what dictates 2/3/4 props on an engine?

[Daveroo]

i was reading the recient posts on the paspsed skylark thread and bill made a comment about how it wouldnt support a 3 bladed prop ..( im not qoating exactly,dont jump me..lol)but just what does dictate the use of props?...horse power? displacement?aircraft size?

i have no clue and have allways wondered...at the recient airshow i attended.we saw a lancair..my dad kept saying that plane is really hot it has a 4 bladed prop...i said but what makes it hot..he said...shup..so he didnt know...he didnt even know what it was..i had to go look at it upclose...he just thought cause it is a realitvly small plane (4 seater) with 4 bladed prop..its "hot"...i now what to know WHY they use the differant props....like why did early allison engines have three blades like the P40/P38/P39..even the early P51 i think...but the merlin used a large 4 bladed prop...is the calculation?
and im rambling again........:jump:

[Bjoern]

The more blades, the hotter. :icon_lol:

Also, the more blades, the quieter and more efficient. Otherwise you wouldn't have all the fancy five to seven blade props on modern transporters/commuters.

[IanP]

I always thought it was a power issue - the more power the engine produces, the more power has to be transmitted through the shaft to the prop and thence to the air. If you can't transmit the power produced by the engine into air shifted by the prop, you're at best wasting your time and at worst going to end up with big problems.

To shift more air, you have two choices - a bigger prop, or more blades. The F4U Corsair is an example of the former, the Spitfire XIV is an example of the latter. In addition, as Bjoern says, a multi-blade prop is a lot quieter than a big fewer-bladed one, because the prop tips are a lot slower. Indeed the Spitfire is a very good example anyway - the Spitfire I had a two-blade prop, the Ia three blades. By the time you got to the IX, you had four blades, the XIV had five. If they had stuck with the two blades from the early 990HP prototype (Merlin C), can you imagine how big the blades would have to be to handle the 2050HP of the Griffon 65 in the MkXIV?

Ian P.

[azflyboy]

IanP is right, it's basically a power issue, but airspeed also plays a major role as well.

The biggest limiting factor with propellers is that their efficiency plummets as the blades exceed Mach 1, so the propellers have to be kept subsonic, which becomes increasingly difficult as engine power increases.

By adding more blades, the individual blade diameter can be decreased, which lowers the propeller tip speed and allows the propeller to rotate faster while still keeping the blades subsonic.

Smaller aircraft tend to use 2 or 3 bladed propellers due to the fact that they don't have enough power for propeller speed to be an issue, but larger aircraft use 4 or more blades simply to keep the propeller blades subsonic.

Using a scimitar shaped blade (like the C-130J or AN-70) allows a further increase in propeller speed using the same principals as as swept wing, and while that design allows higher speeds, it also creates a tremendous amount of noise (more than modern turbofans in most cases), which offsets the additional gains in efficiency.

[Ickie]

very interesting
I was once ask about a P-38 M?? with a 5 bladed prop.

does this mean you could add a 4 bladed prop to a cessna and get more power?

[Brett_Henderson]

The blade-tip speed is a big concern.. but the number of blades is just as much about efficency and smoothness.

A prop with fewer blades of larger area and higher pitch (pitch range), can do just as good a job of translating power into thrust; as more blades of less area, and less pitch. It's a give-n-take thing.

Take for example a 200hp Cessna Cardinal. They can be found with both two, and three bladed props, and produce identical performance.

[azflyboy]

very interesting
I was once ask about a P-38 M?? with a 5 bladed prop.

does this mean you could add a 4 bladed prop to a cessna and get more power?

In theory, yes.

However in the real world, 4 bladed propellers simply aren't made in those sizes (around 65-70 inches for most piston singles and twins), since small aircraft almost never have enough power to need anything more than a three bladed propeller. Even large piston twins (like the Cessna 414 and Piper Navajo) usually have three bladed propellers, since their engines only produce about 350hp each.

Even on small turboprop aircraft (where there's rarely less than 500hp per engine), three blade props are seen on aircraft like the Cessna Caravan and some early King Air models, since propeller/airframe combinations are decided after taking a huge number of factors (weight, noise, vibration, ground clearance, maintenance requirements, etc...) into account.

