News:


Advertise Here

  • May 23, 2022, 08:49:55 AM

Login with username, password and session length

Author Topic: Where should the heavier blade be?  (Read 4301 times)

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1704
Where should the heavier blade be?
« on: September 24, 2015, 08:43:28 PM »
Hi all,
       I read with interest in the recent Aeromodeller magazine that single bladed props are best placed at 90 degrees to the piston at TDC or 0 degrees.
This was in a speed model but I am assuming that it applies regardless to any single cylinder IC engine with a heavier blade.

The article mentions heavy maths involved as proof but basically the rocking couple produced by placing the heavier blade at 0 or 180 degrees is worse than suffering the out of phase at 90.

Any thoughts on this?
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1704
Re: Where should the heavier blade be?
« Reply #1 on: October 02, 2015, 02:44:31 AM »
Wow!

No replies about a hobby that predominately uses a single cylinder IC engine that is notoriously impossible to balance and along comes an article that places an imbalance that crosses the plane described by the piston at TDC, counter to previous articles and suggestions that I have read that only advocate keeping any imbalance on the same plane - usually a heavier blade placed at 180 degrees or 0 degrees (but never ever 90.)

Just for the record I do not know why a perfectly balanced prop is preferred at the end of a perfectly imbalanced single cylinder engine and have always wondered if a counter balance around the prop area may in fact be a better system to investigate anyway.

Just saying .............
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline PerttiMe

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1098
Re: Where should the heavier blade be?
« Reply #2 on: October 02, 2015, 03:11:03 AM »
I thought single-blade propellers were also balanced, using a counterweight.
I built a Blue Pants as a kid. Wish I still had it. Might even learn to fly it.

Offline GregArdill

  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 142
Re: Where should the heavier blade be?
« Reply #3 on: October 02, 2015, 05:10:05 AM »
Chris,
I've spent some time with speed fliers, even flown a couple of my own, and the only place to position a single bladed prop is so that it is upright when the engine stops.

The reason for this is practical rather than theoretical, it's so the prop won't break on landing.

End of story.

I can't think of any one of the previously mentioned speed fliers who would not go to any length to ensure the prop was totally balanced. Apart from them being very strong on attention to detail, I would think that they would want to remove one possible source of vibration from an admittedly unbalanced basic unit.

Unless the article was written by someone like Pete Halman, I wouldn't take that part of it too seriously. Other parts of the story may have merit.

Come flying soon Chris?

Offline Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 12583
Re: Where should the heavier blade be?
« Reply #4 on: October 02, 2015, 06:40:10 PM »

I can't think of any one of the previously mentioned speed fliers who would not go to any length to ensure the prop was totally balanced. Apart from them being very strong on attention to detail, I would think that they would want to remove one possible source of vibration from an admittedly unbalanced basic unit.

  If nothing else, the energy used to shake the airplane with an unbalanced prop has to come from somewhere, and it comes out of the energy that might otherwise be used to propel the airplane forward. It was my understanding, talking to some of our speed experts, that stiffening up the airplane to vibrate less was a big step forward in performance, for much the same reason.

    Brett

Offline Phil Krankowski

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1031
Re: Where should the heavier blade be?
« Reply #5 on: October 02, 2015, 06:50:53 PM »
If it "seems rough" loosen the prop and turn it one blade (1/2 turn for 2 blades) and try it again.  If the engine runs better then go with it.  If there is no improvement, try a different prop and take this one back to the balancer.

Yes, I balance all my props except the Cox Rubber Ducky props.  After the rubber ducky props eat dirt they ain't in balance anyways.

Phil

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1704
Re: Where should the heavier blade be?
« Reply #6 on: October 02, 2015, 11:32:22 PM »
I thought single-blade propellers were also balanced, using a counterweight.
I believe that they are statically balanced but how does one dynamically balance any prop for use at 30 000 rpm?
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1704
Re: Where should the heavier blade be?
« Reply #7 on: October 02, 2015, 11:43:57 PM »
Chris,
I've spent some time with speed fliers, even flown a couple of my own, and the only place to position a single bladed prop is so that it is upright when the engine stops.

