How do you take your Propeller? One blade or two?

[old man emu]

I didn't want to "Off topic" Skippydiesel's thread on propeller shape, but while researching material to put in that thread I was called back to the idea of a single blade propeller.

 

We don't give much thought to the fact that propellers fitted to low horsepower engine have at least two blades. It's normal. It's been the way things have been done since Wilbur and Orville got their pilot's licences. But does a propeller for low horsepower engines really need two blades?

 

This thought occupied the mind of Baltimore-based inventor Walter Everts in the 1930's. While most engineers of the time were debating the performance advantages of three-blade props over two-blade props, or four-blade over three-blade, Everts must have been a contrarian, arguing that a one-blade propeller was the most efficient. He calculated that the single-blade propeller not only had less drag than its two-blade cousin. Everts took his eccentric innovation one step further, creating not just a counterweighted single-blade propeller, but one with a blade that freely pivots on an ingenious hub, allowing it to automatically change pitch in flight. As the center of pressure on the blade changes, the propeller’s pitch changes with it. At full power on takeoff, the blade pivots to fine pitch. At reduced power in cruise, it pivots to coarse pitch. 

 

Here's a short video showing the operation of the prop and a comparison of takeoff performance in the 1930s. 

 

The Everel propeller got a lot of publicity, but that publicity did not translate into a lot of sales. Only a hundred or so were sold. One of the issues with the propeller was that it was heavier than a two-blade propeller—a major drawback for an airplane with a 40-horsepower engine. A bigger problem was its cost. Where a two-blade Sensenich at the time cost around $27, the Everel cost around $270—a hefty price given that the cost of a Cub in 1938 was only around $1,000. 

 

Modern testing seems to contradict the 1930's hype. Was the other pilot in the 1930s video pulling his punches?

 

 

The patent for the hub was licensed to the Koppers Corporation of Baltimore, which made the two-blade Aeromatic propeller with automatic pitch control. The Aeromatic design later was built by Univair, and it is manufactured by Tarver Propellers today for homebuilt aircraft only. By the mid-1940s, the Everel name was pretty much forgotten, and today the one-blade propeller has been relegated to a topic shared among RC modelers.

 

 

[skippydiesel]

Thanks OME - you seem to have stolen my non existent audience - however in support of your conversation;

 

It seems to me that while there theory of propeller efficiency leans toward a single blade , in aviation we like structural weight to perform a positive function/gain. So, for some aircraft, having lead in your tail, to balance out some forward system, is considered to be a necessary evil - the lead not contributing to efficiency/gain as such.  Similarly. the single blade prop has this humongous counter weight, which does nothing to translate power into forward motion/gain.

[old man emu]

 

5 minutes ago, skippydiesel said:

Thanks OME - you seem to have stolen my non existent audience

Maybe we should meet at a phone box and carry on this conversation there. There would be room there for all interested parties!

 

The argument was that, if you label one blade "A" and the other "B", then B must pass through air disturbed by A, reducing the efficiency of B. However, that is only true when the relative airflow is not moving, that is, when the aircraft is stationary on the ground. Due to the forward motion of the aircraft, the path traced by a prop is a corkscrew-shaped in side view. That shows that after following A, B moves into undisturbed air.

 

[skippydiesel]

2 hours ago, old man emu said:

 

Maybe we should meet at a phone box and carry on this conversation there. There would be room there for all interested parties!

 

The argument was that, if you label one blade "A" and the other "B", then B must pass through air disturbed by A, reducing the efficiency of B. However, that is only true when the relative airflow is not moving, that is, when the aircraft is stationary on the ground. Due to the forward motion of the aircraft, the path traced by a prop is a corkscrew-shaped in side view. That shows that after following A, B moves into undisturbed air.

 

Just guessing (bad habit) but your disturbed/undisturbed air observation, would likely be true at course pitch & relativly high forward speed but what happens at fine pitch on TO role & climb ??