[mfitch]

This is a nice science (aeronautical engineering) lesson. Thanks all for the interesting reading. I am actually designing a course (remedial mathematics) for aviators. Many tidbits are working their way in as examples.

[Snave]

In theory, yes.

However in the real world, 4 bladed propellers simply aren't made in those sizes (around 65-70 inches for most piston singles and twins), since small aircraft almost never have enough power to need anything more than a three bladed propeller. Even large piston twins (like the Cessna 414 and Piper Navajo) usually have three bladed propellers, since their engines only produce about 350hp each.

Even on small turboprop aircraft (where there's rarely less than 500hp per engine), three blade props are seen on aircraft like the Cessna Caravan and some early King Air models, since propeller/airframe combinations are decided after taking a huge number of factors (weight, noise, vibration, ground clearance, maintenance requirements, etc...) into account.

Excellent technical reply. Also an additional factor is that the efficiency of a propeller varies along its length, usually (but not always) being most efficient at about 1/3 `span`. Trouble is, on many aircraft this also puts the area of peak efficiency in a region where the flow past the prop is impeded by the engine, cowling or airframe behind, so that efficiency advantage cannot be fully transferred. Also, three or more blades usually are more transient-efficient, whereby they cope with changes in rotation speed and power application so they work better across a wider total range, whereas a two-bladed prop will be more efficient during one or other phase of flight - hence the expression `climb` or `cruise` prop often used with fixed-pitch two-bladed propellers.

Finally, as you add more blades there is less time for the airflow to pass before the next bade `arrives` and unless you slow the rotation speed down and use very complex blade designs (think six or more bladed turboprops) there is a reduction in efficiency caused by the following blade hitting the turbulent airflow of the former. For simple cost reasons, most fixed-pitch props will be two-blade, there's a crossover point with power/performance as azflyboy describes where constant-speed or adjustable pitch can favour either two or three, and then you move into the high-power, high-performance envelope where four or more are simply necessary to pass the sheer power the engines produce over a wide (air)speed range.

Hope this helps!

[nio]

I flew twice as A passenger in a Boeing Stearman G-AROY many years ago.It was in the livery third down.

http://www.abpic.co.uk/search.php?q=G-AROY&u=reg

I believe the engine had been uprated.

It was flown a lot at UK air displays by the pilot who took me up.He was ex -WW2 ,Berlin Airlift etc.. I could swear that I was told the prop tip exceeded the speed of sound in certain modes.Is my mind playing tricks on me ?

Anyhow the whole experience was a blast.I was sitting in a converted seed tank instead of a seat and had no safety belt.The pilot did a lot of his display routine with me in that tank.Positive g maneouvres only of course. Those loops was still butt-clenchers though:eek:.

Best

nio

[IanP]

Blade tips exceed the speed of sound quite regularly, nio, depending on what you are doing with them. It's more a problem with turboprops than piston engines and as many people have said already, it ain't going to happen on a C150, but if you have a high-revving engine, with a big prop, the blade tips have to be travelling at many times the rotary speed of the engine shaft. The Stearman has a big prop, but I always thought the engine was quite low speed at the shaft? It wouldn't be the first time I was wrong on that.

Ian P.

[cheezyflier]

ok then, how about the 182? in fs9 it has a 3 bladed prop. the carenado version for fsx has a 2 blade prop. both are variable pitch. for that plane, what is the determining factor for 2 vs. 3 blades? if 3 is better why wouldn't all of the 182's have 3?

[nio]

Thanks IanP

It was no big deal then.
It was still quite a prop to sit a couple of feet behind though.

Best

nio

:wavey:

[Snave]

ok then, how about the 182? in fs9 it has a 3 bladed prop. the carenado version for fsx has a 2 blade prop. both are variable pitch. for that plane, what is the determining factor for 2 vs. 3 blades? if 3 is better why wouldn't all of the 182's have 3?

http://en.allexperts.com/q/Aeronautical-Engineering-1809/2008/10/3-BLADE-vs-2.htm

[cheezyflier]

thanks :applause: