Plane + Engine = propeller

[oakranger]

I always wonder how people know what is best in relations to the AC plus the type of engine and how do they know what is best used when it comes to propeller? For example, is the paddle shape propeller best for the 190's? How would the performance change if some AC have one less or one more propeller added? B-17 four props vs three, P-51 three props vs four, 109 four props vs three, Moss three props vs four.

[Krusty]

[edit: in terms of this game...]

It's not about what's best. It's about what was used. What was most common. HTC makes a call on what prop to use. Sometimes there's no question (only 1 historic choice), other times it may be a gameplay balance issue (2 planes with same prop would be nearly identical, so give 1 a different prop with different performance, to set them apart more -- maybe like the P-47 line).

[Saxman]

As far as historical, sometimes the engine came first and the rest of the aircraft was designed around it (as in the case of the Corsair. The designers wanted to build the smallest fighter possible around the largest and most powerful radial engine available at the time, which was the R-2800-8. The airframe and wing plan was a direct result of meeting these requirements). I'm sure the number and type of propeller blades was partly to do with aeronautical knowledge available at the time the aircraft was designed. Note that most pre and early-war aircraft utilized three-bladed props, with four blades being introduced as the war continued. There was probably experimentation and trial and error involved with discovering four blades were better than three, and wider blades were better than narrow-chord ones. I'm sure supply and manufacturing time factored into it as well. Case in point, IIRC there were concepts aimed at giving the P-38 four-bladed paddle props, but this would have required a complete revision of the spinner and engine cowling to accomodate it, as well as retooling the assembly lines to facilitate the modification which would have meant shutting the plant down to make the necessary changes. The resulting production delays were ruled to be not worth it, so the P-38 kept its three-bladed narrow-chord props.

[Krusty]

There's also the situation where propellor design took a while to catch up. The Hellcat's prop was one of the largest at the time it was introduced, I think.

The engine has X amount of horsepower, but that horsepower has to be converted into thrust. The prop can only make N% of that X hp into Y thrust, etc... There are ratios involved. The prop, however, gets heavier and heavier the more blades you put on it, and eats up more of that engine horsepower just to turn it.

On the very high hp griffon variants you saw 5 prop blades to attempt to make better use of the extra horsepower. However, other variants, instead of adding a 6th blade, tried 2 contra-rotating 3-blade designs, etc.

That's why even most late war monsters stopped at 4 blades. It was one of the more efficient setups. It was a balance between massive weight (also of the gearbox needed for such a heavy prop and powerful engine) and the thrust needed for the plane.

[hitech]

The best prop is always 1 blade infinitely long turning very slow.

Obviously we are limited by landing gear as to how long we can make the prop. Hence we start into trade offs.
The prop must be large enough to handle the HP of the engine. With a constant speed prop (I.E. variable pitched) the blades turn to compensate for the torque driving the prop. When the plane is standing still or traveling very slow, the prop is taking a big bite of air, so much so that the prop is  stalling and the HP is not generating thrust. As the speed increases the AOA of the prop to the slip stream will decrease and more of the HP will go into creating thrust.

Adding blades creates turbulence for the other blades hence creates less efficiency, but adding blades is preferable to stalling the prop as described above.

As more HP is added more blades are added, with more then  4 blades the efficiency starts dropping off faster plus the mechanics of rotating the blades become more difficult. Now your stuck you can't add blades and you can't make them longer, hence you add more surface area to the blade.

But the issue is that at high speeds, the prop does not need as many blades or much area to turn all the HP into thrust. But at slow speeds more area (I.E. more blades or more area is needed) this is exactly like adding or removing wing area of a plane.

Less area means go faster but very high stall speeds. More area = go slower but performs better at slower speeds.

The prob tradeoffs are the same.


HiTech

[Baumer]

Just as with aircraft and engines, there was a tremendous amount of development with propeller designs during this time frame. The fundamental problem was the massive increase in drag the propeller generated (starting at the blade tips) as it approached supersonic speeds. This lead to the developments that Krusty mentioned, like more (shorter) blades, and counter-rotating designs. So with an aircraft like the Corsair you can see that even given the the large increase in power between the F4U-4 with a 2,100hp P&W R-2800-18W to the F2G-1 with a 3,000hp P&W R-4360-4W there was only a 7mph top speed improvement.

With jet aircraft you avoid this problem since jet engines use subsonic airflow (even when the plane is supersonic).

