prop RPM and max cruise speed.

[Herne]

I remember seeing a while ago that yak has a higher top speed with less than full RPM.

I've also seen this on xplane, on both piston engines and turbo props, where it easier to observe thanks to on board GPS, so I was wondering if this would also be common for most WWII Fighters. 

 

If you could use full power with less than full rpm would your speed increase. I know you have to worry about detonation, although I am not sure when that would be likely to occur.

I made a vid to record my observations, but cut it in the wrong place. GS eventually rose to 215 knots, but I cut it short at 214.
 

 

Edited October 20, 2018 by =FEW=Herne

[Legioneod]

Maybe it just gets to the point where it's just not efficient so it's better off at lower rpms or settings.

Just a guess as I have no knowledge of how it really works.

[Bremspropeller]

Keep in mind that power-required is cubed over speed - even when not accounting for Mach effects.

At the high RPM-settings, you'll probably kill a lot of your power-budget just turning the props at the demanded RPM.

If you reduce tip speed by lowering your RPM, you might actually send more power to the air (read: your prop might produce more net thrust) than when running at higher PRM.

 

As Legion said, the tips might be close to (or above) Mach 1, which wrecks havoc on your lift (thrust) disribution across the prop disc.

Many CSD props are close to Mach 1 on take-off already and adding forward speed doesn't help you much here.

 

This article digs into it somewhat:

https://www.avweb.com/news/airman/184483-1.html

 

[Herne]

6 minutes ago, Bremspropeller said:

Keep in mind that power-required is cubed over speed - even when not accounting for Mach effects.

At the high RPM-settings, you'll probably kill a lot of your power-budget just turning the props at the demanded RPM.

If you reduce tip speed by lowering your RPM, you might actually send more power to the air (read: your prop might produce more net thrust) than when running at higher PRM.

 

As Legion said, the tips might be close to (or above) Mach 1, which wrecks havoc on your lift (thrust) disribution across the prop disc.

Many CSD props are close to Mach 1 on take-off already and adding forward speed doesn't help you much here.

 

This article digs into it somewhat:

https://www.avweb.com/news/airman/184483-1.html

 

 

Very good read. Thanks !

[ZachariasX]

In general, to be fastest, you need lowest rpm while applying highest torque.

 

If you stay put. at 3000 rpm, the tips of your prop (Yak style prop) will go at about 310 m/s (with a reduction gear of 0.66).

 

You can see that at full rpm, there is very little margin for the tips to approach the speed of sound (~340 m/s). Any part of the prop going faster than the speed of sound will not really be part of what is pulling your aircraft forward, meaning the faster you go, the smaller (for practical purposes) your prop will get. Making matters worse, it takes a considerable amont of energy to move the blades at supersoninc speed, further bleeding usable power from your aircraft.

 

Now, let's see how fast you can go (in this example) for the speed differential of 30 m/s speed differential (between 0 m/s forward movement) and tips going supersonic.

 

So it is basically this:

 

a=300 [m/s]

b=unknown

c=340 [m/s] as c is the distance the prop tip will go when you move the aircraft over the distance b.

 

b= 160 [m/s]

 

Seeing this, you can understand the importance of the prop gearing, as this one sets the window for the usable engine rpm. With the gearing of 0.66, you can go 576 km/h until prop efficiency really starts to drop.

[6./ZG26_Klaus_Mann]

Also Consider your Supercharger and Torque Curve. 

 

1. Superchargers hate being Throttled, so at lower RPM the Centrifugal Supercharger will produce less Boost, so the MAP Regulator can open the Throttle to a higher degree, allowing for more Efficient Running, and thus more Power. 

2. Centrifugal Compressors (simplified) will produce roughly an Exponential Amount of Fluid Flow/RPM without increasing Drive Power by a lot (certainly less then by Gearing them up more), so more Revs means Exponentially more Air/Boost. What does that mean for the Engine Designer? He of course wants good Altitude Performance but also good Performance low Down, as well as good Economy. 

 

The 105 Engines thus have a rather Conservative Cam and Timing, giving the Engine Maximum Power at 2550 at it's Maximum Boost Level, however, as the Aircraft Climbs at the lower RPM the Supercharger is spinning too slowly to maintain that Boost for long, thus increasing the Engine RPM allows more Boost and thus more Power.

 

In a Way the Power Chart Curve is Shifted to a Higher RPM, until the Supercharger is shifted into Second Gear. Theoretically, while producing Maximum Boost in Second Gear, 2550 RPM will also produce More Power than 2700. 

 

 

The Germans did the Same, looking for example at the 109E-7 which had a normal Peak RPM of 2400RPM, but after the Battle of Britain Daimler Benz decided to allow the use of up to 2600RPM once above FTH to maintain 1.3ata, giving it almost 1500m additional FTH. 

Same for the 109F-2, which originally was limited to 2600RPM, but ingame we have the 2800RPM Limit which also gave it almost an additional 1000m FTH. 

[Talon_]

Brits found the Corsair II was faster with reduced RPM at WEP during testing.

[ZachariasX]

1 hour ago, Talon_ said:

Brits found the Corsair II was faster with reduced RPM at WEP during testing.

There is a reason why you have torque meters installed in airliners featuring these large radials... You know how much torque your engine can tolerate and you fly it at that value. It is one more step toward milking the best mileage from your ride.

