# Propeller Shapes

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Actually, whether centrifugal force is a "real" force or an "apparent" force depends on your point of observation.

The difference between centripetal and centrifugal force has to do with different 'frames of reference,' that is, different viewpoints from which you measure something. Centripetal force and centrifugal force are really the exact same force, just in opposite directions because they're experienced from different frames of reference. Centrifugal force is an outward force apparent in a rotating reference frame. It does not exist when a system is described relative to an inertial frame of reference.

If you are observing a rotating system from the outside you are observing from an inertial frame of reference. Standing watching a kid swing a ball around his head on a string, you see an inward centripetal force acting to constrain the ball to a circular path. The ball is kept in its circular path by the centripetal force exerted along the string to the kid which is preventing the ball from going away from the kid. There is no source of a force to resist the pull towards the kid, except the inertia of the ball due to its velocity.

Ball in circular motion – rope provides centripetal force to keep ball in circle. If the rope breaks the ball continues in straight line with velocity at the time of cutting the rope, in accord with Newton's law of inertia, because centripetal force is no longer there.

However, if you are part of the rotating system, such as if you are in an aircraft doing aerobatics, you experience an apparent centrifugal force pushing you away from the center of the circle, even though what you are actually feeling is the inward centripetal force that is keeping you from literally going off on a tangent.

The whole misconception that centrifugal force exists is because people do not specify the frame of reference from which the observations are being made.

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We have been making these propellers since 1993.

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

"  The wooden prop I had on my aircraft worked OK and it looked good but wasn't that efficient ".   No wooden prop will be good in comparison to a metal prop. Composite is between

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The answer to skippy's question has to be in the testing. As we know from postings here, that is a difficult thing to do.  But if you do it properly, you will sure find out the best prop for you.

Personally, I think it strange that we don't make our own props more. I would like to buy plans for the laminations and the data to give to a computer-router lot. Then I would buy the wood and cut out the laminations, glue them together and send them off to the router company.

When they came back, I would sand and paint and balance the new prop.

This should we way cheaper than buying and fun too. And you could try out different ideas like aspect ratio...  ie fat or thin chord.

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We have been making propellors for well over 100 years including those used in water. You would think that the one perfect design would have been found by now. The problem is that it all depends on what you want from it, power, speed, reliability, efficiency etc or a combination of those things and more. It seems to me that's what is driving people to try and produce the best prop for them. Personally I am happy to accept what reputable manufacturers provide & if it fails to live up to their claims send it back.

The wooden prop I had on my aircraft worked OK and it looked good but wasn't that efficient and I was forever fixing stone chips to the leading edge and tips. The replacement Bolly Bos-5 is heaps better & being ground adjustable I was able to set it up for my sort of flying. It also has a torus on the trailing edge near the hub which is supposed to provide a bit more airflow into the cooling intake nacelles. Other than that it looks like a pretty ordinary propeller but flawlessly manufactured. I am a happy camper prop wise.

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Blade or hub failure is not easy to just accept and cope with, especially with a pusher. If you lose one blade the motor will probably depart the plane or seriously damage the cowl. Where an engine has departed, usually the plane is too tail heavy to be controllable.. Wooden props are probably the most reliable and safe  in this respect except in heavy hail which you should just avoid at all costs anyhow. . Most props have procedures and inspections for  safe operation. How do you determine the safe life of these props? I'd trust people like Sensenich and it's not something to just say she'll be right. Mate. Any curved blade is going to be harder to design structurally as the inside part of the curved structure will be in tension more than "normal". as revs increase.  Nev

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The wooden prop I had on my aircraft worked OK and it looked good but wasn't that efficient ".

No wooden prop will be good in comparison to a metal prop.

Composite is between the two.

BUT

Lots of slow revving motor's won't make use of that efficient metal prop !.

SO.

It's a choice of composite or wood : revs 3500 pm : aircraft speed of less then 100knots. Wood is in it's element,

A little more revs : speed up as well. GO composite.

much better aerodynamics in metal, as well as composite. that are very hard to put into a wooden prop.

BUT

I will say a " Scimitar " prop will outperform our type of composite propellers .

( l see someone Has started making small Scimitar pros ).

spacesailor

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In nature, where similar demands/objectives/environments are imposed, we see a tendency for animal shapes to "conform"  ie be very similar.

The inference (possibly wrong) is that as animals evolve,  to meet the demands imposed by similar environments/demands, so only a small range of body shapes/abilities will be successful.

Success in this case can be viewed as efficiency, leading to survival and dominance, so those body shapes (no matter the species) will become, over time, the norm.

So over millennia we see reptiles that look like modern mammals. We see mammals that look like fish. To a lesser extent mammals that look like birds (this may be the exception that proves the rule).

