Question about propellers

[HandyNasty]

Often times I cruise the forums and suddenly see a remark like "yeah, but the broad propellers make it accelerate faster at slow speeds"   (just inventing here). 

Or when you look at differences between some models of planes, the only difference is the propeller (example being P-39Q-21 which is a  P39Q-20 fitted with a four-bladed Aeroproducts propeller - source : wikipedia). 

 

This piqued my interest - i know there is a difference between wing designs, with benefits and tradeoffs for different designs. Concerning propellors, I wonder if someone could explain (or post link to explanation) the effects of different propeller specifications/properties (length, are the blades broad/thin, 3-bladed, 4-bladed, blade curvature, why are the blades thinner near the hub, then broader?, difference between wood or metal props??, ...) on the aircraft's performance. What are some typical tradeoffs concerning propellers on ww2 warbirds?

 

Note that I have mathematical background so if you link a course of propellor theory in which the subject is approached from maths, i can possibly understand it :D

Also note that I'm not asking the difference between variable speed props or constant speed props, but really more about the properties of the blades themselves!

 

 

HandyNasty

 

[Finkeren]

Bookmarking this thread. Propellers is a topic I'd very much like to know more about.

[Herne]

I don't know anything about the subject but did a quick search and found this PDF that made my eyes glaze over, but you might find it interesting.

 

https://www.researchgate.net/profile/Anthony_Colozza/publication/2517460_High_Altitude_Propeller_Design_and_Analysis_Overview/links/00b7d535eaeb6368f2000000/High-Altitude-Propeller-Design-and-Analysis-Overview.pdf

[schurem]

Here's some quick googlage:

 

https://www.grc.nasa.gov/www/k-12/airplane/propanl.html

 

http://s6.aeromech.usyd.edu.au/aerodynamics/index.php/sample-page/propulsion/blade-element-propeller-theory/

 

Those two are way over my dumb ol' head so I just stick with this:

 

http://www.explainthatstuff.com/how-propellers-work.html

[=X51=VC_]

The blade is a wing, so it should have the same trade-offs in terms of lift efficiency with shape. Blades look narrow near the hub because the tangential velocity is low there and it won't produce useful lift (thrust) so it twists to give lower frontal area (lower axial drag) while maintaining strength and rigidity. It's not actually narrower, it has a shorter chord and eventually is closer to a pipe than an aerofoil.

 

There are practical limits to how fast you can spin a propeller and indeed how big you can make it. After a point, increasing blade area either by wider blades or more blades is the only way you can continue to convert more engine power to thrust. Assuming your engine has enough torque to spin it against the greater tangential drag. But increased blade area nets greater axial drag, offsetting some of the thrust gain at high speed. And wider blades are less efficient as by definition they are low aspect ratio aerofoils.

 

Once you get to modern props, swept tips are there to reduce issues arising from transonic tip speeds.

[von_Tom]

The propeller is the spinny thing at the front of my plane.  Sometimes it's at the back or going around and around as I slowly spiral into the ground.  It goes faster and this make my plane go faster.  If I am ever in a bomber *shudder* and one engine dies, the propeller on that engine usually makes me slow down, stall and spin into the ground.  This happens usually when I am trying to feather it.

 

That's all I now about propellors.

 

von Tom

[schurem]

Isnt the propeller there to keep the pilot cool? They sure start sweatin' when it stops... <badabum pssh!>

[HandyNasty]

 

 

There are practical limits to how fast you can spin a propeller and indeed how big you can make it. After a point, increasing blade area either by wider blades or more blades is the only way you can continue to convert more engine power to thrust. Assuming your engine has enough torque to spin it against the greater tangential drag. But increased blade area nets greater axial drag, offsetting some of the thrust gain at high speed. And wider blades are less efficient as by definition they are low aspect ratio aerofoils.

 

So, broad blades decrease top speed. Is it right then to say a 109 equipped with longer and thinner prop blades would have a higher top speed than the a 109 with rather short and broad prop blades (that is, supposed the longer blades would fit the 109)?

