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Light airplane propeller imbalance — Parallax Forums

Light airplane propeller imbalance

SUIRE2SUIRE2 Posts: 4
edited 2011-04-18 15:31 in Accessories
Hello,
I try from a long time to make a device for measuring the imbalance of a light airplane propeller.I think I can do that with a Memsic 2125,( only one vertical axis is needed ), plus a Basic stamp and a LCD display.I would like to know the max imbalanced level ( IPS /inches per sec. ) and in which position of the blade( from 0 to 360°).
I need a light beam sensor taped to the cowl of the plane, and aimed at a reflective patch, on the back of one propeller blade.Like this I can measure rpm and blade position, in order to put small counterweights, to counteract out of balances forces.
Could you help me to design such installation?
Thanks in advance.
Michel

Comments

  • kwinnkwinn Posts: 8,697
    edited 2011-01-10 15:12
    If this is a 2 blade propeller you really only need to know which blade is heavy and which is light. That is a much simpler thing to accomplish. The reflective spot on one blade only will allow you to distinguish between the two blades. If you put the memsic near the light sensor on the cowl and record both signals you should be able to see which blade is heavy and which is light.
  • ercoerco Posts: 20,256
    edited 2011-01-10 15:18
    The prop on my Cessna 152 gets R&R'ed at every annual inspection and sent out to a specialized prop shop for magnafluxing and balancing. All that's certified aircraft mechanic A&P stuff that is well worth paying for. I don't think you really want to get involved at that level, I'm sure it's totally illegal and a generally bad idea, even if you knew exactly what you were doing. There isn't any provision for adding weights to a propeller. It's not like tacking tire weights onto a wheel. Your weight will fly off at some point, then you have 2 problems!

    Much as I like to do things myself, I wouldn't even think about going down this road. I highly suggest you leave this job to the professionals.

    Edit: At some point your prop was in perfect balance. How is it now out of balance? If you hit something and damaged the prop, that's further evidence that your prop needs professional maintenance.

    And there is so much cowl vibration even on a properly balanced prop that you will have significant background noise on an accelerometer.
  • wjsteelewjsteele Posts: 697
    edited 2011-01-11 07:12
    Our VirtualHUD Heads Up Display uses a laser range finder system to determine the shape, speed and direction of the propeller on a plane. Within the data we get back, we can actually see the "wobble" of the blades as the engine fires for each piston pulse. We're working with a couple of propeller manufactures to build testing rigs for examining the performance of propellers for Q/A and engineering in real time with this equipment.
    There isn't any provision for adding weights to a propeller.

    That's correct, normally propellers are filed down to the correct weight instead of having to have weights added.
    And there is so much cowl vibration even on a properly balanced prop that you will have significant background noise on an accelerometer.

    Actually, using an accelerometer is quite adaquate for finding the imbalance. Generally, any noise in the system will cancel itself out if properly filtered and the magnitude of the imbalance will clearly exceed the values of the noise. Using the optical encoder combined with the accelerometer will work just fine... in fact, there are several units on the market already that do exactly that. (That's how the professionals do it on a plane without having to remove the prop.)

    Here is an example of one of the professional units we evaluated: http://www.rpxtech.com/rpxweb/Dynavibe.asp

    Bill
  • SUIRE2SUIRE2 Posts: 4
    edited 2011-01-11 10:01
    kwinn wrote: »
    If this is a 2 blade propeller you really only need to know which blade is heavy and which is light. That is a much simpler thing to accomplish. The reflective spot on one blade only will allow you to distinguish between the two blades. If you put the memsic near the light sensor on the cowl and record both signals you should be able to see which blade is heavy and which is light.

    Yes, you are right, but, I would like to know, in dynamic test,How many grams or milligramms, I need to put on the flange of the propeller and where.
    Normally for this task, we put one or two washers under one of the bolts in order to get balancing of the propeller.
    I know also that electronic tools does exist , but the price is very high.
  • SUIRE2SUIRE2 Posts: 4
    edited 2011-01-11 10:09
    You can add some milligrams with one or two washers under one of the bolts of the propeller . The problem is to determine on which bolt; that's why we need to synchronise with photocell.
    Michel
  • kwinnkwinn Posts: 8,697
    edited 2011-01-11 20:50
    Synchronizing with the propeller can be done by determining when the peak in the memsic reading occurs in relation to two of the photocell signals. Determining the weight or number of washers to add can be done empirically. Calibrate that by measuring the memsic signal peak with 1, then 2, then 3, etc. washers added.
  • SUIRE2SUIRE2 Posts: 4
    edited 2011-04-18 15:31
    I got a BOE with a Memsic 2125, and the program works well, by scrolling all the g values during a rotation of the propeller. But my problem is to find a command in PBasic to extrct the maximum value . Is there a command like "MAX" to do the job ?
    Thanks for your help.
    Michel
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