So if nature is the ultimate engineer/designer and it steers living organisms toward very similar shapes, for a given environment, why do our propellers have so many variations in shape - perhaps we lack the science or are we all being conned?

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18 hours ago, kgwilson said:

We have been making propellors for well over 100 years including those used in water. ......

We sure have...I once came across one of these on a launch hulk in the Kuril Islands. But much less clunky, a lovely hollow centered double helix in bronze, tapered at start and finish so wider at centre than at the ends, and with only the steel propshaft through the centre. The shaft was all but rotted through, and I bent and eventually snapped it, dragged the screw in through the surf.
Sadly, it was far too heavy to fly home with me.......it was a beautiful piece of work.............

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20 minutes ago, skippydiesel said:

In nature, where similar demands/objectives/environments are imposed, we see a tendency for animal shapes to "conform"  ie be very similar.

Form follows function.

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1 hour ago, old man emu said:

Form follows function.

One would think - but air screws seem to be much about form, function being a close but definite second.

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• 6 months later...

Three forms of blades are enough for us: classic paddle, saber-shaped and elliptical. The saber-shaped one prevails, it is quite logical for the existing circumferential speeds. Noise is reduced, its tonality changes, and efficiency increases. There are different opinions: Hungarian pilots measured an increase in thrust of 25 kg, American pilots-by 10%. It is difficult to accurately measure yourself. The important thing is that there is a clear increase. But the blade does not only create traction. It also fights abrasive wear, it bends and tends to break in conditions of changing temperature, humidity and ultraviolet light. To do this, we cover the blade with a solid gelcoat, protect it with stainless steel, and a powerful spar is required inside the blade. For a propeller, the resource, strength and rigidity are important. Today we have removed the resource restrictions. Each operator, after the development of the assigned resource, has the right to independently extend the resource for the next 100 hours.

The elliptical blade has so far few admirers. But its form is the most correct. I hope that the comparative tests will explain a lot to us: https://www.stolspeed.com/nid/46

Edited by meglin
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The pictures show:

1) a saber-shaped blade with a diameter of 1600 mm for Rotax-503/2, 58. It works perfectly on both 582 and 912-100.

2) a straight classical blade on a Czech aircraft with a Rotax-912-100, diameter 1880 mm. Huge thrust on takeoff.

3) elliptical blade. I don't have exact data from the operators. They say it's no worse than a saber-shaped one.

Edited by meglin
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11 hours ago, meglin said:

It also fights abrasive wear, it bends and tends to break in conditions of changing temperature, humidity and ultraviolet light. To do this, we cover the blade with a solid gelcoat, protect it with stainless steel, and a powerful spar is required inside the blade. For a propeller, the resource, strength and rigidity are important. Today we have removed the resource restrictions. Each operator, after the development of the assigned resource, has the right to independently extend the resource for the next 100 hours.

Hi Meglin, I am interested in what you have written here, but I am not understanding part of it.

I am guessing by resource you mean working life?

Also 'bends and tends to break...'?

Can you check your translation? Thanks.

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Thank you for your comments. It is customary for us to separate the service life and the resource. The service life is usually measured in years according to the calendar. And the resource is measured in operating hours. Perhaps you use a different word? I was trying to describe the operation of the blade. The blade is subjected to external loads for bending, stretching, torsion, abrasive wear. During static tests of our old blades, we brought them to break, the forces were 9-12 tons. Now these efforts are even greater.

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Thank you, Meglin.

I think here that would also be calendar time

and what you call resource would be flight hours.

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• 1 month later...
On 05/02/2021 at 3:16 PM, Old Koreelah said:

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.

I agree with Nev, some basics never change, glider wings, short span fighter type wings. You will get some interesting variables. Swept or crescent prop blades and tips I understand, are mostly to reduce tip vortices and shockwave formation at the tip area. Sweeping the leading edge back, creates a series of weak spanwise shockwaves ahead of the leading edge, which slow the airflow down ahead of the wing or blade, so at very high Mach number speeds, the wing will behave like a in subsonic airflow, this means aerofoil shapes good for subsonic flight, will still perform very well at high speed. Bearing in mind, the dilemma designers have, is Supersonic aerofoils do not fly well at all, at low speeds but you need to fly slow to land! very close to Mach 1, when normally, shockwaves would be shock stalling the wing. Considering, as Mach numbers get up to around M .8, initially a “normal” or 90 deg shock wave will form, behind this shockwave, boundary layer airflow separates from the wing surface, in effect stalling the wing behind the shock and leading to high drag. This is why the Handley Page Victor bomber had a crescent wing, so airflow slowed through the shockwaves on the outer wing section.