Why also would the late-war 109's (or 190's) with ±2000hp engines not fit 4 bladed props? Or same question posed otherwise : Why did griffon spits have 5 bladed props? Why not 3 blades, but broader blades? The engine output is roughly the same, ±2200hp or so. Why the different approach between both ?(both planes have quite low ground clearance so "longer prop blades" is practically not feasible to convert the extra HP's into thrust)

[=X51=VC_]

More prop area won't decrease top speed, it just won't increase it as much as you might think relative to how much more engine power you're putting in.

 

A thinner prop blade is a more efficient lift generator in the same way a high aspect ratio wing is. So I think the Griffons prop is the better solution. But slapping bigger blades on an existing propeller hub is relatively easy compared to designing a whole new constant speed unit for more blades, and a 5-blade unit is mechanically very complex. Speculating a little, I assume that had an impact in design choices given Germany's industrial state vs. Britain's.

[Bilbo_Baggins]

Great topic.

 

I'd be interested to hear how the VDM propellers changed on the F4 from F2, since the F2 is faster on the deck with less power than the F4, same airframes.

[=X51=VC_]

Wiki says wider blades for better altitude performance on the F-4, so if you're right I guess wider blades do reduce top speed, at least on the deck. But since they move more air they're better higher up.

 

How much faster is the F-2 than the F-4 at sea level?

[Finkeren]

I know that Wikipedia says, that the wider blades increased performance at altitude, but is it possible, that the change was simply done because the narrow-bladed prop was unable to harness the increased power of the DB601E? 

 

From the discussion so far it seems, that widening the blades of the prop is the simplest and easiest way to allow it to produce more thrust at the expense of reduced efficiency. Maybe the wider blades were just a simple solution to an immediate necessity after the change of engine?

[Finkeren]

If you look at pictures of museum examples of the Yak-9U with the powerful VK-107, the prop blades appear significantly beefier than on the Yak-3 with the weaker VK-105PF-2:

 

 

[HandyNasty]

On another forum I found the theory that the germans didn't increase the number of propeller blades because they used synchronized guns in the nose. Adding extra blades would decrease the rate of fire of those nose-mounted guns. 

[Finkeren]

On another forum I found the theory that the germans didn't increase the number of propeller blades because they used synchronized guns in the nose. Adding extra blades would decrease the rate of fire of those nose-mounted guns. 

 

Sounds like conjecture to me. At the RPMs WW2 props usually spun, the synchronisation gear would give off far more firing impulses than even the fastest firing MGs of the era would need.

 

If it had something to do with the sychronisation, it was more likely simply that the German industry produced units  that worked for 3-bladed props and would have to halt production for a significant amount of time to make the switch to a 4-bladed system.

[AndyJWest]

From the discussion so far it seems, that widening the blades of the prop is the simplest and easiest way to allow it to produce more thrust at the expense of reduced efficiency. Maybe the wider blades were just a simple solution to an immediate necessity after the change of engine?

 

This. The most efficient way to use increased engine power in a propeller-driven aircraft is to increase the propeller diameter, while adjusting the gearing to keep tip speed down (having the propeller tips travelling at transonic speeds is a very bad idea if you can avoid it). This is seldom an option with an existing fighter aircraft design though, so instead you can only really increase the number of blades or the area of each blade. This will result in a propeller which is less efficient, but capable of absorbing more power.

Edited January 23, 2018 by AndyJWest

[Finkeren]

This. The most efficient way to use increased engine power in a propeller-driven aircraft is to increase the propeller diameter, while adjusting the gearing to keep tip speed down (having the propeller tips travelling at transonic speeds is a very bad idea if you can avoid it). This is seldom an option with an existing fighter aircraft design though, so instead you can only really increase the number of blades or the area of each blade. This will result in a propeller which is less efficient, but capable of absorbing more power.

 

So there is some truth to the notion, that by the end of WW2 propeller driven aircraft really were nearing the limit of their potential development performance-wise? The engineers were constructing more powerful engines than conventional propellers could handle. Sure, for a while you might be adding additional propellers (or even multiple individual props, but in the end you'd inevitably reach a point, where the prop became so inefficient, that it negated any gains in power. Jet engines really matured at the best possible time.

[=X51=VC_]

On another forum I found the theory that the germans didn't increase the number of propeller blades because they used synchronized guns in the nose. Adding extra blades would decrease the rate of fire of those nose-mounted guns.

