A question about the advantages of more than 3 propellor blades.

I visited Renishaw probes in Gloucestershire some years back and remember that they had a variable pitch RR fan engine somewhere in the facility. From what I recall it was similar in size to an RB211, and designed to reduce engine noise in built up areas.

That would make sense as the RB211 was the engine they were talking of using it on in the article I read, just wish I could remember what publication I read it in. I’m not sure if they ever test ran an engine fitted with it.

If I read it correctly, the early Shackletons did not have contra-rotators?
Tell me more. I only met them in 1955. Soon after which I smashed 8 of them!!
A great thread!
= Tim

Every Shackleton from VW126, the first prototype, to XF730, the last MR3/3 had contra-rotating propellors, it was the only way of hanging the always specified Griffons off the existing Lincoln / Lancaster Merlin engine positions.

Hopefully you mean eight props…..and not eight Shackletons!

Tell us more. 🙂

If I read it correctly, the early Shackletons did not have contra-rotators?

Tell me more. I only met them in 1955. Soon after which I smashed 8 of them!!

A great thread!

= Tim

Of course most of us tend to overlook the most common many bladed prop, the bypass section of a turbofan. There was an idea years ago, before turbofans had such high bypass ratios, of making the bypass fan variable pitch. Don’t know what became of that though.

I visited Renishaw probes in Gloucestershire some years back and remember that they had a variable pitch RR fan engine somewhere in the facility. From what I recall it was similar in size to an RB211, and designed to reduce engine noise in built up areas.

At a guess I would say the economics would figure quite prominently in the thinking even in war time. On that basis I reckon it is quite an accurate guess.

… it begs the question, why weren’t Havards fitted with 3 blades in the first place, as a trainer I would have thought it would have been an advantage to the student to have a quiet ride.

My guess would be that the training of green pilots in wartime consumed a fair number of propellors through nose-overs caused by sloppy landings and ground handling

A two blader was almost certainly cheaper to replace.

But I stress, this is a guess, not knowledge.

Moggy

The Harvard and Stearmans’ ghastly rasping comes from the fact the blade tips are supposedly travelling at supersonic speed, I thought this stemmed from the fact that the props are directly driven by the engine crank and not via a reduction gearbox like most later piston types, so this is more to do with drive and speed rather than amount of blades.

The CAC Wirraway has a 3 blade variable position geared prop, and, based on the NA-16 design, is from an earlier North American machine than the Harvard I or later.

The Wirraway was expected to undertake a wide range of duties than the pure trainer Harvard, so the different prop, I suspect, was chosen for a greater flexibility.

The Wirraway’s prop remains subsonic. Because of the prop, and other changes, it is generally held to be a trickier and more dangerous aircraft than the Harvard T-6 family to fly, IIRC, some of that is a slower control response due to prop-power factors – but I may mis-remember.

HTH.

With RR Merlins RCAF, BOAC and CPA used three bladed props on their Canadair Fours

TCA used four bladed on the trans Canada fleet and three blades on the trans ocean although sometime four bladers were used sometimes

I read that the three blades were faster…..so perhaps that means the fours were more economical and maybe without the same competition on the Candian routes were a better bet where a little longer time was not an issue.

Tudors and Lincolns always seem to have used the fourblade.

The Harvard and Stearmans’ ghastly rasping comes from the fact the blade tips are supposedly travelling at supersonic speed, I thought this stemmed from the fact that the props are directly driven by the engine crank and not via a reduction gearbox like most later piston types, so this is more to do with drive and speed rather than amount of blades.

…like trying to drive a car with one gear, a fixed pitch prop has to be a bad compromise on a high performance aircraft…

That is a very good way of thinking about variable-pitch propellers; as the aircraft’s (almost) infinitely-variable transmission (gearbox).

I’m sure this is a very complex issue with no simple answer…..but it is very good ‘brain exercise’ to think about.

