Dumb propeller question.

Status
Not open for further replies.
Joined
May 12, 2009
Messages
5,532
Location
Canada
What determines the # of blades on an airplane propeller?

I think it's because each blade must operate in 'clean air'
(not in the vortexes of the blade ahead)
Also, prop diameter, might also be a factor, for the same reason.
But really I don't know!
21.gif


Is Air speed or designed altitude part of the equation?

Reason: I have seen similar aircraft with more or less blades on their props.

How does a super sonic prop work?

Can props incur cavitation?
 
Cavitation is a phenomenon in liquids, when turbulence and "suction" cause a phase change from liquid to gas...not something that can happen in props (or turbos).

Fans/impellers etc. can stall, and surge depending on environment.
 
Our flight department has an interesting Aerobatic aircraft, an experimental Extra 300L.

It's available with 3 or 4 bladed props.

We installed a more powerful engine. The 3 bladed prop was, at times, unable to absorb all the HP and the engine would overspeed.

The solution was to install an experimental 4 bladed prop with the "larger" blades from the 3 bladed unit. That way, all the HP could be converted into thrust.

It's not exactly what you might think either. On the ground, the 3 bladed prop would easily absorb all the HP, by increasing the pitch to extreme levels. Think of paddles going through the air at 90 degrees. But on climb out, where angle of attack is high (nose is pitched up and airspeed is low) , and full power is requested, the prop would allow an overspeed. (remember, the prop varies it's pitch according to need) .
 
C-130's have had 3 blades (a-b models), 4 blades(e-h models), 6 blades (j model), and may end up with 8 blades (h model retrofit).

It depends on design. If you look at their shape each model has different shaped blades.
 
Thanks Guys, some interesting reading to do in this area.
I guess it's one case where there is "more than one way to skin a cat"

Shannow, Thank you, I know a little about cavitation in liquids, I just threw the term out there, as I suspected there may be some other unseen Bug Boo to propeller design. Seems Transonic shock waves might be the equivalent design headache
33.gif
 
And of course the a/c has to have the applicable STC for any prop other than factory spec to go on. It's matched with the engine too. With the right engine you can actually go backwards with a 3 bladed prop. I will admit a 3 bladed prop looks cooler than the common 2 blade.
 
Here's an example of what you don't want. The tips of the long, skinny, two blade prop on the AT-6 go supersonic. Lots of noise and fury, not much go. They're actually louder than most of the unlimited racers.



Ed
 
The blade tips on the Huey and Cobras did as well. Thus the famous "chop chop". GD nice sound to hear when the rock and roll started, though the clank and rattle of an M48 was also welcome.
 
Some of the extremes that come to mind: the WW2 Corsair had to have an inverted gull wings to get the landing gear to give enough clearance for the large diameter prop that was required to use all of the horse-power. And I remember the article about the line control model airplane for racing that used the fact that the efficiency decreased with increase in the number of blades, and increased with the reduction of the number of blades. To get maximum efficiency the racer used one blade with a lead weight in the opposite side of the spinner to counter-balance.
 
What was the old TV series ?

Lost Sheep squadron or something like that ?

I've read about the single blade prop and the lack of wake effect, but it would have a massive couple on the thrust and journal bearings due to a rotating lifting surface having no counterbalance.
 
That would be Black Sheep Squadron starring Robert Conrad as Pappy Boyington. A great series that was fun to watch.

Those were BRAVE men that flew those heaps!
 
We operate a Pilatus PC-12, a single engine, 1200HP turboprop. The aircraft came with a 4 bladed aluminum prop. Very conventional in design.

MT-Propeller came out with a composite 5 bladed flavor, with some very interesting "new" technology. It certainly looks good.

The MT 5 blade is smaller in diameter, and that one feature is valuable, as the PC-12 is a regular on grass and gravel strips.

MT made all sorts of claims about improved performance and so on. I'm not one to believe such claims. Especially in light of the fact that 5 blades have more drag than 4.

Come to find out, the 5 blade program has not been a huge success. As 5 blades make an odd resonant sound on "power off" descent. Pilots and pax find it annoying (according to reports) . Also, the promised improved performance was not noticed by the crew.

Pilatus_PC_12.JPG


pc12_3.jpg
 
Last edited:
Quote:
Also, the promised improved performance


So decresed takeoff and landing runs weren't shorter, nor climb to cruise times decreased?

A smaller diameter propeller with more blades should equal about the same drag as a longer propeller with fewer blades.

Do you know if the chord length and profile/thickness were different between the two propellers?
 
Originally Posted By: MolaKule
Quote:
Also, the promised improved performance


So decresed takeoff and landing runs weren't shorter, nor climb to cruise times decreased?

A smaller diameter propeller with more blades should equal about the same drag as a longer propeller with fewer blades.

Do you know if the chord length and profile/thickness were different between the two propellers?


Pilots report no difference in performance. I have no objective data to compare, our aircraft has the original 4 bladed prop.

The MT 5 bladed prop is made of wood, covered in fiberglass and has a stainless steel leading edge. I suspect it's thicker at the root and thinner at the tip (by visual inspection) . But, once again, I can't verify that.

You are right, all else being equal, a smaller prop with more blades should have similar drag. In this case, since there is no detectable difference in performance, I suspect we've reached the limit of prop efficiency.

Just an FYI, the later model PC-12, with a slightly upgraded engine has significantly better performance. So, the airframe is quite responsive to additional power.
 
The 5-bladed prop is said to cure the vibration problem at 350-950 prop rpm that the 4 bladed prop has. I'm currently flying the PC12 also. Talked to a guy that had flown the 5 bladed prop, he said they gained maybe 2 knots in cruise but nothing on takeoff. He didn't mention any noise on descent.
 
Last edited:
Can someone explain the possible advantages of a Pusher prop over a Puller.

I understand a puller will provide airflow over the wing ( for extra lift)
But the Drones we see on TV all seem to have pushers.
I have also seen amphibians (around here) with pushers.
What's the deal?
 
Pusher props are generally for packaging and noise reasons. To keep the prop clear of something. Possibly a canopy in a sea plane. Or in the case of a Piaggio, it simply keeps the cabin very quiet.

In all cases, the pusher is affected by (sometimes very) disturbed airflow forward of the prop.

Interestingly, a pusher configuration can affect stability, by slightly increasing pitch and yaw stability. It also can stabilize while windmilling.

Engineers can do some very interesting things with aircraft efficiency and engine location. The pusher seaplane you mentioned is certainly not ideal for efficiency. As thrust creates a pitching moment. But, the opposite is also true with other designs. Slight thrust related "pitch up" can, on certain airframes, reduce drag and increase cruise endurance.
 
Originally Posted By: Cujet
Pusher props are generally for packaging and noise reasons. To keep the prop clear of something. Possibly a canopy in a sea plane. Or in the case of a Piaggio, it simply keeps the cabin very quiet.

In all cases, the pusher is affected by (sometimes very) disturbed airflow forward of the prop.

Interestingly, a pusher configuration can affect stability, by slightly increasing pitch and yaw stability. It also can stabilize while windmilling.

Engineers can do some very interesting things with aircraft efficiency and engine location. The pusher seaplane you mentioned is certainly not ideal for efficiency. As thrust creates a pitching moment. But, the opposite is also true with other designs. Slight thrust related "pitch up" can, on certain airframes, reduce drag and





Thank you Cujet for your explanations. I hope to find time this winter to read about some of these subjects, you have given me a good start.
 
Status
Not open for further replies.
Back
Top