Those gears only fired once per propeller revolution i.e. shots always went between the same two blades and as Finkeren said this is already faster than most MGs.

 

The only plausible argument here is that there is inaccuracy in the mechanism and shrinking the space between blades might not have been a good idea.

 

In any case there is enough of an argument from the point of view of technological inertia and 3 blades being good enough to not waste resources coming up with a completely different solution when you're already investing in the next step instead (jets).

 

Still inconclusive as to the effect of broad blades on top speed. Is the F-2 actually faster than the F-4 on the deck?

[69th_chuter]

A slower turning large prop is much more efficient than a smaller prop spinning like crazy.  The problem is you have to fit it on a plane with limited space for a prop so once you have the airframe penciled in you can start fitting the best propeller compromise to it.

 

In the twenties and thirties blades were narrow because they were, for the most part, rigorously tested on the ground where they were found to be the optimal design.  When planes started reaching higher altitudes it was found that broader blades worked much better up there.  Simply throwing on more and/or broader blades is going to increase propeller "solidity" (a function of blade area and number of blades) which has an adverse affect on directional stability.  In the case of the P-39Q-21/25 the four blade prop slightly increased climb performance (maybe) but the loss of directional control was the main thing pilots noted when they flew it so it was recommended to convert delivered aircraft back to three blade as soon as reasonably practical and the final -Q-25s and the Q-30s were delivered with the three blade.  The P-51 experienced the same thing but they stayed with the prop (and motor) and fixed the airframe.

[AndyJWest]

So there is some truth to the notion, that by the end of WW2 propeller driven aircraft really were nearing the limit of their potential development performance-wise? The engineers were constructing more powerful engines than conventional propellers could handle. Sure, for a while you might be adding additional propellers (or even multiple individual props, but in the end you'd inevitably reach a point, where the prop became so inefficient, that it negated any gains in power. Jet engines really matured at the best possible time.

 

I think so. As I suggested earlier, you can increase the diameter of the prop (not usually practical for an existing design), but as aircraft speed increased, it became increasingly difficult to keep the propeller tips subsonic, with the resulting drop in efficiency, amongst other issues. As an illustration of what supersonic propellers are like, consider the ill-fated Republic XF-84H "Thunderscreech" (https://en.wikipedia.org/wiki/Republic_XF-84H). So aerodynamically unstable that they never managed to properly test its performance,  and ridiculously noisy (you could apparently hear it 25 miles away). The Soviets had more luck with the Tupolev Tu-95"Bear",  but even this seems to have been something of a dead-end, design-wise. I'm sure that without jets, aircraft designers would have found a way to get a bit more use out of propellers, but they were running into much the same issues as they were with aircraft design in general: the simple solutions for increased performance they had been relying on for years became irrelevant as they closed in on mach 1.

[blitze]

With regards to blade design and thickness, think of the designers tuning the prop blades for a certain altitude where the aircraft was expected to need best performance.  As one increases altitude air density decreases so the need for more prop area to push the air is required but on the deck, the prop with more blade area can be inefficient and so performance is sacrificed.

 

All quite interesting.  Yet even with the advent of the jet engine, in some modern applications the prop still reigns but in the modern form as the Turbo Prop.  Giving greater endurance and fuel efficiency compared to jets, i.e. the Tu95 Bear as opposed to the B52.  Both designed for the same purpose but the Bear has much better endurance.

[Oubaas]

The most important thing that you need to know regarding propellers is that if you find a prop blade de-laminating on pre-flight after an R.O.N. on a cross-country, over-water flight, and you tape it up with 1,000 mile per hour tape and fly it home, the Maintenance Chief will need around four milligrams of lorazepam to calm him down when he finds out what you did.

[Ehret]

The F4u Corsair design was based on a big diameter propeller, hence the reversed gull wings needed to get clearance from ground with relatively short gears.

[Uriah]

The Wright brothers may have been the first to change the propeller from just air paddles to air foils.  And I have read their propeller was so efficient that only the best modern knowledge could produce a more efficient.  Last night I watched a documentary on the Hurricane and it mentioned that the guns on the hurricane were put on the wings because they could not get that many guns to shoot through the prop.

[Bilbo_Baggins]

Wiki says wider blades for better altitude performance on the F-4, so if you're right I guess wider blades do reduce top speed, at least on the deck. But since they move more air they're better higher up.

