Question about Props

[Terry S]

Hi all,
I want to run my electric motors in soft mounts to keep vibration to a minimum for aerial photography but at high speed they get unstable. The question is, would 3 blade props be any more stable that 2 blade props ?

Thanks for any help, Terry

[Sparky Paul]

At high rpms?
If so, cut the motor power when taking the picture.
Soft mount the camera.
The motor needs to be mounted securely to the airplane so that -all- the power goes into the airplane.

[BMatthews]

Carefully balance the props, mount the motor firmly as SP suggests and mount the camera using light foam as he also suggests.

Mounting the motor on soft mounts can actually promote vibration in some cases. It's a narrow range of flex that helps without promoting.

[Terry S]

It's a narrow range of flex that helps without promoting.
Yes thats what I am finding, soft mounts DO help a lot even when you have spent all night balancing your props. They work great at low to mid rpm's but will reach a point where they get unstable. I have just gone from 2 LiPo to 3 LiPo and at full throttle this is what happens. A friend has said he is sure this instability is due to me using 2 blade props and 3 blade will self centre. im looking for anyone that can confirm or disprove this, not just a guess. Thanks.

Terry

[vintage1]

It's not something that is normally recommended, but you might find that a flywheel, as well as a prop, works. Tends to lower the resonant frequency of the motor/mount.

But for electric and cameras..just chop the throttle..?

[olmod]

Some small cd rom motors that i have pulled apart have a hollow disc type of enclosure with a few ball bearings that float around inside loosly (cant really call it a flywheel) that irons out any imballances in much the same way as do some vertical tub clothes washing machines use a bit of water in the rim of the drum.I dont think it would be to hard for someone with a lathe to adapt the same principle into a spinner base.

[Terry S]

just chop the throttle..?
I can do this but I prefer to circle the subject and take pictures all the way round, chopping the throttle means I have to wait until the trim change settles each time I take a picture.

Im not sure how the ball bearing idea works so I dont know if it will work horrizontal. I have a lathe myself so Im happy to make it if I can work out how to do it.

Thanks for the replys guys, its good to hear your ideas.

Terry

[BMatthews]

Frankly it would be a LOT less trouble to just soft suspend the camera. Because the camera doens't involve a lot of fast whirlling bits it's easy to suspend it in a holder of soft foam rubber or even on a mount that is spring or rubber band suspended in the model. A soft camera mount of this sort will allow the model to shake like a small paint mixer while the camera is still.

I can't help you with the 2 vs 3 blade prop thing but I have heard that 5 blade ceiling fans produce less wobble than 4 bladed ones do.

But regardless of how many blades you have a smooth running electric motor system is still going to depend on the prop being balanced, the motor being balanced and the prop being the same pitch and blade shape along the span on all the blades. There's far more than just the prop balancing involved.

[ghoti]

It's not something that is normally recommended, but you might find that a flywheel, as well as a prop, works. Tends to lower the resonant frequency of the motor/mount.

But for electric and cameras..just chop the throttle..?

Just remember, this is an airplane and you will need to put lightening holes in your flywheel. hehehe

[ghoti]

Three blade props do smooth out the unbalance sometimes just as three-phase electric power has less ripple than single phase. However, the 3-blade offerings are strictly limited in size and pitch. Few sizes are available. And there are few makers. I would think that you can better fine tune performance if there is a wide range of size from which to choose. I better shut up and fly one and see what happens, but I'm too busy making pictures. If it can make better pictures I'm for it. Bill

[Scaledown]

Well its been a while since I learnt this at Uni, but any elastic mount will have a natural frequency of oscillation. If the motor excites it at that frequency, or a harmonic, and its not sufficiently damped, then the amount of movement will increase. At other frequencies it should reduce. So maybe you should try a stiffer or softer mount until it runs smooth at the normal cruising rpm of your motor. I don't think the number of blades will make any difference. An imbalanced wheel on a car will shake the steering wheel at a particular speed.

[Terry S]

Thanks for your surgestions guys. for now I have stiffend ou the mounts slightly but I am still interested in 'the ultamate mount'.
Scaledown, you are 100% right about it being a particular speed that is the problem as proved by the soft mounts working well on 2 LiPo but not 3. If I could make them stiff for takeoff and soft for cruise it would be great.
BMatthews, In my case I think your wrong in saying it would be a LOT less trouble to just soft suspend the camera. The camera will be mounted on a tilt servo inside the fuse and will be able to be setup to look right, left or down before takeoff. With the limited space a simple motor mount solution would be much easier.

Cheers, Terry

[BMatthews]

Well, I think you'd be surprised at how easy it would be to make a mount for the camera even if it does have a range of panning built into it. But why bother with a servo if you're not going to alter it in flight? Just make it a lever or reach in under the wing. It sounds like you're making this model specifically for AP work. Making the fuselage just a little fatter to provide room for a swivel mount that still provides vibration isolation would still be the way to go.

[Sparky Paul]

In-flight tiltable mounts work. They need to be stiff, to keep the mass of the camera from vibrating the mount whle the motor is on.
I've gotten good videos with tilt mounts.
And poor ones.
Vibration can be controlled with some experimentation.

[Brandano]

if the spinner can hold them and you don't mind the look of the thing you could try stacking two smaller 2 bladed props to get a rough 4 bladed prop. Increasing the number of blades does increase the flywheel effect, so it should reduce vibration a bit. However, there's quite a few "incognita" to this approach. More blades mean more gyroscopic precession, so the flight behaviour of your bird will change. At a certainspeed the leading blade might blank the trailing blade, so you may have to adjust the angle between the two props... sounds like an interesting experiment, but I'd try it without the camera onboard first. I think that 3 and 4 bladed props can result in fewer vibrations because they are easier to balance "across" the prop, rather than tip-to-tip.
[edit] another thing that might make odd-bladed propellers and rotors less vibration prone might be that their mass distribution is less prone to inertial coupling. in a two bladed prop all vibrations will tend to sum up around the axis of major mass distribution.