control) for $35 ea and what every Propeller that comes with it - there are combos available from China (GWS) I think that can just meet your critera. And since efficient lift is pretty much proportional to the prop disc you probably want a geared motor with a larger prop. Then there is the battery - Li Polly and a charger - that is going to run you about $50 and you'll have to figure out how many mhr and weight when you actually test the lift of your motor and prop combination. (don't just believe the manufactures specs)

Its not trivial - thats what your going to learn with this project.

Just my 2 cents (as an old Aero engineer)
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Old Sep 19, 2006, 08:57 PM
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You could try two of these. You will have to modify one so that they rotate in opposite directions.

http://hobby-lobby.com/evp-pro.htm
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Old Sep 19, 2006, 10:20 PM
Ego varius quis.
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Quote:
Originally Posted by Sean Witte
You could try two of these. You will have to modify one so that they rotate in opposite directions.

http://hobby-lobby.com/evp-pro.htm
This does look like the best option. A skilled R/C pilot can fly a plane with one of these in the nose straight up and hover it; with a bit of work and a gyro, you should have no problem.
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Old Sep 19, 2006, 10:42 PM
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Wow, did you ever take on a big project for not having any R/C experience!! If you plan on trying to fly your creation, be warned that you need a very experienced heli pilot on the sticks. The tilt rotor craft fly like a heli in hover mode, and flying a heli is nothing like flying a plane. The best analogy I've heard is hovering is like trying to balance by standing on a board that is on top of a large ball. It's a real juggling act on the sticks.

You might want to do some reading in this forum (VTOL):

http://www.rcgroups.com/forums/forumdisplay.php?f=360

Also research all posts by Vtolman, here is a good place to start:

http://www.rcgroups.com/forums/showthread.php?t=440470

He is one of very few people that has actually been able to develop a VTOL design for R/C that really works.
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Old Sep 20, 2006, 11:02 AM
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Just as a thought thrown in from the sidelines - could a somewhat simpler to fly vehicle be accomplished by combining the Osprey concept with something from the "auto-stable", fixed pitch helicopters ? Keep the outboard motors and props ala V22 but add a pair (?co-axial or 1 front, 1 aft?) of lifting blades (or fans) in the fuselage. In hover mode they provide some (?most?) of the lift and the outboard motors then do; (some) lift, yaw rotation via differential tilting, forward/aft translation via coordinated tilting and lateral translation (if desired) via differential speed control. During transition to forward flight the fuselage fans speed up to provide for the lost lift when the outboard motors tilt. As the vehicle comes to speed the wings provide more lift and the fuselage fans can be slowed or shut off. The fuselage fans, motor(s) and ESC(s) are dead weight when flying but this might be an inefficiency that could be tolerated. Anyway just a thought for your consideration and perhaps something semi-novel (though kinda a takeoff on the Harrier with it's puffer jets or an X-UFO with tilting "side" motors).
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Old Sep 20, 2006, 03:51 PM
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Hello again, thanks for all the input!

To start I will make one important detail clear!: It is too late to significantly change the design of this craft. It must be a tiltrotor-style craft. I am open to suggestions on variable pitch props, as I can understand the concern here about feedback time. Considering we will have a microcontroller that can make corrections to all motors hundreds of times per second, do you still believe we would need the shorter response time offered by a variable pitch propeller? I would have assumed just altering the RPM would be sufficient, even though it is not the fastest.

As for rotating the whole motor pod, I think I understand what you mean by cross torques while the angle is changing, but I am uncertain how this would cause a problem. The arm which would rotate a prop would be connected to a servo-motor, which could resist any opposing torques. The reimaining torques would be absorbed by the aircraft frame.

I should also make one more thing clear: Yes we know this project is challenging! We are engineers, if we all had experience with RC craft our supervisors would not have let us choose this project. Engineers must pick up new concepts very fast and implement them, that is the primary goal of all university-based project work for engineering students. Learning a highly complex idea is something we do every day at school, we are not opposed to hard work and learning! That's why I am here to ask for your help.

Now as for using a helicopter head, is there a particular reason why using props would not work? I can of course switch to helicopter heads, but I must be able to reason why that decision was made. My ideology behind using props is that I have seen single-engine planes be controlled by a pilot to hover, so as of right now I don't understand why this route would fail.

