Propeller Shapes

[skippydiesel]

We all know that the  shape/dimensions, angle/pitch and length influences the behavior/properties of an airfoil. We also know that some airfoils have a twist over their length to better accommodate differences in speed range  (the swept arc of a prop).

 

A logical assumption would be, that for a given speed (RPM) there would be an optimum  blade design, modified only by limitations on ground clearance (length of blade) and the ability to translate the power of the engine into thrust (number of blades).

 

I would guess that forward speed  and in flight adjust may also have some influence but again logic would suggest a fairly straightforward /consistent end result in design - bit noooo!

 

So why do blades differ so much, from thin, straight fence picket, types to graceful sweeping  scimitar's, thin to broad, ?

 

By way of an example , here we have a 4 different blades, all from the same manufacturer/supplier. My question is what benefits (other than aesthetic) does blade shape confer on the performance of a propeller????

 

[old man emu]

What is the link to the site that contained the picture?

 

If you want to get into the nitty-gritty, sit yourself down with a refreshing drink of your own choosing and read the attached pdf.

62174-244239-1-SM.pdf

 

Here's another reference : https://www.sciencedirect.com/topics/engineering/propeller-blade

 

This diagram is from an article in the above reference. Can you C the error in the vector digrams?

 

[skippydiesel]

OME - got the pic from the Rospeller web site http://www.rospeller-aero.de/the-propellers-in-english.html- very little information went with it.

 

With regard to your pdf - Brain Overload !!!! however after reading , the message to me, was that there are two valid calculation systems, that produce consistently functional propeller designs.

 

I did not get the message that explains the wide range of shapes/designs available for the same hp, similar application & aircraft speed.

 

Check out the Warp Drive Propellers https://warpdriveinc.com/ and compare with the Rospeller blades. The WD's (picket fence) would seem to be at one end of the shapely spectrum and the Rospellers (paddles) at the other.

 

It still seems to me that, in theory, a given power input, operating in a small rpm range, in a very limited forward speed range, should deliver almost identical propellers (if we disregard aesthetics).

 

True advent of more flexible blades (composite) that are designed to distort (change shape) in predictable ways and are durable with it, may change this perception but by how much?

 

There are plenty of other examples and they all seem to have their following and do the job (but how efficiently?).

 

The plethora of propeller suppliers would also suggest that there is still a great deal of room for different offerings (not just on price & materials) 

 

How can this be??

[old man emu]

Not that my post answered the original question of what benefits (other than aesthetic) does blade shape confer on the performance of a propeller????

[skippydiesel]

Just you & me OME - had hoped this topic would get the conversation FLYING!!!

[old man emu]

59 minutes ago, skippydiesel said:

Just you & me OME

 

I think I scared people off with the algebra again! But let's press on, ever hopeful of igniting a discussion.

 

First of all, you have to recognise that the propeller is a complex wing, so the design of the blade is an application of the same Lift formula that is used in wing design.

What we have to consider is the velocity of the blade at various positions along the blade.

This equation tells us that the velocity of a point circling a central hub is dependant on the distance from the hub. Therefore, at each point along the blade the value of TAS in the lift formula is different from all the other points along the blade.

 

We would like the velocity (TAS) of each point, when applied to the Lift formula, to be the same, so, leaving air density as a constant for calculation purposes, and assigning it the numerical value of "1", the things that can be played with are Coefficient of Lift and the surface area of the blade at each point.  

 

In simplest terms the area of a wing is given by (Span x Chord). That's for a rectangular wing as on a PA-28. For a trapezoidal wing, we need to know the semi-span (s), which is the distance from the root to the wing tip, and the chord length at the root (cr) and at the tip (ct). Then from the equation for a trapezoid, the area is one half the sum of the tip and root chords times the semi-span, A = .5 * [ ct + cr ] * s.

