Don‘t want to say much more to this to not create anger with technical arguments anymore.
Only wondering, if we are not forgetting about propellor efficiency here.
AFAIK propellors get less efficient with very high RPM, due to the tip getting into progressive compressibility effects.
So at least some of the engine power increase from higher RPM would be neutralized from a less efficient prop. In that case it’s not a linear relationship between RPM and thrust. Hard to get data for the specific C310R prop (moderators take note, which is why we have to discuss it in general here) but in general it seems to be the accepted wisdom, that above a prop tip speed of about 500mph/~ M0.67 - for C310R’s 76” prop that’s approx @ 2300 rpm, certainly below max. rpm - prop efficiency declines progressively…
(prop tip speed is the vectored sum of rotational speed and airframe speed. airframe speed only contributing in small proportion to the overall tip speed.)
Seems blurry, as long as we have no specific data for the C310R (anyone? Milviz/Blackbird?) but it correlates with my testing.
And not surprising, since the primary design goal for prop efficiency and tuning the fixed parameters of the governor in the C310R was efficient high speed cruise, not low speed performance (which is still sufficient).
Where did you get that number from? As far as I know, Mach 0.8-0.9, just before the first bit of mach=1 appears on the airfoil of the propeller, is considered the most efficient and highest performing tip speed.
http://www.epi-eng.com/propeller_technology/selecting_a_propeller.htm
Blackbird show several performance charts in their product manual starting at page 60. While there is no dedicated chart for propeller efficiency, one can repeatedly observe that higher prop rpms, in this case 2500 rpm, have at least as much additional BHP as one would expect from the linear increase in rpm compared to say 2200 rpm, and in many cases a fer percent more.
This suggest that propeller efficiency plays no major role here, and might have the opposite behaviour as you expected in your post. As this type of data is derived from speeds at around 180 KTAS, we should be able to transfer it our case with lower airspeeds by increasing the rpm a bit.
here, a propeller manufacturer.
they talk about two effects.
1.) onset of “compressibility” of the air above relatively moderate tip speeds of 475 mph which begins to “degrade the performance of your propeller.”
2.) the disruption of airflow approaching Mach 1
first point seems to be empirically proven by also the C310R losing in fact speed when RPM is higher than the most efficient 2200 to 2400 range in cruise (!).
At lower airframe speeds that RPM efficiency sweet spot - as related to tip speed as the vectored sum of rotational prop speed and airframe speed - should go up, but not by very much, since the vectored airframe speed component is only in the one digit percentage range approx. compared to the rotational prop speed.
Hard to impossible to find hard data on the actual C310R for this apparently.
In my test flights with the Blackbird C310R I always reached the highest airspeeds in cruise (at low altitude and “top of green” 24.5 MP) in the 2200 to 2400 RPM range. Going higher in RPM always resulted in loss of airspeed. Make out of it what you want.
That’s expected behavior as the effect and benefit of prop efficiency comes into play at higher airspeeds, as explained above, in the C310 and just about every other piston GA aircraft.
I would absolutely expect to slow down with props at fine pitch at cruise speed. That’s why the ability to select a more coarse setting and a lower RPM exists (along with engine wear, noise, vibration, and fuel efficiency)
Still doesn’t make up for the fact that power, not efficiency (which is a negligible difference at approach speeds), is the primary goal for sustained climb at takeoff and go-around, until well safe of terrain and obstacles, which was the original back and forth yesterday.
I thought we had been further in our argument. Max power for best sustained climb rate was not argued. It was argued how large and relevant the difference in thrust between 2500 and 2700 rpm at low airspeeds actually is. You and TheMightyFish argued the relation between engine power, rpm and thrust was linear. I wasn’t so sure.
A moderator had already mentioned that this topic was getting into very technical details and did not belong in the Official Milviz Thread which is for general feedback and discussion. I’ve created this new thread for your discussion, but please keep it civil between each other.
But notice how they quote 0.8 to 0.92 as “Maximum performance”, suggesting that the drop in efficiency is in no way large enough to offset the gain in engine power, as long as you do not exceed 0.92 or so. Which, in turn, means that max. RPM gives the most thrust by quite a margin, as long as you do not approach high airspeeds, where the vectored sum exceeds the Mach-Limit.