[IBob]

Would not the rotating centre of thrust impart a corkscrew motion to the nose of the engine/aircraft (requiring reinforcement of bearings and mounts)?

[old man emu]

22 minutes ago, IBob said:

Would not the rotating centre of thrust impart a corkscrew motion to the nose of the engine/aircraft (requiring reinforcement of bearings and mounts)?

In the AOPA video at time 2:18 you see that the prop seems to be attached to some form of universal joint so that it can move. I would think that the universal joint would allow the blade to come to an equilibrium position. Vibration is a result of a rotating object being out of equilibrium. Once that equilibrium was established, I would expect that the forces on the crankshaft of the engine would be of the same order as those caused by a regular multi-bladed prop.

 

Apparently the universal joint allowed the pitch of the prop to alter with the  power output of the system, so that the blade would go to a finer pitch at high revs, then coarsen off as revs decreased. I think that is what the 1930s video is trying to show - the plane with the finer pitch accelerates faster and climbs quicker.

 

 

[IBob]

Ah, I think I've got it: forward thrust generated by the blade would tend to tilt the blade forward on the hub...but this is offset by the counterweight, which will tend to track around a max possible diameter, which is the plane perpendicular to the hub.
That suggests that, in flight, the blade is tilted a little forward of the hub, rotating as a sort of cone shape, rather than a flat disk......

[old man emu]

32 minutes ago, skippydiesel said:

your disturbed/undisturbed air observation, would likely be true at course pitch & relatively high forward speed but what happens at fine pitch on TO role & climb ?

You have to consider the path of the blade. If you agree that in simple terms the path of the blade when the aircraft is moving forward describes a helix, Geometrically, a screw can be viewed as a narrow inclined plane wrapped around a cylinder. We can compare the interaction of the blades with those of the threads of a screw. 

The blade of a propeller can be likened to the flank of a screw thread.

The flank of one thread does not pass collide with the flank of the following thread. Thus the the "following" blade of the prop does not pass through the disturbed air of the "leading" blade.

[old man emu]

10 minutes ago, IBob said:

forward thrust generated by the blade would tend to tilt the blade forward on the hub.

I don't know the answer to that.

 

As for the counterweight, I believe that is simply there to provide a balancing force for the single blade. Just like you put a multi-bladed prop on a balancer to make sure that it doesn't vibrate. 

[facthunter]

Quite a few larger thrust props have 4 blades and are fitted to FASTER planes  The Electra prop on the Allison runs at the same rpms all the time, (after the start cycle) Ground fine on some turbo props will sometimes produce distinct sounds when the blades are obviously tracking in each others wake. Nothing much else happens except you get a burnt kero smell through the airconditioning system.  Nev

[spacesailor]

The Scimitar prop also moves forward to adjust the blades to a finer pitch.

BUT 

It's hard to make and harder to balance.

spacesailor

[Blueadventures]

7 hours ago, old man emu said:

I don't know the answer to that.

 

As for the counterweight, I believe that is simply there to provide a balancing force for the single blade. Just like you put a multi-bladed prop on a balancer to make sure that it doesn't vibrate. 

Prop walk, maybe? That’s what it is sometimes referred to by the maritime tribe.  Mass of prop also plays a part.

[spacesailor]

MASS !

Not for Aero's. lightest weight & max strength is needed in our little world, until you " strike" that prop !, then you need it all reversed,

NO strength need today thank you. LoL

spacesailor

[Bruce Tuncks]

A prop that flexed so as to run finer for take-off and coarser for cruise would be great. As long as it was strong enough.

Alas I am not clever enough to design one.

But single-bladed props have been used on rubber-powered models. They did not have a mechanism to prevent ibob's shaking, and I still don't understand how you could do that.

A 2 bladed prop also had an unbalance except when the inflow air is exactly perpendicular to the prop disc.

 

[facthunter]

More noticeable on a taildragger before the tail is raised or full power near the stall. (similar situation as far as the prop is concerned) The descending blade presents to the airflow at a greater angle.  This gives an assymetric thrust effect. Nev

[kgwilson]

The theory that a single bladed prop is more efficient is all well and good but they look stupid & seem to be heavier so what's the point.

