Prop blade AOA is it really this simple or have I missed something??

JETLAG03

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Jul 6, 2019
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france
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300+ flexwing (pendulaire) newly trained on gyrocopter ((autogire)11h)
I have on occasions had to re-set my propeller AOA and find the setting tool can allow small errors in adjustments, spirit levels are good, but there is an inherent error trying to place the bubble in exactly the same position a number of times.

During last winter whilst stuck on the ground I was playing with some bits and bobs in my garage and may have stumbled on an easier and extremely accurate way of setting props, I did post my idea on a facebook microlight forum in February 2019. (search laser pens in Microlight pilots on facebook)

I Set my prop blades AOA to a give max engine rpm with flewing (I fly both) static on the ground therefore I achieve slightly higher revs in flight.

I believe if each blade does not have exactly the same AOA then vibration may be created and my idea suggests high levels of accuracy in blade to blade adjustments.

I am NOT an engineer, nor very clever, if I've overlooked something very simple, please be gentle with me .... cos I may cry

My idea is to ensure that each blade is set as closely as possible to each other. This requires firstly to set one blade with your tool supplied by the manufacturer and then use my contraption to set the remaining blades to the exact same AOA. I actually did mine by guess work for the first then worked back from there.

The parts are two laser pointer pens such as these
https://www.ebay.co.uk/.../New-405nm.../142312686510...

There are two pieces of plastic one like this square channel mine is 10mmx10mm


Cut to 1x30cm 1x8cm 2x2cm the 30cm piece will balance along the top of the prop, the two 2cm pieces will be the saddles you will glue under the 30cm piece,the saddles will sit across the prop as a saddle, clever eh?? The third part (8cm) will be glued to one end of the 30cm piece at 90° cut a groove in the centre of the 8cm piece for 15mm these will glue on each side of the 30cm piece and will face in a downward direction, this will sit on the tip of the prop and ensure that the tool sits in exactly the same postion on each blade. The “actual” position is not important,we are setting each blade relative to the other blades.

On the opposite end of the 30cm piece cut a thin slot slightly off centre and to side of the blade facing away from the engine, this needs to be just big enough to accept a tongue made from the end of the square section channel listed next.

And another like this an angle plastic channel mine is 15mmx15mm


Mine I cut to 25cm long and then cut 15mm down each side of one end to create a tongue to pass through the hole (slot) created in the 30cm angle piece in the previous section.

All sizes will depend on your prop, bigger blades will require bigger pieces. It is important that the square section that carries the laser pen to the right is longer than the prop is wide so that passes and rests on the trailing egde.

You now have 5 pieces of plastic and two laser pens you also need two elastic bands.

Piccy AOA 1 shows 1x30cm 1x8cm piece glued on the end and the 2x2cm pieces to create the saddles. The last piece is shown prior to gluing in place.

Piccy AOA 2 has all parts glued together and depicts how the tool would sit on the propeller with the stay on the left hand side snug up to the prop tip.

Piccy AOA 3 Shows the 2 laser pens in situ and would be needed to be secured, elastic bands work OK.

The laser on the top points to the left and will eventually shine on a surface (wall) as far away as reasonable for you to clearly see the laser point.

The laser to the right can point either down to the ground or, if you have a high hanger even better to the roof. The greater the distance to the point where it shines, the greater the accuracy.

Picky AOA 4 is simply another view point.

On the first blade, turn on the lasers and place the tool on the blade and rotate the blade to near level, exacts at this point do not matter, we are simply creating a “point of reference”

The top laser is shining on the wall place a piece of card on the wall at that point and mark a cross where the laser shines. The second (right) laser is shining on the floor or the roof depending which you chose, create a reference point with cross there, if it's on the roof ... careful.. We now have two points of reference. Turn the prop so the next blade is near level, install the tool on the blade and slowly turn the blade till the left pointing laser shines on the first reference point which is the wall. Now adjust the blade AOA till the second laser points to the second reference point. Do the same for all the blades.


This method reduced the prop vibes on my machine and I “think” I get better performance but have no data to prove it.

Cost about 10 euros
 

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