 

How much faster is the F-2 than the F-4 at sea level?

F2 528kmh

F4 522kmh

 

That's with 25HP less than the F4 too.

Edited January 23, 2018 by Mcdaddy

[Hirachi]

This is rather old and might not be as current 

 

Basic Propeller Principles

   The airplane propeller consists of two or more blades and a central hub to which the blades are attached. Each blade of an airplane propeller is essentially a rotating wing. As a result of their construction, the propeller blades are like airfoils and produce forces that create the thrust to pull, or push, the airplane through the air.

   The power needed to rotate the propeller blades is furnished by the engine. The engine rotates the airfoils of the blades through the air at high speeds, and the propeller transforms the rotary power of the engine into forward thrust. 
 

An airplane moving through the air creates a drag force opposing its forward motion. Consequently, if an airplane is to fly, there must be a force applied to it that is equal to the drag, but acting forward. This force, as we know, is called "thrust."

   A cross section of a typical propeller blade is shown in Fig  17-38. This section or blade element is an airfoil comparable to a cross section of an airplane wing. One surface of the blade is cambered or curved, similar to the upper surface of an airplane wing, while the other surface is flat like the bottom surface of a wing. The chord line is an imaginary line drawn through the blade from its leading edge to its trailing edge. As in a wing, the leading edge is the thick edge of the blade that meets the air as the propeller rotates.

 

   Blade angle, usually measured in degrees, is the angle between the chord of the blade and the plane of rotation (Fig. 17-39) and is measured at a specific point along the length of the blade. Because most propellers have a flat blade "face," the chord line is often drawn along the face of the propeller blade. Pitch is not the same as blade angle, but because pitch is largely determined by blade angle, the two terms are often used interchangeably. An increase or decrease in one is usually associated with an increase or decrease in the other.

   The pitch of a propeller may be designated in inches. A propeller designated as a "74-48" would be 74 inches in length and have an effective pitch of 48 inches. The pitch in inches is the distance which the propeller would screw through the air in one revolution if there were no slippage.

   When specifying a fixed pitch propeller for a new type of airplane, the manufacturer usually selects one with a pitch which will operate efficiently at the expected cruising speed of the airplane. Unfortunately, however, every fixed pitch propeller must be a compromise, because it can be efficient at only a given combination of airspeed and RPM. The pilot does not have it within his power to change this combination in flight.

  
 

When the airplane is at rest on the ground with the engine operating, or moving slowly at the beginning of takeoff, the propeller efficiency is very low because the propeller is restrained from advancing with sufficient speed to permit its fixed pitch blades to reach their full efficiency. In this situation, each propeller blade is turning through the air at an angle of attack which produces relatively little thrust for the amount of power required to turn it.

To understand the action of a propeller, consider first its motion, which is both rotational and forward. Thus, as shown by the vectors of propeller forces in Fig. 17-39, each section of a propeller blade moves downward and forward. The angle at which this air (relative wind)  

strikes the propeller blade is its angle of attack. The air deflection produced by this angle causes the dynamic pressure at the engine side of the propeller blade to be greater than atmospheric, thus creating thrust. 
 

 

   The shape of the blade also creates thrust, because it is cambered like the airfoil shape of a wing. Consequently, as the air flows past the propeller, the pressure on one side is less than that on the other. As in a wing, this produces a reaction force in the direction of the lesser pressure. In the case of a wing, the air flow over the wing has less pressure, and the force (lift) is upward. In the case of the propeller, which is mounted in a vertical instead of a horizontal plane, the area of decreased pressure is in front of the propeller, and the force (thrust) is in a forward direction. Aerodynamically, then, thrust is the result of the propeller shape and the angle of attack of the blade.

   Another way to consider thrust is in terms of the mass of air handled by the propeller. In these terms, thrust is equal to the mass of air handled, times the slipstream velocity, minus the velocity of the airplane. The power expended in producing thrust depends on the rate of air mass movement. On the average, thrust constitutes approximately 80% of the torque (total horsepower absorbed by the propeller). The other 20% is lost in friction and slippage. For any speed of rotation, the horsepower absorbed by the propeller balances the horsepower delivered by the engine. For any single revolution of the propeller, the amount of air handled depends on the blade angle, which determines how big a "bite" of air the propeller takes. Thus, the blade angle is an excellent means of adjusting the load on the propeller to control the engine RPM.

