Propeller Question

[Stationair8]

The discussion in the crew room got around to why do some aircraft have two bladed props and some have a three bladed props.

The discussion/arguement was based around the original Piper PA-31/310 being equipped with only a two bladed prop and being slightly faster than a three bladed version that was owned by the same company.

Does a two bladed prop offer any advantage over a three or four bladed prop on a Piper Navajo?

Does the number of blades change the Vmca speed?

[sevenstrokeroll]

some planes have more blads on their props to have greater clearance with the ground...more blades, shorter blades

did you know the US army even experimented with a ONE bladed prop for their light liason/spotter planes?

some props give better cruise performance, some better climb performance...if fixed pitch

Lindbergh had a ''cruise'' prop on the spirit of stlouis.

[minimumunstick]

Choosing a two-bladed propeller will indeed make the aircraft have a faster cruise speed like you say (due to less drag). Whether a propeller is chosen to be two- or three-bladed depends on the performance the designer/manufacturer wants to achieve. If you want to compromise cruise performance for extra take-off and climb performance you would rather have a three bladed prop.

I don't know any specific details regarding the PA-31, but I guess the reasoning might be as simple as that extra take-off and climb performance was preferred rather than a few knots extra cruise speed?

I am not sure how this affects Vmca, but if I were to give an answer I think since three blades gives more drag, Vmca would be higher (higher speed needed to have sufficient control-authority to overcome yawing moment). Also, since having more blades will increase power absorption from the engine at low speeds, this should also increase the yawing moment causing Vmca to increase.

[Landroger]

I'm guessing here, so no throwing stuff please. Is there not an element of 'Four wheel drive' here? By which I mean given the same engine output, less power is routed to each wheel (blade) and thus the slip angles are less, while all of the power is employed usefully.

As the RR Merlin/Gryphon family got more and more powerful over its development, the Spitfire grew more and more blades on its prop, did it not? I'm not certain about four blades, although I'm pretty sure I made a 4 blade Airfix model. Ultimately the Seafire had five blade props to prevent 'deck pecking' and still be able to use the circa 2,400 hp available and later Seafires even had contra-rotating props, as did the Shackleton. In that case, probably because of spacing along the wing, structural strength and asymmetry.

Obviously drag is going to be a major issue with five or six blades, but the trade off must be that the increased power available can be used and still 'get a grip' on the air. A mighty engine like the Gryphon or the R4360 would simply cavitate on just two or three blades, wouldn't it?

Interestin question though. :ok:

Roger

[QJB]

More blades absorb more power without the need for increased propeller diameter or RPM which is limited by tip speeds and ground clearance. That is my understanding. Less blades are more efficient due to less interference in the airflow.

[Irish Steve]

Prop diameter is also a factor of max engine rpm, in that to avoid noise and performance issues, the prop tips must not go sonic, it was a "suggestion" on the PA39 Twin Commanche that as soon as possible after take off to reduce the RPM to 2500 in order to reduce the noise footprint, as 2700 RPM made for a significant noise footprint increase, as we discovered many years ago when night flying training at Lydd :*. By 23:30, with my Twin and 2 Cessna 150's in the circuit, the phones were hopping in the tower. The vast majority, when asked if they took overseas holidays got off the phone very fast, and those that didn't had the facts of pilot training explained to them very simply, along the lines of "they wouldn't be doing it if it wasn't a requirement of being able to fly at night". It was a long night, we did something like 15 landings, most of them full stops, as touch and go landings were not recommended with the twin, and there had to be a specific number of full stop landings to get the rating..

The much larger engines on long haul piston aircraft operated at significantly lower revs, or were geared to reduce the prop RPM, which tended to make this an issue for the smaller aircraft. which were normally direct drive.

Ground clearance was also an issue, not only because of avoiding damage from actual contact, but also to avoid damage on unmade runways from small stones and the like picked up by the prop wash.

[barit1]

The Consolidated PB2Y Coronado flying boat was originally fitted with 3-blade props on all four engines. However, the inboard props encountered heavy spray and thus erosion.

In response to this, the inboard props were replaced with reduced-diameter 4-blade props. Strange-looking bird, but for good reason.

[27/09]

Two for go, three for show as the old saying goes, though it doesn't really tell the whole story.

Three bladers are generally a bit quieter and can give better acceleration however two bladers generally give a faster cruise speed.

More blades are generally used to give better ground clearance and or to absorb more power where it isn't practical to run a two blade prop.

Interesting Piper had to increase the power on the PA31-310 to 325 when they installed the CR engines in order to get the same cruise speed. Non CR aircraft are faster for the same horsepower than CR aircraft.

[aviatorhi]

To "complicate" things a bit further, IIRC the Navajo "Panther" conversion slings 350 HP (CR I believe, long time since I flew a Navajo) engines on the wings and 4 bladed Q-tip props, and is faster than the 2 and 3 blade versions (by more of a margin than you would expect for 60 more HP).

[stuckgear]

Why is it that a more powerful engine would need more propeller blades? To understand this problem, it is important to understand 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.Unfortunately, many of these options create more problems than they solve and are generally impractical for a variety of reasons.

Blade angle: The pitch of the blade is set by the angle that optimizes the aerodynamic efficiency of the blade. If this angle is changed, one kind of efficiency is lost in order to gain another. This tradeoff makes changing the blade angle a very unattractive alternative.
Blade length: While increasing tip speed is a significant issue (see the next point), size constraints are usually the biggest problem with this option. As the propeller size increases, the landing gear must become longer to avoid scraping the blade tips on the runway. This change has a domino effect on a number of other 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, one kind of efficiency is again sacrificed for another. Increasing camber may also result in blade structural problems.We are now left with the final two options, increasing the blade chord or the number of blades. Both have the effect of increasing the solidity of the propeller disk. Solidity simply refers to the area of the propeller disk occupied by solid componenets (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 the aspect ratio of the blades is decreased 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. Two good examples are the Tu-95 bomber and Tu-114 airliner. These Russian aircraft were equipped with the most powerful turboprop engines ever built, and both designs feature a total of eight propeller blades per engine.
- answer by Jeff Scott, 12 August 2001