I noticed the same, but Milviz literally show us the charts they try to go after. I trust the charts much more, as the sim often limits the developers ability to truly match RL performance and behaviour in all situations. At least thats what I heard on their respective Discord channels.
ok now I saw it. yes, but they don’t say “max. efficiency”.
They say above 475 mph (approx M0.62 / RPM 2100 for the 76" C310R prop blades) tip speed a (gradual) degradation in efficiency starts, and subsonic above M 0.92 it drastically goes south. That’s what I read there.
Now what I would like to know, how much the degradation in efficiency toward higher RPMs actually neutralises the higher power/thrust (“performance”) from higher RPM. Somewhat…
Really hard to say, and apparently also not exactly similar between real life and sim C310R.
Were the effect of tip-speed compressibility a significant factor in loss of thrust, more than the gain in power that any further increase in RPM would provide, the RPM at which that occurred would be built into the design, operating limitations, and/or instructions.
It could be both, significant AND not more than the gain in absolute power to 100% neutralise it.
So bottomline max RPM would still give a small power increase, but not linear to RPM increase.
Would make sense in correlation to real world data, but I can’t investigate further, since I lack a C310R in real life for testing. 
And again, it is reasonable to assume that the Cessna engineers designed the prop and governor primarily for efficient high speed cruise, not low airspeed high RPM procedures.
Another assumption: even if the offset from loss of prop efficiency would swallow 100% of power increase going from 2500 to 2700 rpm at normal go around speeds, probably the Cessna engineers would STILL have written the procedure checklists with FULL RPM, since the relevant speed for which the RPM should be optimised in the emergency procedures would be the worst case scenario close to the ground: loss of one engine or wind shear
Ever look at the prop diameter and takeoff/max RPM of a Piper Archer? A Cessna 172 SP? Do you think there isn’t a significant gain in thrust/power when going from 2500 to 2700 RPM? Hint: there is.
I’ve got the plane next week and I’ll tell ya what I’m not going to do: I’m not going to throttle up to 2500 RPM and go “eh, that’s good enough, not going to gain much more by going to 2700.” Not by a longshot.
sure. don’t do that.
but - I know I sound like a broken old record - would you still at a half mile out at vREF +20 going around for reasons not related to your airplane or volatile winds - e.g. previous landing plane too slow in vacating the runway - crank the RPM from 2500 up full forward, noisy, excess fuel burning, engine stressing, for no realistic safety gains?
200’ off the ground, fully configured? You bet your and my life I would.
I’d have to be much higher off the ground before I considered any of those factors over the need to climb to a safe altitude. And if I were higher enough off the ground, I’d be farther from the runway and likely not yet fully configured at fine pitch, dirty, and slow anyway.
But yes, posing that scenario - 1/2 mile final and the normal things that go with that, absolutely, yes. The power is that necessary.
I would like to know how much more thrust there REALLY is. I’m afraid lacking the real world hard data, we have to leave it at that inconclusive state.
Again, in the sim the difference was negligible, very small at best. But that’s just a sim.
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Those aircraft have fixed-pitch propellers… pilot-selected RPM, for a specific scenario (takeoff/climb/cruise…etc.), are the result of throttle settings (manifold pressure).
So… for this example (takeoff)… choosing other than max RPM, would mean choosing less than “full throttle”… which would of course yield (significantly) less thrust.
At level cruise, fixed-pitch prop, again the selected prop RPM is a function of changes in throttle setting. I.E… level @ 6,000msl (where you’re already down ~6" of MP)… going from 2700 to 2500 would require a further lessening of manifold-pressure (reduced throttle setting).
By comparison… level @ 6,000msl, with a constant-speed prop, the throttle, wide-open, yields ~23" manifold pressure… you can reduce RPM, leaving the throttle wide open.
Universal point to this discussion… most GA piston engines resemble automotive engines, in that their peak horespower actually occurs somewhere above 3,000 RPM… SO… at any given manifold-pressure, any increase in RPM, will yield more horsepower.