 

The hard part is working out the Coefficient of Lift of the aerofoil at each station along the length of the blade. 

http://www.thaitechnics.com/propeller/tg8/prop_element.jpg

 

 

[Marty_d]

13 hours ago, skippydiesel said:

We all know that the  shape/dimensions, angle/pitch and length influences the behavior/properties of an airfoil. We also know that some airfoils have a twist over their length to better accommodate differences in speed range  (the swept arc of a prop).

 

A logical assumption would be, that for a given speed (RPM) there would be an optimum  blade design, modified only by limitations on ground clearance (length of blade) and the ability to translate the power of the engine into thrust (number of blades).

 

I would guess that forward speed  and in flight adjust may also have some influence but again logic would suggest a fairly straightforward /consistent end result in design - bit noooo!

 

So why do blades differ so much, from thin, straight fence picket, types to graceful sweeping  scimitar's, thin to broad, ?

 

By way of an example , here we have a 4 different blades, all from the same manufacturer/supplier. My question is what benefits (other than aesthetic) does blade shape confer on the performance of a propeller????

 

I reckon C and D go backwards.

[Kyle Communications]

You could always stick this prop on your faeta and see how it goes 😂

 

[skippydiesel]

What took you so long Kyle - this is a discussion custom tailored just for you (& others).

 

As for the above: Is it actually meant to fly or just do high speed/noise taxi runs to please the crowd ??? - has a bit of the monster buggy about it - great fun/spectacle but little practical application.

[old man emu]

9 hours ago, Kyle Communications said:

You could always stick this prop on

Clearly that prop is a compromise between torque application and blade span so that the prop has good ground clearance. Since the span is reduced, the chord length must increase to provide the same wing area as a "regular" prop.

[pmccarthy]

The prop on Scrappy came from an airboat. It has done test runs but there was a recent hint that it will be changed for something more aviation like.

[Kyle Communications]

Yes I watch all of Mark Patey's videos...He is unbelievable in what he can do and achieve..then again limitless pockets almost helps you do that. Do yourself a favour Skippy and subscribe to Mike Patey's youtube channel..its worth it I have learnt a lot more about glassing  and other stuff from his videos.

 

As far as not hearing from me...some of us still actually work you know so sometimes it can be a day or two until I see whats been posted...other times I can fire back straight after you post. 

 

I do not profess to know all the maths involved all I know is what I see with my own two eyes. So yes what you think should work to your eyes and brain doesnt necessarily do so....The first time I saw the Eprop blades I couldnt believe they had the rapp that so many have given them...well so far everyone who has them now has a marked improvement in performance...so thats all I have to say on blade shapes...except for asthetics...the rest of it I leave up to much smarter people than myself

 

 

 

[skippydiesel]

Sounds to me you've gone a bit shy Kyle - feeling OK?

[IBob]

You raise an interesting question, Skippy.

It does seem, from tests carried out, that a whole variety of blade shapes will give much the same result for a given engine power setting.

(Which is not the same as saying they give a similar result at the same RPM, a point that JG here tries very hard to get across.)

However, discussions round this generally seem to devolve into 'well I took off prop Y and put on prop Z and it works better, so prop Z must be better'. Which doesn't line up with test results.

So I guess you take your pick: free lunch or no free lunch???

[Kyle Communications]

Hi Bob

It depends on how you setup your prop. A fixed pitch prop needs to be setup differently to a variable pitch prop. JG did his tests as per a variable pitch prop setup and this is why you see little difference between all the props he has tested. The Eprop is setup differently being fixed pitch...how it is setup is how I setup my original Bolly and how everyone I know basically...setup up their props. When you set it up like that you get much better perofmance than before with a different prop setup the same. So all things being equal the Eprop outperforms every prop so far...over 20 props now fitted and not one customer who has said it wasnt worth every cent in the extra perfomrance they have now. I cant say anymore than that really..the proof of the pudding is in the eating

 

[IBob]

If that is the case, Mark, there would seem to be two possible explanations (or a a combination of the two):

1.  The Eprop is more efficient in converting engine power into thrust.

2. The engine is working harder, delivering increased power at a given RPM, when fitted with the Eprop.

It ought to be possible to determine if it's #2 by comparing before and after fuel consumption?

