Panman
Topic Author
Posts: 603
Joined: Mon Aug 09, 1999 8:25 pm

Props Theory

Wed Nov 13, 2002 10:24 pm

Just going to quickly explain Centrifugal Twisting Moments (CTM - also known as centrifugal twisting force in some areas) and Aerodynamic Twisting Moments (ATM - also known as Aerodynamic Twisting Force) on a propellor any input/corrections (regarding the theory/terminology) would be appreciated. Oh obviously this is dealing with VP props. I'm trying to explain it without drawing a diagram (which would make it way too easy to explain).

CTMs:

As a prop rotates centrifugal force acts outwards through the center of mass trying to a) pull the prop out of the hub, b) turn the prop to a finer pitch about the pitch axis. To explain point b) further. Visualise a prop blade and divide it into half. Each section now has it's own center of mass. Once again centrifugal force is acting radially outwards from the hub center through the center of mass of both sections. Basic trigonometry tells us that a force canbe divided into component parts (triangle/rectangle of forces) thus centrifugal force can be broken down into one force that acts parallel to the plane of rotation of the prop and one parallel to a line drawn through the center of the prop. Now visualise a cross section of the prop, the two centres of mass each have a force acting out towards the leading/trailing edge of the prop and parallel to the plane of rotation. This force tries to pull the prop towards a finer pitch. This force is called Centrifugal Twisting Moment. Factors that affect the value of CTMS are 1) Blade mass, 2) Speed of rotation of the prop assembly, 3) Blade chord length (longer moment arm), 4) Blade aspect ratio (longer blades have longer moment arms).

ATMs:

A wing moving through air creates lift due to a pressure difference between the upper and lower surfaces. Air flowing over the top of the wing has a greater distance to travel than air beneath the wing and has to move faster in order to "catch up" to the air beneath. This creates an area of low pressure over the wing (think I should mention Bernoulli's here) which creates lift. The amount of lift and the direction that it acts (in regard to a point on the chord of the wing in the region of 1/3 of the chord's length from the leading edge) is a function of the centre of pressure (the center of this lower pressure area). Likewise as a prop blade moves through the air, an area of low pressure is created just in front of the prop. The forces generated by a prop as it rotates are a force for torque that is parallel and opposite to the direction of rotation and a force for thrust which acts in the direction that the aircraft is moving (which in most instances is forward). The resultant force (once again rectangle of forces comes into play here) and acts through the centre of pressure. This resultant force is termed the aerodynamic twisting moment and tends to turn the prop towards a coarser pitch.

PanmaN
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Props Theory

Thu Nov 14, 2002 9:10 pm

And now that I’ve had time to read it with more care, I’ll try to provide some feedback:

Isn't the centripetal force by definition always in the plane of rotation of the propeller? That makes it rather hard to break it up into one component in the plane of rotation and one component in any other plane… You asked for input on terminology, and one point is that it might be better sticking with vectors/forces being “in the plane of…” or “parallell to the [line/vector/axis]”? A force being “parallell to a plane” is somewhat confusing IMO. The sentence “parallel to a line drawn through the center of the prop” is not as good as it could be, I think.

How about breaking it up into one component parallell with the propeller blade's pivot axis (assuming this axis to be in the plane of rotation and largely parallell with the centripetal force on the blade as a whole) and one component also in the plane of rotation but perpendicular to the blade's axis and acting in the half-blade’s CoM (splitting the blade in two made for a very nice explanation, BTW)? With one half of the blade forward and one half aft of the pivot axis, that explains the force couple/torque driving the blade towards fine pitch.

As for ATMs, your explanation of lift is off. I guess you are aware of this but are trying to make things simpler. I still don't think it is a good idea to refer to the old theory of the upper airstream playing catch-up with the lower however, it makes further understanding too difficult for no real reason.

Perhaps it would be of interest to mention the counterweights which are put in place to give the propeller the correct default behaviour on loss of controlling oil pressure as well?

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Props Theory

Thu Nov 14, 2002 9:17 pm

As for the propeller spelling issue, five out of five printed dictionaries I have available only give propeller, one of those being Jane’s Aviation Dictionary (3rd edition). One gives both and that’s the online www.dictionary.com (listing the American Heritage Dictionary as its source).

I know where I stand. If Jane’s can’t be trusted as official information, where will you find it? But I will give you the benefit of the doubt. Where did you find “propellor” listed? I suspect it might be a survivor from the early days of aviation (‘Heritage Dictionary’?) and a contemporary of the airscrew - which is of course technically correct, but hardly the right term to use if you want to be respected by your peers as a knowledgeable person, up to date on the latest and greatest in aviation.

