Propeller question - my curiosity is getting the better of me

[Ghery]

It has been stated on the boards (this one? and the other one) that the most efficient propeller design would be a single blade. Obviously, this isn't a practical design. As we add blades, it has been stated that efficiency falls off. Obviously, there must be some benefits to additional blades as we mostly fly behind a two bladed prop (balances better than a single blade - duh!). However, I also fly a plane that has been retrofitted with a three bladed prop. There are planes with 4 or more blades.

So, what is it that we are gaining in exchange for the reduced efficiency of a multi-blade propeller?

 

[Jim Chumley]

Obviously, there must be some benefits to additional blades as we mostly fly behind a two bladed prop So, what is it that we are gaining in exchange for the reduced efficiency of a multi-blade propeller?

One benefit is the additional ground clearance for the propeller. Three or four blades, instead of two, are typically shorter blades than those on a prop with two blades.

Jim

 

[Maverick]

It has been stated on the boards (this one? and the other one) that the most efficient propeller design would be a single blade. Obviously, this isn't a practical design. As we add blades, it has been stated that efficiency falls off. Obviously, there must be some benefits to additional blades as we mostly fly behind a two bladed prop (balances better than a single blade - duh!). However, I also fly a plane that has been retrofitted with a three bladed prop. There are planes with 4 or more blades.

So, what is it that we are gaining in exchange for the reduced efficiency of a multi-blade propeller?

My Cardinal had a three blade prop when I bought it so I don't have a before and after comparison for my plane. But, in talks with other Cardinal owners who have made the switch this is what I've learned.

1) There is a speed penalty ranging from 3 to 10 knots depending upon who you talk to, this is obviously not based on objective evidence.

2) Many people believe the three blade prop provides better climb performance.

3) The engine runs smoother and quieter in cruise.

4) A plane with a three blade prop looks sexier than one with a two blade prop. Sorry I couldn't resist throwing that in.

In my experience in my Cardinal here's my take on the above points.

1) The POH tells me my cruise should be 148 Knots at the power setting I normally use, the best I've seen is 140 and that's at 8,000 in cool temps.

2) I have no real comment on this one because I don't have a prior reference with a two blade prop on my airplane.

3) Definitely smoother and quieter than a comparable airplane with a two blade prop and I say this even though I did not have a two blade on this particular airplane.

4) Yeah, I think it is a sexier look than a two blade.

Jeannie

 

[Maverick]

One benefit is the additional ground clearance for the propeller. Three or four blades, instead of two, are typically shorter blades than those on a prop with two blades.

Jim

Oh yeah, what Jim said too.

 

[bstratt]

I recall it also has something to do with blade tip speed. The blades cannot go supersonic so on a shorter blade the tips are slower, which could allow for a higher RPM.

 

[Iceman]

Blade design makes a big difference too. We recently put a 3 bladed hartzell (from a two blade) on our mooney and gained 35-40% on ground roll and initial climb and did not give up any cruise speed. The best investment on the plane yet!

 

[Lance F]

There's an article about this in the latest Aviation Consumer. With 3 blades some improvement in climb and at least a minor loss in cruise speed can be expected. Yep, 3 blades are "sexier." Summary was that with less than 200hp more than 2 blades wouldn't make sense from an engineering standpoint. Above 200, maybe depending on the owner's priorities. In Europe I've seen a bunch of 4 even 5 bladed props on GA airplanes because of the very strict noise standards. My Mooney, 2 blades...oh well.

 

[Ed Guthrie]

So, what is it that we are gaining in exchange for the reduced efficiency of a multi-blade propeller?

A prop blade is a wing. Think of HP as weight. You want to lift heavy weight you need a bigger wing. IOW, the more horsepower the prop must put into the air the larger the "wing" must be. High aspect ratio wings (long and thin) are more efficient than short and fat. You could make the prop longer in order to make it possible to absorb more horsepower, but eventually you run into some practical limitations. One obvious practical problem is the ground (prop clearance). Another not so obvious problem is prop tip speed; the longer the prop blade at a given RPM the faster the tips are moving (2*Pi*r*RPM*60=tip speed in miles/hour). AFAIK, a transonic region on the prop is a structure issue, but I do know that supersonic prop tip(s) is a noise issue.

Taking all of this (and other things) into consideration the perfect prop would be single bladed and long enough to effectively couple the engine horsepower into the air, yet not so long as to strike the ground or have supersonic tip speeds. In reality I think such a prop could maybe couple ~30 HP on a J-3 Cub (I didn't do the math, total SWAG). So, with nowhere else to turn when increased HP is a must, the aircraft designer starts adding blades and accepting multiple blade inefficiency penalties versus having the prop blade strike the ground, build a beefier prop to handle transonics on the blade, and have his aircraft design banned throughout Europe as a noise nuisance.

 

[Dave Krall CFII]

It has been stated on the boards (this one? and the other one) that the most efficient propeller design would be a single blade. Obviously, this isn't a practical design. As we add blades, it has been stated that efficiency falls off. Obviously, there must be some benefits to additional blades as we mostly fly behind a two bladed prop (balances better than a single blade - duh!). However, I also fly a plane that has been retrofitted with a three bladed prop. There are planes with 4 or more blades.