As for the "experienced pilot" can of worms, keep in mind this plane will be computer controlled. An onboard microcontroller will be the pilot, an actual RC remote will only be added for fun if there is time (and money :P). So inconsistent piloting isn't an issue here, we will experiment to program the computer to fly it and it will do it exactly the same way every time.
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Old Sep 20, 2006, 04:19 PM
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Quote:
Originally Posted by mojo_4664
Hello again, thanks for all the input!

To start I will make one important detail clear!: It is too late to significantly change the design of this craft. It must be a tiltrotor-style craft. I am open to suggestions on variable pitch props, as I can understand the concern here about feedback time. Considering we will have a microcontroller that can make corrections to all motors hundreds of times per second, do you still believe we would need the shorter response time offered by a variable pitch propeller? I would have assumed just altering the RPM would be sufficient, even though it is not the fastest.

As for rotating the whole motor pod, I think I understand what you mean by cross torques while the angle is changing, but I am uncertain how this would cause a problem. The arm which would rotate a prop would be connected to a servo-motor, which could resist any opposing torques. The reimaining torques would be absorbed by the aircraft frame.

I should also make one more thing clear: Yes we know this project is challenging! We are engineers, if we all had experience with RC craft our supervisors would not have let us choose this project. Engineers must pick up new concepts very fast and implement them, that is the primary goal of all university-based project work for engineering students. Learning a highly complex idea is something we do every day at school, we are not opposed to hard work and learning! That's why I am here to ask for your help.

Now as for using a helicopter head, is there a particular reason why using props would not work? I can of course switch to helicopter heads, but I must be able to reason why that decision was made. My ideology behind using props is that I have seen single-engine planes be controlled by a pilot to hover, so as of right now I don't understand why this route would fail.

As for the "experienced pilot" can of worms, keep in mind this plane will be computer controlled. An onboard microcontroller will be the pilot, an actual RC remote will only be added for fun if there is time (and money :P). So inconsistent piloting isn't an issue here, we will experiment to program the computer to fly it and it will do it exactly the same way every time.
You're doomed.


Now that we've gotten this out of the way....

I do not know how to fly a heli, but I do know that it's very, very difficult to manage pitch and roll without a proper heli rotor-head. A variable-pitch propeller at least allows you to adjust rotor pitch for flight speed, another major advantage.

I suppose that you could use a set of accellerometers and gyros to keep one in the air, but it's not going to be easy.

My advice? Build a heli with a folding prop on the front. Take off like a heli, fly it like a gyrocoptor.
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Old Sep 20, 2006, 05:10 PM
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Hahaha, now now, don't be a wet blanket! We are not being unrealistic here, we do understand this isn't a cakewalk and there is a large possibilty this will not be perfect, or even maintain flight, we are just doing the best we can. Professors are impressed by innovations, which is why we never even considered the 3 or 4 propeller models beacuse they have already been done.

To put this in perspective, a group last year did a 4 propeller model, and it couldn't stay stable for more than .5 seconds. They got the highest mark in the class, because their research and design was extremely impressive.

I think it is safe to assume that we will be using a heli rotor head though, as you people seem to think that would offer greater control. Can anyone explain why a propeller doesn't offer such control? Just out of curiosity here.
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Old Sep 20, 2006, 10:12 PM
It's a really big number.
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A helicopter has cyclic pitch control (varies the lift produced in opposite sections of the blade arc) and this induces precession that pushes on down on one side of disk and up on the other. The disk rotates under precession, and the airframe rotates under it as it is attached 90 to the disk via the rotor head suppressors (these allow the disk to flex a small amount relative to the shaft and also supply vectored thrust to some degree before the airframe follows the disk).

A propeller has no ability to do this, it can only produce thrust and no angle changing forces. When you have only two like a V22, you have no way to control the pitch of the aircraft. If it starts to nose down and you have no way of inducing correction forces (extra prop in tail etc), that's it. With cyclic control, the rotor disks will provide this themselves. Rotating a motor pod to cancel out torque will induce precession and the net result would be the maching twisting on an axis 90 degrees out of phase, plus the angular momentum of the propeller will probably mean the airframe moves in reaction and absorbs a lot of control input. Cyclic pitch doesn't have this issue as it uses the forces, not fight them.

Now, looking through my toybox, I had an idea that'll solve all the issues and will certainly be 'innovating' - get your mitts on a pair of cheap indoor coaxial helicopters (as TheKnife suggested) like the E-Sky Lama's and you're problems are (mostly) solved for around $90 IF you get them sans radio / battery / charger / mixer boards (build them from parts basically). The transmission block out of each becomes your rotor transmissions for each side with complex axis control, and you need a single servo on each side for fore / aft cyclic control.