   The blade angle is also an excellent method of adjusting the angle of attack of the propeller. On constant speed propellers, the blade angle must be adjusted to provide the most efficient angle of attack at all engine and airplane speeds. Lift versus drag curves, which are drawn for propellers as well as wings, indicate that the most efficient angle of attack is a small one varying from 2 to 4 degrees positive. The actual blade angle necessary to maintain this small angle of attack varies with the forward speed of the airplane.

   Fixed pitch and ground adjustable propellers are designed for best efficiency at one rotation and forward speed. They are designed for a given airplane and engine combination. A propeller may be used that provides the maximum propeller efficiency for either takeoff, climb, cruise, or high speed flight. Any change in these conditions results in lowering the efficiency of both the propeller and the engine. Since the efficiency of any machine is the ratio of the useful power output to the actual power input, propeller efficiency is the ratio of thrust horsepower to brake horsepower. Propeller efficiency varies from 50% to 87%, depending on how much the propeller "slips." 
 

Propeller slip is the difference between the geometric pitch of the propeller and its effective pitch (Fig. 17-40). Geometric pitch is the theoretical distance a propeller should advance in one revolution; effective pitch is the distance it actually advances. Thus, geometric or theoretical pitch is based on no slippage, but actual or effective pitch includes propeller slippage in the air. If you wonder why a propeller is "twisted," the answer is that the outer parts of the propeller blades, like all things that turn about a central point, travel faster than the portions near the hub (Fig. 17-41). If the blades had the same geometric pitch throughout their lengths, at cruise speed the portions near the hub could have negative angles of attack while the propeller tips would be stalled. "Twisting," or variations in the geometric pitch of the blades, permits the propeller to operate with a relatively constant angle of attack along its length when in cruising flight. To put it another way, propeller blades are twisted to change the blade angle in proportion to the differences in speed of rotation along the length of the propeller and thereby keep thrust more nearly equalized along this length.

 Usually 1 to 4 degrees provides the most efficient lift/drag ratio, but in flight the propeller angle of attack of a fixed pitch propeller will vary - normally from 0 degrees to 15 degrees. This variation is caused by changes in the relative airstream which in turn results from changes in aircraft speed. In short, propeller angle of attack is the product of two motions - propeller rotation about its axis and its forward motion.

A constant speed propeller, however, automatically keeps the blade angle adjusted for maximum efficiency for most conditions encountered in flight. During takeoff, when maximum power and thrust are required, the constant speed propeller is at a low propeller blade angle or pitch. The low blade angle keeps the angle of attack small and efficient with respect to the relative wind. At the same time, it allows the propeller to handle a smaller mass of air per revolution. This light load allows the engine to turn at high RPM and to convert the maximum amount of fuel into heat energy in a given time. The high RPM also creates maximum thrust; for, although the mass of air handled per revolution is small, the number of revolutions per minute is many, the slipstream velocity is high, and with the low airplane speed, the thrust is maximum.

 

   After liftoff, as the speed of the airplane increases, the constant speed propeller automatically changes to a higher angle (or pitch). Again, the higher blade angle keeps the angle of attack small and efficient with respect to the relative wind. The higher blade angle increases the mass of air handled per revolution. This decreases the engine RPM, reducing fuel consumption and engine wear, and keeps thrust at a maximum.

   After the takeoff climb is established, in an airplane having a controllable pitch propeller, the pilot reduces the power output of the engine to climb power by first decreasing the manifold pressure and then increasing the blade angle to lower the RPM.

At cruising altitude, when the airplane is in level flight and less power is required than is used in takeoff or climb, the pilot again reduces engine power by reducing the manifold pressure and then increasing the blade angle to decrease the RPM. Again, this provides a torque requirement to match the reduced engine power; for, although the mass of air handled per revolution is greater, it is more than offset by a decrease in slipstream velocity and an increase in airspeed. The angle of attack is still small because the blade angle has been increased with an increase in airspeed. 

[ACG_daffy_]

Prop radius effects the potential for thrust production.  (RPMs)

Prop pitch effects the thrust produced per revolution.  