[MK-84]

So if I understand the basic principle:

Take a P47 for example.

Switching to paddle props, all else being the same (for sake of argument)  would mean that the aircraft with more surface area on the prop blades would have more power at slower/climb speeds, but would lose efficiency at higher speeds.  So the direct effect would be better takeoff/climb performance, and reduced top speed?  or am I totally missing the concept?

[oakranger]

So if I understand the basic principle:

Take a P47 for example.

Switching to paddle props, all else being the same (for sake of argument)  would mean that the aircraft with more surface area on the prop blades would have more power at slower/climb speeds, but would lose efficiency at higher speeds.  So the direct effect would be better takeoff/climb performance, and reduced top speed?  or am I totally missing the concept?

I am getting the same understanding too. That means that the 190 famous paddle blade is used to climb fast to get to the bombers. Where the 109 where not giving that option, if it has been even consider, but instead to used them on fighting mode to have the speed and retain it.

[hitech]

So if I understand the basic principle:

Take a P47 for example.

Switching to paddle props, all else being the same (for sake of argument)  would mean that the aircraft with more surface area on the prop blades would have more power at slower/climb speeds, but would lose efficiency at higher speeds.  So the direct effect would be better takeoff/climb performance, and reduced top speed?  or am I totally missing the concept?

Exactly. But the assumption is that the prop was loosing efficiency because of lack of size at slow speeds.

HiTech

[Charge]

"Where the 109 where not giving that option, if it has been even consider, but instead to used them on fighting mode to have the speed and retain it."

I recall that wide blade props were used on both late 190s and late 109s.

BTW, is it possible that a radial engine has actually more mechanical torque than a V engine due to their geometry?

-C+

[hitech]

"Where the 109 where not giving that option, if it has been even consider, but instead to used them on fighting mode to have the speed and retain it."

I recall that wide blade props were used on both late 190s and late 109s.

BTW, is it possible that a radial engine has actually more mechanical torque than a V engine due to their geometry?

-C+

Ahhh, dosn't make any difference.

xxx HP at YYY RPM = ZZZ torque.

Unless you change the HP or rpm, there will be no difference in torque between a radial,V engine, steam engine, electric motor, or hamster powered tread mill.

I.E. Power = Torque * RPM.

HiTech

[Ardy123]

Ahhh, dosn't make any difference.

xxx HP at YYY RPM = ZZZ torque.

Unless you change the HP or rpm, there will be no difference in torque between a radial,V engine, steam engine, electric motor, or hamster powered tread mill.

I.E. Power = Torque * RPM.

HiTech

HiTech,
The 'torque' felt by a pilot is caused by the weight of the prop swinging around correct? so for a constant RPM, the longer the prop blade (given that the prop blade is uniform), the greater the centrifugal force at the tips of the prop, and thus the greater the yaw to the direction of rotation, correct?

[oakranger]

"Where the 109 where not giving that option, if it has been even consider, but instead to used them on fighting mode to have the speed and retain it."

I recall that wide blade props were used on both late 190s and late 109s.

BTW, is it possible that a radial engine has actually more mechanical torque than a V engine due to their geometry?

-C+

What 109 got the paddle shape blade?

[hitech]

HiTech,
The 'torque' felt by a pilot is caused by the weight of the prop swinging around correct? so for a constant RPM, the longer the prop blade (given that the prop blade is uniform), the greater the centrifugal force at the tips of the prop, and thus the greater the yaw to the direction of rotation, correct?

Nope.  Simple engine torque causes the plane to ROLL opposite the direction of the prop, not yaw.

Slip stream, Pfactor, and gyroscopic produce yaw, and gyroscopic is is yaw with a pitch change.
Torque creates a roll not a yaw.

Plus Charger I believe was simply speaking abut engine output shaft torque.

HiTech

[Charge]

"What 109 got the paddle shape blade?"

I recall that 109K has bigger blades than earlier versions.

I thought that with paddle blade it is meant just the general shape that was used in 109s and 190s? I.e. the tip is narrower than the base and the base in generally the part that pushes air past the cowling. In P51 and P47 paddle props can be identified as they used "cuffs" to increase and even the airflow around fuselage? Or what is a "paddle prop blade"?

http://www.warbirdinformationexchange.org/phpBB3/viewtopic.php?f=3&t=19774&start=0

-C+