Since Coaxial helicopter transmissions produce no resultant torque with rotors spinning at the same reletive speed (as the upper and lower disks cancel out), you have the ideal system IMHO. Roll is controlled by varying the throttle settings on each side (there is no torque coming out of each rotor stack, so no rudder issues). Pitch is controlled by fore / aft cyclic working in unison, and yaw is in opposition (the opposite twisting actions of the cyclic input is cancelled out, and the vectored thrust effect as mentioned above will turn the machine). Vertical motion is controlled by throttles in unison.

If could be built and fly hands free with off the shelf components too for RC toy use. Hmmm, is that a plan I have now myself? Indoor V-22?
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Old Sep 21, 2006, 06:03 AM
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Quote:
Originally Posted by mojo_4664
I think it is safe to assume that we will be using a heli rotor head though, as you people seem to think that would offer greater control. Can anyone explain why a propeller doesn't offer such control? Just out of curiosity here.
The two designs are mutually exclusive, and since you're set on a tilt-rotor design it doesn't matter. If you use heli heads they should be mounted in a fixed position. The reason I suggested a variable pitch prop is response time and motor efficiency. You can use fixed pitch props and it'll probably work fine. Aircraft speed controls only have a limited number of steps, maybe 16, so it'll be hard to get a stable hover without continually adjusting the motor speeds. A change in prop pitch will result in a more linear response than changing the motor speed. If you choose not to use an RC gyro check out the Analog Devices ADXRS150 or 300.
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Old Sep 21, 2006, 10:02 AM
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Tilting the rotors by "brute force" fill fight against the gyroscopic precession of them, and will mean that the fuselage will try to tilt in the opposite direction instead. The osprey combines the nacelle tilting servos with cyclic inputs to the rotor to take care of this problem. You should think of the rotors as if they are tilting themselves. The only way to achieve this without using a full-blown helicopter head I can think of is by having trailing vanes solidal to the rotor nacelles and immersed in the prop wash, and using these to passively tilt the rotors leaving the fuselage unaffected. A bit like having two tailsitter models strapped together with a fuselage between them. I'd still go with a variable pitch prop instead of speed control, since it's likely that a fixed pitch prop won't be able to both hover the plane (low pitch, high speed) and fly it horizontally (high pitch, lower speed).
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Old Sep 21, 2006, 03:35 PM
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Okay, these are some excellent issues you've all brought up!

Googleplex, I see what you mean. So are you saying the E-sky Lama has the ability to adjust the blades such that we could maintain the pitch? If so, can we purchase this system elsewhere? I'm concerned that the motors and rotorblades from the E-sky Lama will not be large enough for our needs. Basically, the larger the motors and rotorblades, the less significant the weight of our other components such as the onboard microcontroller and servo motors will be.

Sean Witte, I think we have decided to go with variable pitch if possible. Every little bit of extra control ability helps!

Now, we have come up with another idea which may solve the problems we have with controlling the pitch, and also the problem brought up by Brandano where the aircraft will rotate rather than the rotorblades when attempting to convert to forward flight mode. If there was a small balancing weight inside the aircraft, it could be put on some sort of track such that if the nose dipped, the weight could by positioned at the rear of the aircraft to counterbalance this and bring the nose back up, then move the weight a bit forward to stop the tail from dipping. When converting to forward flight mode, the weight could be positioned at the front of the aircraft to force it to stay down. The weight added here would be minimal, as the motor required to move the weight would be extremely small, and the weight required wouldn't need to weigh much if the aircraft was balanced well. We do run the risk of going overweight with this solution, but our plan for failure would be to run up power lines as to not need batteries. Does this sound viable?
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Old Sep 21, 2006, 04:03 PM
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mojo, you are in the wrong forum. These guys only know heli's.
You need 'vtols' right here

It is possible to create a tiltrotor from a couple of outrunners. The expense comes from the gyro's and a decent computer transmitter that will manage the complex mixing.

Particularly look out for threads by a chap called vtolman. the only person I've seen that has built very impressive working tiltrotor's using simple outrunner motors, a couple of gyro's and a shed load of mixing.

Check out this early thread and a really nice looking kit he's planning to build here

The video links (if they are still working) are particularly impressive.
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