[skippydiesel]

20 minutes ago, kgwilson said:

The theory that a single bladed prop is more efficient is all well and good but they look stupid & seem to be heavier so what's the point.

Well it was a concept that looked good, in theory but just didn't work out in practice.

 

Coupled with the dismal performance in real life, is our species need for symmetry - we tend not to favour asymmetrical objects (unless you are into some forme of rarefied artistic expression) particularly those that are functional.

 

I think you will find that throughout modern history we have experimented wit asymmetry in marine & air craft - some are successful, most are not and few survive the test of time/consumer prejudice.

[old man emu]

Don't forget that the Everel propeller was designed and made (by Sensenich) in the 1930s. You can see that the propeller was made from wood as was usual at the time. It would be quite simple to make the propeller from modern materials (carbon fibre) which would make it so much lighter.

 

Just look at the video of the single-blade props for the RC model. They are plastic. If not made from carbon fibre, the prop could be made from the same stuff as RC nylon props, which hold together at extremely high revolutions.

[Yenn]

A single blade prop made of a very light material such as carbon fibre, would need less balance weight, or the same weight at a lesser radius, so maybe they could be usable nowadays. Damage to the prop blade resulting in its departure would be less of a problem than with a two bladed prop maybe.

[facthunter]

You'd still have about the same imbalance which is what causes the problem. Nev

[old man emu]

9 minutes ago, facthunter said:

You'd still have about the same imbalance which is what causes the problem. Nev

Can you please explain where the imbalance originates? 

 

If a "normal" two-bladed propeller is balanced statically and dynamically, then one would think that the only source of vibration in an engine would be as a result of the intermittent application of force within the cylinders during the combustion sequence. That's why a six cylinder engine appears "smoother" than a four, and an eight smoother than a six.

 

I seemed to have forgotten to post Walter Everel's description of the principles of his propeller so here it is. The only thing that I would edit is his saying that "Aerodynamic thrust is balanced by the centrifugal force of the propeller". You know how I hate "centrifugal force". I'm pretty sure that the correct term is angular momentum of the propeller. 

everel.pdf

[skippydiesel]

2 minutes ago, old man emu said:

Can you please explain where the imbalance originates? 

 

I..................................... That's why a six cylinder engine appears "smoother" than a four, and an eight smoother than a six....................................................

 

 

I always understood it to be the uneven numbers , 9, 7 & 3 cylinders, are the smoothest. The problem is the customer likes even numbers 4, 6, 8 & even 12.

[old man emu]

Odds or evens, the more cylinders the smoother the engine. 

 

Think of it this way. If you are driving your car along a dirt road and hit a single pothole, you really feel it. But if you drive along a road with a corrugated surface, you can find a speed where the on and off again contact with the road surface appears smooth.

 

 

[kgwilson]

Radials have to have an odd number of cylinders because of the single crank so the firing order is every other cylinder. If it is a 2 stroke it doesn't matter so you could have an even number of cylinders.

[skippydiesel]

51 minutes ago, kgwilson said:

Radials have to have an odd number of cylinders because of the single crank so the firing order is every other cylinder. If it is a 2 stroke it doesn't matter so you could have an even number of cylinders.

I dont have the learning to debate the issue with any assurance but I will tell you a very learned engineering friend of mine told me , many tears ago, that the odd cylindered engines, confer some sort of balancing/smoothing out in their firing pattern and he wasn't talking about radial engines as such..

Soooooo far many years I collected Mercedes Benz W123, 300D's - a five cylinder single overhead cam engine that ran like the proverbial sowing machine - truly one of the great cars.

Had to sell them all when I retired (it was them or the aircraft).

As a consolation, I now drive a Ford Ranger, PX 1 with a 5 cylinder turbo diesel - not bad at all but not quite as smooth or easy to work on as the old MB's and will never even come close in "ambiance"