So I think as prop tips start compressing, shock waves form, which stalls the tip, leads to high drag at the tip and creates a lot of noise, so holding off shock wave formation is important.
Prop design will be making the root area strong enough to handle blade and centrifugal loads, twist and spanwise taper, will make sure lift is equally generated along the blade span. Rounded tips will reduce tip vortices and drag. Remember some Mooneys had 90 deg Winglets on their prop tips? They didn’t seem to last long…Prop blade length will be a clearance and strength compromise. The only reason the F4 Corsair had gull or cranked wings, was so the biggest possible propeller could be used without very long gear legs.
To me that’s the key, a lot of prop blade shape will be a compromise, to get the best performance possible, within constraints of engine power, prop clearance and rotational speed (Mach. number), Prop mass.

As airspeed increases, reducing blade angle of attack.That’s why coarse pitch works well at high or cruise speeds. Taking off in coarse pitch, results in high angle of attack on the blade, due the low forward speed. leading to a stalling of the blade in the blade root area, only the outer span of the blade will be installed. Thrust is lost and the aircraft accelerates very slowly. Watch movie clips of the Supermarine S6 seaplane taking off….it took forever! Simple force vector diagram below shows speed change on angle of attack of blade. That’s why a fixed pitch prop is so restrictive. But cheap!

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3 hours ago, F10 said:

I agree with Nev, some basics never change, glider wings, short span fighter type wings. You will get some interesting variables. Swept or crescent prop blades and tips I understand, are mostly to reduce tip vortices and shockwave formation…

An interesting discussion, but probably not relevent to my quoted words; I guess I didn’t explain too well.

The prop I was referring to had a distinct bend about 100mm from each tip, as if someone had heated the composite prop and bent it almost 90 degrees. At high revs, this would put enormous twisting loads on the outer part of each blade;

I presumed the aim was to cause the blade to twist slightly, resulting in an increase in pitch.

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The effect you describe is certainly possible. High stress in certain areas is also able to be anticipated.. Nev

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56 minutes ago, Old Koreelah said:

An interesting discussion, but probably not relevent to my quoted words; I guess I didn’t explain too well.

The prop I was referring to had a distinct bend about 100mm from each tip, as if someone had heated the composite prop and bent it almost 90 degrees. At high revs, this would put enormous twisting loads on the outer part of each blade;

I presumed the aim was to cause the blade to twist slightly, resulting in an increase in pitch.

Yes I mentioned at one stage Mooneys having this 90 deg bend to the prop tips. I think it was a “winglet” concept, to reduce the prop tip vortices. This would reduce rotational drag and noise. But yes, they didn’t seem popular, I agree, this will put a lot of centrifugal stress on the tip area. Also agree with Ol Emu about age old centripetal/centrifugal force debate. To me what you are feeling is the change of direction. Your body wants to keep going straight, as per Newton 1, centripetal force is forcing it to turn and you feel the opposite and equal reaction, Newton 3.

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An interesting fact is, during the Second World War, probably due to shortages of strategic materials, aluminium alloy in this case, most British fighters had wooden propellers. The bombers tended to have metal ones, maybe due to the blade size, but Hurricanes and Spitfires, in fact all models of Spitfire, had wooden prop blades. An early composite you could say. Because yes, they had brass leading edge abrasion strips. I think the Typhoon and Tempests also had metal blades. But interesting. Maybe it was also a contract thing with prop manufactures?

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F10, I think you'll find most of the British WW2 fighters had laminated wooden props. Aluminium was needed for fuselages and wings and was in short supply and quite expensive.

Aluminium scrap drives were a constant feature of WW2 and millions of pots and pans, and aluminium components from scrapped vehicles, were sought eagerly by the Govts of the day, for aircraft manufacture.

In addition, aluminium propellor construction involved more technical skills than wooden propellors. Futhermore, there were hundreds of thousands of carpenters and woodworkers who had all the necessary skills to make wooden propellors.

Aluminium propellors became necessary as aircraft weight increased enormously (as in fully-loaded bombers), and engine HP kept increasing from the 1000 HP range of the early Spitfires, to the 2500 to 3000 HP of the engines in the later stages of WW2.

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Anything that was designed for war was short lived. Longevity wasn’t their primary concern. Wood is a great medium, still is👍

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We have been making these propellers since 1993.

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On 04/02/2021 at 8:54 PM, old man emu said:

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.

What Skippy is asking though is that if it's so scientific, why do you get seemingly different results? Propeller design seems almost like Feng Shui- the result seems to depend on however the individual designer feels at the time 😄

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What about a flexible prop which automatically changes pitch in the right way?

I've been trying to think of how you would do this, but I'm not smart enough.

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Bruce.

It,s called a ' scimitar ' and the more preasure it receives makes it lower the standard pitch, when getting to cruise speed it moves to a corser pitch.

Tried my wooden home chopped scimitar & notice it had a higher rpm than the standard straight prop, even with an extra One degree pitch increase.

(  same Dia ).

spacesailor