So, what is it that we are gaining in exchange for the reduced efficiency of a multi-blade propeller?

The single blade design was said to be satisfactory, but the owner got real tierd of so many questions asking if he knew his propellar was broken.

 

[Ghery]

A prop blade is a wing. Think of HP as weight. You want to lift heavy weight you need a bigger wing. IOW, the more horsepower the prop must put into the air the larger the "wing" must be. High aspect ratio wings (long and thin) are more efficient than short and fat. You could make the prop longer in order to make it possible to absorb more horsepower, but eventually you run into some practical limitations. One obvious practical problem is the ground (prop clearance). Another not so obvious problem is prop tip speed; the longer the prop blade at a given RPM the faster the tips are moving (2*Pi*r*RPM*60=tip speed in miles/hour). AFAIK, a transonic region on the prop is a structure issue, but I do know that supersonic prop tip(s) is a noise issue.

Taking all of this (and other things) into consideration the perfect prop would be single bladed and long enough to effectively couple the engine horsepower into the air, yet not so long as to strike the ground or have supersonic tip speeds. In reality I think such a prop could maybe couple ~30 HP on a J-3 Cub (I didn't do the math, total SWAG). So, with nowhere else to turn when increased HP is a must, the aircraft designer starts adding blades and accepting multiple blade inefficiency penalties versus having the prop blade strike the ground, build a beefier prop to handle transonics on the blade, and have his aircraft design banned throughout Europe as a noise nuisance.

Thanks, Ed. I thought it was something like that. Explains why those WW II fighters from Europe and Japan with motors around 1000-1500 hp had three blade props, while US fire breathers with 2000 hp had 4 blades. Did we up the hp in the Corsair with the newer models? Early ones had a three blade prop while I recall later versions had 4 blades. Never much question about that big P&W in the nose of a P-47.

 

[Stache]

It is not unusual to observe that aircraft powered by piston or turboprop engines often feature different numbers of propeller blades--ranging from two or three on many light general aviation aircraft to four or more (I believe eight is the maximum ever used) on large bombers or transport planes. The reason for this is most often related to the power produced by the engine in question--more powerful engines require more propeller blades.



So why is it that a more powerful engine would need more propeller blades? To understand this problem, we need to realize that a propeller must be tailored to the specific needs of an engine. The job of the propeller is to "absorb" the power produced by the engine and transmit that power to the air flow passing through the propeller disk. Thus, energy is added to the air to speed it up and generate a thrust force on the propeller blades. If the propeller and engine are not properly matched depending on the power of the engine, both become inefficient and performance suffers.



As engine power increases, the aircraft designer has a limited number of options to design a propeller capable of efficiently absorbing that greater power:

• Increase the blade angle (or the pitch) of the propeller blades. In so doing, the angle of attack of the blades increases allowing the blades to impart greater energy to the air flow.
• Increase the diameter of the propeller disk, i.e. make the blades longer. The blades will therefore transfer more energy by affecting a larger volume of air.
• Increase the revolutions per minute of the propeller. The same amount of energy is transferred to the air but in a shorter time.
• Increase the camber (or curvature) of the blade airfoil. A propeller blade is composed of airfoil shapes just like a wing is. Increasing the camber of a propeller blade creates a greater thrust force just like increasing the camber of a wing creates a greater lift force.
• Increase the chord (or width) of the propeller blades.
• Increase the number of blades.

Although we have discussed a number of possible solutions, many of these options create more problems than they solve.

• Blade angle: The pitch of the blade is set by the angle that optimizes the aerodynamic efficiency of the blade. If we change the angle, we lose one kind of efficiency in order to gain another making this a very unattractive alternative.
• Blade length: While increasing tip speed is a significant issue (see the next point), size constraints are usually the greatest problem with this option. As the propeller size increases, the landing gear become longer to avoid scraping the blade tips on the runway, and this change has a knock-on effect on a number of structural and weight issues.
• Revolutions per minute: For the same propeller diameter, the blade tips travel faster and faster as the rotational speed increases. Eventually, the blade tips become supersonic where shock waves form, drag increases substantially, and efficiency plummets.
• Airfoil camber: The blade airfoils are chosen for optimum aerodynamic efficiency. By changing sections, we again sacrifice one kind of efficiency for another. Increasing camber may also result in problems with the blade structure.

This leaves us with the final two options, increasing the blade chord or the number of blades. Both result in increasing the solidity of the propeller disk. Solidity simply refers to the area of the propeller disk that is occupied by solid components (the blades) versus that area open to the air flow. As solidity increases, a propeller can transfer more power to the air.



While increasing the blade chord is the easier option, it is less efficient because we decrease the aspect ratio of the blades resulting in some loss of aerodynamic efficiency. Thus, increasing the number of blades is the most attractive approach. As the power of engines increased over the years, aircraft designers adopted increasingly more propeller blades. Once they ran out of room on the propeller hub, designers adopted twin contra-rotating propellers on the same engine, such as those used on the Tu-95 which feature a total of eight blades per engine.

Stache