Broadly aero-engines run at a fixed RPM and so do propellers (constant-speed). The tips of the blades mustn’t go supersonic and (at constant RPM) the longer the blade the faster the tip. When the engine produces more power the angle of the blade must increase to produce more drag (and more thrust of course) or the RPM would rise (but the blade length is limited by tip speed). There must be an optimal blade angle for all engine-power and aircraft-speed conditions and increasing the blade angle beyond that must be inefficient (imagine blades almost flat to the direction of rotation – high-drag but low thrust).

So as engine powers increase the blade length, and blade angle, are limited by these factors and the only (efficient) way for the propeller to convert more engine power into thrust is to have more blades!

I know there are a lot of generalities in there…..just thinking out loud. 🙂

Of course most of us tend to overlook the most common many bladed prop, the bypass section of a turbofan. There was an idea years ago, before turbofans had such high bypass ratios, of making the bypass fan variable pitch. Don’t know what became of that though.

Some interesting replies thank you very much, I expected some Mosquito and Allison variants would have 4 bladers.
It answers another question too, the Harvard with it’s 2 blade prop is a very noisy beast, I always understood that it was due to the prop tips going supersonic, however, when Gary Numan was entertaining us in his Harvard/Zero conversion it was fitted with 3 blades and was much quieter and didn’t produce that familiar rasping sound, now I understand why, though it begs the question, why weren’t Havards fitted with 3 blades in the first place, as a trainer I would have thought it would have been an advantage to the student to have a quiet ride, makes a good noise for the film makers though.

Look at the American aircraft, Allison powered aircraft only ever had 3 props…

American Allison-powered (V-1710) aircraft that mounted four-bladed props:

P-39Q
P-63
P-82 (in its later marks, but wartime design)

The Spitfire Mk.I and Hurricane were originally equipped with two blade fixed pitch props. The fixed pitch was far more significant a shortfall than the number of blades – as is often said, like trying to drive a car with one gear, a fixed pitch prop has to be a bad compromise on a high performance aircraft of this type, as it has to cope from landing speed to top speed to some degree.

The Mosquito never had more than 3.

In the interests of pedantry, Sea Mosquito and the target-towing mk 39 had four-blade props.

The reason for the prop rotation on the A400M is to smooth the airflow behind the aircraft to give parachutists a smoother ride and fewer collisions.
They could of course have just staggered the para doors (like the Hastings) to avoid para collisions but they didn’t want to listen.

Prop design is somewhat similar to wing design, just a lot more complex. If you look at the Spitfire as a handy example, more and more power was shoehorned into a mostly unchanged airframe. If you add more power to the same prop, it will turn faster. More speed means more power but at some point the tips will try to go supersonic and you’ll need to change something, either gear it down or add blades. More blades mean more drag, keeping the rpm in a reasonable range, and more lift i.e. more thrust which is why you added the power in the first place. The basic idea behind it is still the lift formula, in that you can get more lift (=thrust) by adding speed or surface area. To add surface area you can create longer blades, broader blades or more blades. On the spitfire the last one was the best option as longer blades were not possible without changing the airframe/gear and broader blades wouldn’t be enough (there are performance penalties as well with those).

Another thing to keep in mind is that as you add blades, the hub becomes more complex as well. Also, as said before, blades influence each other. Because of this more than five blades didn’t come into fashion until our knowledge of aerodynamics caught up.

Hmm, not a very clear story now that I read through it again but it’s the basics in a nutshell I think.:cool:

As others have said, more blades to absorb more power.

Two interesting examples of many blades/contraprops….

The new A-400M has 8-bladed scimitar blades on each of the four engines – but the two engines/props on each wing are counter-rotating (not conta-rotating) – presumably to counter torque ?/ or smooth the airflow over the wing ??

The Antonov An-70 still holds the record for the highest number of propeller BLADES – 56 in total 😮

Each Progress D-27 propfan engine has a contraprop – with eight blades on the front and six blades behind – 14 in total.

Ken

I can’t beleive you wrote that Moggy, the contra-prop arrangement on the Shackleton is nothing to do with countering torque and the effects of ‘swing’.

You are quite correct in that sloppily using the phrase ‘four times’ was not even close to accurate.

However I did not intend to convey that correcting the famed swing to port under take off power was the purpose of the contra-props on the Shackleton. It was merely a handy side effect.

Moggy