[Kyle Communications]

Totally agree

 

No one though has reported any real extra fuel consumption but that is because their original prop has been set up the same

 

[IBob]

So, you're saying that aircraft are climbing faster/flying faster etc without any increase in fuel consumption?

[Kyle Communications]

Well it certainly appears so with all the reports back

 

[IBob]

Okay. There have certainly been efficiency improvements in things like big industrial fans in recent years (though traditionally many of those were horribly crude in design). So I can entertain the idea of improved/more efficient prop design.

What I am struggling with is JG's tests which, simply put, suggest that for a given engine power output, there is very little difference in the performance from various (same diameter) props.

Assuming, that is, that I am understanding him correctly.

[skippydiesel]

If this be true (I hope it is) the E-prop designers must have developed what appears to be the optimum combination of science & art ,for it would seem that despite my original question/hypothesis, most props are developed this way.

 

Again I ask - how can this be?

 

• For the purpose of this discussion we assume air is a consistent medium.
•  
• We are dealing with aircraft that are generally RAA compliant (similar weight range say 250-600 kg empty to fully laden).
•  
• Engines range from 80 -120 hp, my guess being that most are in the 80-100 hp. Direct drive and gear box, may make some difference as the former tend toward significantly higher propeller speeds.
•  
• Speed range has been increasing, for the last 20 years or so but most will still be in the 90-120 Cruise band. 
•  
• True we have quit draggy  airframes at one end and slippery little beauties at the other.
•  
• Fixed pitch props are by far the most common - dont have any idea as to the proportion that might be ground adjust.

 

Despite are often heated debates ,the above list sagest our aircraft have more in common (than their owners) than not - so why do our props not look very similar??

[facthunter]

I can't think of an all pervading "thing" that would dictate only ONE shape would be efficient. Not ALL are CS either. At the high end of efficiency, only incremental gains can be made with specialised designs. "Bent" blades create structural issues . Gliders have high aspect ratio wings pretty much universally. Supersonic has angular shapes for shock waves. Many angled back tails are only for style on low speed planes.  Highwing and low wing have their own efficiency aspects. Midwing are difficult structurally. Engines are placed "everywhere". Nev

[APenNameAndThatA]

On 04/02/2021 at 10:29 AM, old man emu said:

What is the link to the site that contained the picture?

 

If you want to get into the nitty-gritty, sit yourself down with a refreshing drink of your own choosing and read the attached pdf.

62174-244239-1-SM.pdf 698.21 kB · 2 downloads

 

Here's another reference : https://www.sciencedirect.com/topics/engineering/propeller-blade

 

This diagram is from an article in the above reference. Can you C the error in the vector digrams?

 

I can't see the error in the diagram. From an engineering point of view, centrifugal force does exist because it opposes the centripetal force. Stated differently, if it were not for the centrifugal force, centripetal  force would zap the blade tips into the propeller hub. Stated differently again, centrifugal force is the force exerted by the blades because of their momentum. The idea that centrifugal force does not exist, as I think you have stated, is because if the blade were separated, it would fly in the direction of delta Q rather than the direction of delta C. Where this idea falls down is that the moment the blade breaks off, the centripital force stops, and the moment that happens centrifugal force stops, and the blade stops accelerating. So, if this is what you were referring to, delta C in the diagram is perfectly correct.

[facthunter]

Yes the blade goes off  in a straight line at a tangent. Is that called blade  drift?  Nev

[Old Koreelah]

I know of a gyro with a fixed composite prop which has a distinct bend about 40mm from the end of each blade.

I presumed it’s to induce a bit of twist at higher revs; a crude but automatic pitch change.