[-SF-Disarray]

9 hours ago, 6./ZG26_Klaus_Mann said:

...

 

The 105 Engines thus have a rather Conservative Cam and Timing, giving the Engine Maximum Power at 2550 at it's Maximum Boost Level, however, as the Aircraft Climbs at the lower RPM the Supercharger is spinning too slowly to maintain that Boost for long, thus increasing the Engine RPM allows more Boost and thus more Power.

 

In a Way the Power Chart Curve is Shifted to a Higher RPM, until the Supercharger is shifted into Second Gear. Theoretically, while producing Maximum Boost in Second Gear, 2550 RPM will also produce More Power than 2700. 

...

 

So this means that in a Yak with a 105 engine it is best to climb and maximum RPM and power settings but to get the best strait line speeds running at 2550 RPM with max power settings will be better? Also would it be fair to compare it to driving a car up a steep hill: keeping it in a lower gear, thus lower RPM, will result in better climbing performance?

[Bremspropeller]

1 hour ago, Disarray said:

keeping it in a lower gear, thus lower RPM, will result in better climbing performance?

 

Lower RPM equals a higher gear.

Higher RPM equals a lower gear.

 

High RPM means a fine pitch-angle of the blade, which makes it accelerate quicker if you pour on the coal (good for take-off and go-arounds and great for additional braking (flat blades produce a lot of drag in idle) during landing). A higher RPM also means more thrust (thrust scales with velocity squared). That same velocity will case problems at higher airspeeds, as the drag of the blades increases and the required power to turn the blades starts to surge (power scales with velocity cubed).

 

Low RPM means coarser pitch, which means a laggier response to throttle-movements. Also, thrust is now produced by the AoA of the blade, rather than the rotational speed. AoA only scales linearily in thrust. If the blade-angle is too coarse, the drag might become too high and the required power also gets too large for the engine. The TPE331 turboprop needs the blades in full fine pitch to start, or it will hang up and have a hot start, killing the hot section. But that's just trivia here...

 

 

1 hour ago, ZachariasX said:

There is a reason why you have torque meters installed in airliners featuring these large radials

 

And it's the primary power-instrument on most turboprops.

[6./ZG26_Klaus_Mann]

No. The Engine produces its Peak Power at the given Manifold Pressure at 2550RPM.  However, as you Climb the Supercharger cannot keep up with the Air Demand anymore, so the Power at 2550 starts to Drop, however at 2700RPM the Supercharger can provide more Air, and thus the Torque Curve shifts Max Power to a later RPM at that Altitude. 

 

Centrifugal Compressors produce Exponentially more Air the faster they Spin, so while the Engine requires a  Linear Amount of Air/RPM the Supercharger can give it an Exponentially Climbing Amount, thus more Air at higher RPM, and thus more Power. 

[unreasonable]

On 10/21/2018 at 5:09 AM, 6./ZG26_Klaus_Mann said:

No. The Engine produces its Peak Power at the given Manifold Pressure at 2550RPM.  However, as you Climb the Supercharger cannot keep up with the Air Demand anymore, so the Power at 2550 starts to Drop, however at 2700RPM the Supercharger can provide more Air, and thus the Torque Curve shifts Max Power to a later RPM at that Altitude. 

 

Centrifugal Compressors produce Exponentially more Air the faster they Spin, so while the Engine requires a  Linear Amount of Air/RPM the Supercharger can give it an Exponentially Climbing Amount, thus more Air at higher RPM, and thus more Power. 

 

That is also what Stepanets says in "How to Fly the Yak"   http://www.airpages.ru/dc/doc110.shtml           Google translate does a passable job on this.

[PainGod85]

On 10/20/2018 at 11:23 AM, =FEW=Herne said:

I remember seeing a while ago that yak has a higher top speed with less than full RPM.

I've also seen this on xplane, on both piston engines and turbo props, where it easier to observe thanks to on board GPS, so I was wondering if this would also be common for most WWII Fighters. 

 

If you could use full power with less than full rpm would your speed increase. I know you have to worry about detonation, although I am not sure when that would be likely to occur.

I made a vid to record my observations, but cut it in the wrong place. GS eventually rose to 215 knots, but I cut it short at 214.
 

 

 

It was true for all WW2 planes that were fitted with the B series R-2800 turning as larger diameter prop as the prop replacement did not modify the reduction gear. Hence you had a larger prop turning at the original smaller one's optimum RPM, which ended up costing you performance.

 

It's documented for the P-47D as well as the F4U-1.

[JtD]

The Hamilton standard 6507 13" prop as used on later P-47's gave better speed and a lot better climb performance than the early Curtiss electric 714 12.2" prop.

 

http://www.wwiiaircraftperformance.org/p-47/p-47d-75035-fig2.jpg

 

While prop tip mach numbers matter, the more relevant point is the optimum lift-drag ratio of the propeller blade. Just like wings, propellers are nothing but airfoils moving through air. Typically, the prop loading at high speed is too low to achieve optimum lift-drag ratio (too low angle of attack), while at climb it is too high to achieve optimum lift-drag ratio (too high angle of attack). This means that reducing rpm will improve efficiency at high speed, but reduce it at climbing speeds.

 

It's also possible to improve lift-drag characteristics by changing and improving propeller design, as they did on the P-47.