The Propellers radius is more often matched according to the power-plants abilities.

Obviously, the deeper the pitch, the smaller the radius and visa versa.

[=X51=VC_]

For the Hurricane, it's not so much that you can't synchronise that many guns, just that you can't physically fit them in the nose of a small fighter so they decided it was easier to put them all in the wings and keep fuselage frontal area low. The US persisted with synchronisation and had mixed nose/wing armament in some designs e.g. P-39, early P-40 and early P-51/A-36. Late war designs like the Ta-152C and La-9 each had 4 cannons firing through the propeller arc, so it's more a fuselage size limitation than anything else.

 

In the case of the F-2 and F-4 it does indeed seem the cons outweigh the pros of the wider prop on the deck, but the difference is small and the wider prop gives better performance over a wider range of intended altitudes. All design compromise I guess.

 

Great detailed post above. That really illustrates why props reached a limit. Consider how pitch has to increase to maintain an angle of attack with significant forward speed, but that you're getting diminishing returns since the lift from the blades is now at an angle, generating less thrust. You can't increase the angle past a point, and you can't increase rotational velocity either without dealing with supersonic tips. It's a design corner with no way out, if you want to go faster you need a completely different propulsion system.

 

Having said that, pops are still generally better for fast power response and thrust at low speed, which is why they get so much use on things that need short take off and landing.

[Dakpilot]

I imagine there is lots of info on the P-47 'needle' and 'paddle' paddle prop versions that is easily available 

 

if indeed those would be relevant to this conversation , I am rusty on P-47's and looking forward to it in B-platte, have to do some revision   in the next year

 

Cheers, Dakpilot

[=X51=VC_]

So there is some truth to the notion, that by the end of WW2 propeller driven aircraft really were nearing the limit of their potential development performance-wise?

Just for a fun bit of math. Assume you don't want to push your prop much past 45 degree pitch at the tip as the thrust loss is too great after. At this pitch the forward speed of the plane is about the same as the rotational velocity (assumption for easy math but not unreasonable). So, trigonometry:

 

Prop tip speed squared = tangential velocity squared + forward speed squared

 

Let's allow the tips to go a little supersonic, say Mach 1.05. Do the calculation, you'll see tangential velocity and the aircraft forward speed come out at approximately Mach 0.75.

 

The fastest operational prop plane (Tu-95) flies at approx. Mach 0.8 with supersonic prop tips. Coincidence? End of WWII superprops are just under Mach 0.7 in level flight. The Bear has a jaw-droppingly disgusting amount of power (14k hp PER engine!!!) and makes more noise than a small volcanic eruption... all to scrape 0.1 more Mach.

 

So there's your limit of conventional prop development, give or take.

Edited January 24, 2018 by VC_

[Bilbo_Baggins]

Just for a fun bit of math. Assume you don't want to push your prop much past 45 degree pitch at the tip as the thrust loss is too great after. At this pitch the forward speed of the plane is about the same as the rotational velocity (assumption for easy math but not unreasonable). So, trigonometry:

 

Prop tip speed squared = tangential velocity squared + forward speed squared

 

Let's allow the tips to go a little supersonic, say Mach 1.05. Do the calculation, you'll see tangential velocity and the aircraft forward speed come out at approximately Mach 0.75.

 

The fastest operational prop plane (Tu-95) flies at approx. Mach 0.8 with supersonic prop tips. Coincidence? End of WWII superprops are just under Mach 0.7 in level flight. The Bear has a jaw-droppingly disgusting amount of power (14k hp PER engine!!!) and makes more noise than a small volcanic eruption... all to scrape 0.1 more Mach.

 

So there's your limit of conventional prop development, give or take.

Very interesting, thanks.

[GridiroN]

The G2/G4 and presumably G6 also have different propellors than the BF109 F4, which is modeled in game. Obviously the G2 is significantly better at altitude, but it also moves more air and "helicopters" less. 

 

In any event, I'm also unsure why the 3 bladed propellor was such a successful design for the 109. I know many companies during and after the war tried to retrofit the 109 with different engines and propellors when the DB60x was no longer available and it always resulted in turning the 109 into a brick. 

Edited January 25, 2018 by GridiroN