Hopefully, this time the post will be left as it stands. Apparently there are varying opinions on the terminology and a discussion could be interesting. In this field, agreeing on the terminology used is quite important as misunderstandings can have severe consequences.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Panman
Topic Author
Posts: 603
Joined: Mon Aug 09, 1999 8:25 pm

RE: Props Theory

Thu Nov 14, 2002 10:05 pm

Fred,

Did you mean Centripetal or Centrifugal in your first paragraph, I only ask as I was always referring to centrifugal force. Which does act outwards from the center of rotation and in this instance it would be outwards in the plane of rotation. (Isnt centripetal force towards the center and centrifugal away from the center?) But yes I should not have mentioned the force being "parallel to a plane" or "parallel to a line drawn...." but "parallel to the pitch axis" and "perpendicular to the pitch axis". Thanks for that, exactly the type of input I was looking for.

It was easiest to visualise the effects of centrifugal force on the blade by splitting it in half along the pitch axis.

I hear what you say about the counterweights BUT....

Not all prop blades have counterweights. Only single-acting bracket type props have the counterweight (hmm could be wrong I will need to check my notes - but I don't recall double acting bracket type props) - the counterweight being attached to the bracket. During prop rotation the counterweight has the effect of turning the blade to coarse pitch - CTMs and CSU (constant speed unit) oil turn the prop towards fine pitch. Now in these types of props with feathering ability should the CSU fail then the the counterweight would move the prop towards coarse pitch and once the prop revs have not fallen below a specific value (in the region of 800 rpms) then the centrifugal latches will still be open permitting the prop to go through the coarse pitch setting and into feather (whether auto-feathering or manual by selecting turning the feathering pump on after moving the relevant prop control lever to feather).
(The full range of settings on a prop with reverse/brake settings reverse->brake->ground idle->flight idle->coarse->feather - but then that would be taking me on to another subject re: pitch stops).

Double acting props however don't have brackets & counterweights (see comment above re: may be wrong), relying on the CSU to provide controlling oil to move the prop in both directions. CTMs & CSU oil move the prop to fine, a combination of spring/gas (nitrogen)/CSU oil move the prop to coarse.

I could be a bit off the mark there, I will know once I review my prop notes over the weekend. This is just off the top of my head not having looked at props for the better part of 7 months so very rusty). I could go into all the specifics over the weekend if you wish?

PanMan

pro·pel·ler also pro·pel·lor Pronunciation Key (pr -p l r)
n.
A machine for propelling an aircraft or boat, consisting of a power-driven shaft with radiating blades that are placed so as to thrust air or water in a desired direction when spinning.
 
Guest

RE: Props Theory

Fri Nov 15, 2002 12:05 am

Hi PanMan,

Your description is correct , centrifugal force will be the primary force in the picture , to explain in simple terms if a propellor breaks off it would fly tangentially to the circular motion of the prop . Centrifugal force is the force which is outward and applies to every object in a circular motion . Centripetal force on the other hand is the one which would try to keep the prop in place going towards the center . Now for centrifugal twisting motion or CTM as you put it . The foce which would apply if the props were absolutely flat will be just centrifugal force . Now the prop is not flat and is twisted (bad description but hopefully works) The twist is given in order to pull the air inward in order to provide lift . Now imagin a prop breaking from the engine , becasue of the twist of the prop it is going to spiral outwards while the centrifugal force is going to pitch it outward . Thats exactly waht CTM is . Those were absolutely atrocious examples which I gave about the prop breaking but it explains the forces acting on the prop . Now for how each of the aspects you have mentioned affect these:
Blade Mass : Affects centrifugal force .
Rotation: provides the omega for centrifugal and centripetal force.
Chord length and aspect ratio: directly proportional to the twisting force.

Not much i can talk about ATM . It seems to be very well explained.

Propellor and propeller ? Both correct , check out the american and british dictionaries , American English lists it as propeller and the british dictionary list it as propellor , similar to colour and color .

Hope I haven't screwed up anywhere , been a long time since my grad in physics.

Regards




 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Props Theory

Fri Nov 15, 2002 5:17 pm

Yup, there are double-acting props with counterweights. One is the Hamilton Standard on the SF34 (some came with Dowtys) where the counterweight pulls the prop to 50° pitch IIRC. I did not imply that all propellers have counterweights, but I think mentioning them would fit in rather nicely with what you are explaining. I’m also rather rusty, been out of the prop driven side of things for about as long as you have. It is scary how fast you forget the details if you’re not working with something on a daily basis!

As for centrifugal and centripetal forces, it is my physics education making me allergic to any mentioning of centrifugal forces as they simply don’t exist. OTOH, using this imaginary force certainly makes explaining some things easier. One of those tough-to-make judgement calls, keeping it correct or making understanding easier…

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

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