Constant Speed Prop Dynamic Balance

[skippydiesel]

The DB instructions for my constant speed prop, advise that the DB be done, at Cruise RPM,  with the prop on the fine pitch stop.

 

Having  followed the instructions, I thought I may digress and see what happens when the prop is turned to a Crusie pitch setting.

 

The vibration increased marginally - still well within the recommended level. 

 

I have writen to the prop manufacturer, asking why the prop is not balanced at the Cruise prop setting, where I spend most of my flight time - early days, yet to have a reply.

 

I thought this might be a good topic for the Forum to discuss/nut out.

 

Any thoughts?

 

😈

[FlyBoy1960]

more cavitation ?

loosing the laminar flow because tha AOI is to high and your losing air in the boundary layer ?

[skippydiesel]

18 hours ago, FlyBoy1960 said:

more cavitation ?

loosing the laminar flow because tha AOI is to high and your losing air in the boundary layer ?

Please expand.

 

How might your theory impact on vibration analysis/correction/balance??😈

[FlyBoy1960]

you need to think about a propeller flying through the air, much like a wing on an aircraft.

 

If you increase the angle of attack on the blades by going to course pitch you are effectively doing what you would do by pulling the stick back and getting stall buffeting in an aircraft. That is probably whether vibration is coming from. It doesn't help that you are sitting on the ground just beating the air.

[facthunter]

A prop balance only deals with Mass of the prop structure.  Any turbulence caused by airflow or separation is not addressed. It can't be. It's not the  Aim of the game.  Nev

[FlyBoy1960]

(turbulence)   It has to be if what you are balancing is vibrating.  

 

Changing pitch alters:

Blade aerodynamic center of pressure

Torsional load on crankshaft

Gyroscopic forces

Engine firing pulse coupling

Blade flex characteristics

 

So even though the mass imbalance doesn’t change, the way forces transmit into the airframe does. and for best results you must do the balance at cruise rpm AND cruise pitch, most probably while flying ?
 

If you balance in fine pitch but fly mostly in coarse pitch:  The aircraft will still vibrate in normal flight.

 

From the Dynavibe manual FAQ's

 

Edited February 15 by FlyBoy1960

[skippydiesel]

"....for best results you must do the balance at cruise rpm AND cruise pitch, most probably while flying ?"

 

I am using a DynaVibe dynamic balancer. There are practical difficulties with conducting the balance in the air, primarily fitting the cowling over the sensors.😈

[FlyBoy1960]

I know this, but it came from their FAQ section

[skippydiesel]

19 minutes ago, FlyBoy1960 said:

I know this, but it came from their FAQ section

I liked the idea - testing in flight, just not sure I can do it BUT will definitely explore further.

 

Should it prove possible, with my engine/cowl clearances', it will be very interesting to see how the ground based testing/adjustment translates to the flying aircraft.

 

 

Still waiting for the prop manufacturer to get back to me.😈

Edited February 16 by skippydiesel

[facthunter]

A new vibration will Indicate "something" is wrong. Do you really want it to be masked? If all the Parts are Balanced to the acceptable figures, I consider that to be the Most acceptable situation. Plus you can swap Parts quickly, knowing it will be within tolerences.. Nev

[skippydiesel]

My prop manufacturer has finally responded to my question their recommended fine pitch V my exploratory coarse pitch @ Cruise rpm.

 

"Wrt your question about testing the prop at 5000rpm in MAN vs AUTO:

Yes, 5000rpm (or otherwise your normal cruise RPM) is the most commonly attained RPM during flight. With the constant speed prop in auto/cruise - this is often attained by a reduced engine power setting and the propeller at the fine pitch limit (for flight economy)-  the fine pitch limit is the most commonly reached pitch setting in flight.

 

Your suggestion to alternatively test balance in manual mode at 5000rpm (by adjusting fine/coarse) and at a higher power setting is also acceptable. The reason we recommend the first way is simply because automatic operation of the propeller is considered as 'normal' operation i.e. manual mode is not typically used besides during start up checks."

 

 Do you think they have answered why they recomend fine pitch @ 5000 rpm in their Manual?😈 

[FlyBoy1960]

no

[facthunter]

Don't confuse Limit stops with Pitch  control selection positions. I have difficulty determining what is actually being referred to. CS is Not the SAME as in flight Manually adjusted Pitch which stays at the Position  you set it to.. The Fine Pitch Limit stop (in the Prop Hub) is set so that the engine does not have to Be revved too high for you to be comfortably above the Planes stall speed IF the Prop fails to full fine .  Nev

[facthunter]

The centrifugal turning moment tries to force the Blades to fine Pitch when the prop is turning. Nev

[skippydiesel]

So far I have taken the following from the responses to this question;

• Prop manufacturer sees no reason for not analysing vibration at Cruise rpm & Cruise pitch.
• Doing a vibration test in the air, may result in a better vibration analysis.

I do not think I am any the wiser, as to why the prop manufacturer specifies having the prop at Fine Pitch, for this test.

 

😈

[facthunter]

HIS last line is enough for you to go on. You have Manual pitch adjustment as well?  WE  don't know  How  the question  was Put. The fine CS setting on the control is NORMAL apparently. That doesn't mean the prop is/stays at fine pitch or the Motor would overspeed as your airspeed increases.  C/S means engine revs stay constant (over a certain range of Airspeeds). Nev

[skippydiesel]

From another Forum, for your consideration😈

My question is not about the process as such but why does the CS propeller makers Manual advise conducting the test at fine pitch?

"........ this likely because you are capturing this data on the ground. Because rotary wing aerodynamics. If you set cruise pitch/RPM setting on the ground, sitting statically, the angle of attack on the blade would be far higher than in fine pitch, something it would not be encountering in flight because of the big differences in induced flow velocity (thence the resultant relative wind), therefore the data would not be representative. The angle of attack for your prop blade running at fine pitch on the ground is likely not too far removed from what it would be in flight at your cruise RPM/pitch. This is why the prop pitch setting is minimal on the ground and in takeoff and is advanced for your in-flight regimes (ie climbout, cruise, descent)- so it is operating at its design factor of merit (your prop efficiency) as much as possible...and also the most likely explanation as to why they want flat pitch on the ground. If you were doing this in flight, then there would be an appropriate regime for this to capture the data...in your case, the OEM specified pitch & RPM setting for a given airspeed & pressure altitude.

This video explains, visually, what I am talking about:"

 

[facthunter]

WHY bring Rotor Blades into it.? You are just complicating things.   It's NOT a propeller. It's rotary wing.  Your Prop should be on the fine  Pitch stop till you are doing 1.3 Vs. Nev