The reason for this is practical rather than theoretical, it's so the prop won't break on landing.

End of story.

I can't think of any one of the previously mentioned speed fliers who would not go to any length to ensure the prop was totally balanced. Apart from them being very strong on attention to detail, I would think that they would want to remove one possible source of vibration from an admittedly unbalanced basic unit.

Unless the article was written by someone like Pete Halman, I wouldn't take that part of it too seriously. Other parts of the story may have merit.

Come flying soon Chris?

Hi Greg,
            I get that the blade should always be furthermost away from the ground on the glide but there is nothing to say that the engine orientation 'should' be exactly at 90 degrees to it  - or is there?

And curiously Peter does mention in the article that even he can not get perfect balance all the time so there fore he chooses to make use of the one imbalance to counter another or in other words do the least amount of shaking considering the total system. (To quote" align them so they're out of sympathy.")

Nice of you to ask about flying mate, I will PM you if you like.

Cheers.
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 12583
Re: Where should the heavier blade be?
« Reply #8 on: October 03, 2015, 12:27:06 AM »
I believe that they are statically balanced but how does one dynamically balance any prop for use at 30 000 rpm?

  You might be able to balance is in the mass-properties sense, but there will always be the vibration induced by the single blade moving around. It's conceivable that you could introduce a mass-properties shift to compensate for CP moving around, but I can't see how you would know how far off to make it.

    Brett

Offline Motorman

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2818
Re: Where should the heavier blade be?
« Reply #9 on: October 03, 2015, 10:31:38 AM »
You can dynamically balance by using the Neddy Morris approach. Run the engine at the rpm you want and put a glass of water next to it. When the water stays still you got it.


MM

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1704
Re: Where should the heavier blade be?
« Reply #10 on: October 03, 2015, 05:58:21 PM »
You can dynamically balance by using the Neddy Morris approach. Run the engine at the rpm you want and put a glass of water next to it. When the water stays still you got it.


MM
You will never get a run condition on the bench , it wont get up onto the pipe, cooling conditions will be way off and thrust loads are nothing like a 200kph circuit will give you.
So with speed, when does the glass of water become usefull?




MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Online Lauri Malila

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1502
Re: Where should the heavier blade be?
« Reply #11 on: October 04, 2015, 07:05:28 AM »
But why would you have one heavier blade? It should be balanced.
After that, you can counter-balance the spinner for same end result. That's what they do in F3D, more or less.
Also, you must first decide wether you focus in rotating or reciprocating masses in balancing.
L

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1704
Re: Where should the heavier blade be?
« Reply #12 on: October 04, 2015, 03:22:20 PM »
But why would you have one heavier blade? It should be balanced.

I agree, but its simply not possible even if you take the one factor of the single blade providing an asymmetric thrust load that is well off centre - there will always be a side loading that can not be compensated for fully.
(I am also running on the assumption here that F3D does not use single bladed props?)

And please accept that I am not an engineer but how does one balance a rotating mass for use at all speeds when massively differing diameters are used?
I am laboring under the thought that you can get it balanced or acceptable for a range of speeds but as the speed range is exceeded 'error' becomes worse.

I guess that is the point of magazine article, the builder gets it not perfect but acceptable for use and for his purposes placing the error at 90 degrees is the best that he can do.

Thanks.
« Last Edit: November 23, 2015, 06:53:38 PM by Chris Wilson »
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline phil c

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2363
Re: Where should the heavier blade be?
« Reply #13 on: October 21, 2015, 07:52:03 PM »
A single blade prop is always out of balance while running due to the blade area.  That couple puts a torque on the crankshaft all the time.  It be partially balanced by putting some of the prop mass balance behind the rear face of the prop to partially off set the prop torque.  That might lead to metal fatigue problems in the weight mount though.

The best solution is to make the prop as light as possible to minimize vibration.

Some engines have used cutouts and counterweights in the prop driver to help offset stresses from the crankshaft counterweight.  Since nobody appears to do it anymore it probably didn't help much.

Does anyone want to calculate the P effect from running the prop around the circle?
phil Cartier


Advertise Here
Tags:
 


Advertise Here