Thanks for this - it’s been my point all along.
The post you quoted (in the midst of a very long-running argument spanning two threads), was to refute another’s contention that the propeller tip speed would cause compressibility that would negate the power gained from an increase in RPM from 2500 to 2700, regardless of blade pitch. I was pointing out that if that were the case, then propellers of the same geometric dimension and RPM limitations (such as the PA28 and C172, again regardless of constant-speed prop or not) would also be affected by the same incremental gain, thus the reductio ad absurdum illustration that I wouldn’t need to use full throttle on takeoff/go around (assuming 2700 is even achievable), that 2500 would do. This simply isn’t the case and has dangerous real-world implications that some in the sim world might pick up on (primacy being an effect in the learning process).
To catch you up: the entirety of this thread, now split from another and seemingly in slow mode, came down to a poster’s theory that cruise RPM settings may be better on go-around (initially as low as 2200), for the prime concerns of engine wear, noise, and efficiency. I’ve been arguing since that it’s a dangerous, inaccurate notion, not taught anywhere in the real world*, that max RPM* offers the most power in that low, slow, and dirty situation, that efficiency (THP vs BHP) gains at those speeds by altering prop angle is negligible, and that best rate of climb requires the most excess power to avoid the ground, obstacles, and stalls, with the other concerns not being a factor until you reach a safe altitude.
TL;DR: In this scenario, you’re going to get the most gains in power by increasing to the most RPM you can get, which outweighs any other consideration.
Later we can pitch the props back and let the engines work less hard during cruise climb and cruise, once we’re at speeds where efficiency gains start to matter and altitudes that don’t require avoidance.
*except where indicated by POH limitations or instructions
My (late to the party) pleasure. I’ve learned so much, from discussions like this, that I feel a debt, to participate…
That refutation is largely correct… but introducing a fixed-pitch prop makes it tricky… re: a discussion about propeller efficiency (cannot disregard blade pitch)… How the RPM increase of 200 actually happens ? I’ve no doubt you know this, but I’m compelled to clarify…
With a constant-speed prop (assuming no throttle change), the engine power increase, comes from being 200 RPM nearer max horsepower… the byproduct of variable propeller geometry. Whereas with a fixed-pitch prop, … 200 more RPM is the result of an increase in throttle setting (manifold-pressure)…
My pedantic, argumentative nature aside. 
…It’s just tough, to discuss how potential engine power, gets translated into thrust, through a range of RPMs; when the RPM range itself, is throttle dependent.
I still believe there is a bit of a fallacy in your argument. Procedures and checklists are not only indicative of performance parameters. One big white elephant in the room for instance is liability. Cessna engineers will never publish a POH without it being vetted by the legal department.
would they write in there “full MP and graduated RPM 2500 gives you more than sufficient power to smoothly pull out of a dodged approach with comfortable ground clearance with your airplane functioning normally and calm weather without overstressing the engine and burning a lot of fuel for no good reason”?
They probably would not. They would write “crank all you have full forward so we told you to get the last tiny bit of excess power out of your engine, we don’t care if it stresses your engine unreasonably, we also make money on maintaining those. And you could never sue us for not letting you know that while the power increase from 2500 rpm to 2700 rpm is small, there is some. Even if you don’t need it except for the few cases where things are really going south. let’s keep it simple and our liability perfectly minimized.”
not literally, but in effect.
Take commercial aviation as an example. Economics, graduated performance in procedures, is the reality. It’s a constant struggle of power between public authorities for max. safety and private enterprises for max. economy. Noise abatement on top.
(private flying is a joy for very few, and very annoying for a lot due to its noise)
You as a private pilot might not care about engine stress too much in your moments with it, when you do rentals. But your attitude would probably change drastically, if you owned.
not quite. I did not imply (full) negation. I simply wondered about the effect. You and MightyFish claimed a simplified 1:1 relation between RPM increase and available thrust. That seems clearly not to be the case. It’s more complex than that. The thrust gain is smaller.
Meanwhile instead of arguing in circles it might be interesting to fly actual procedures anf test the C310R performance with sensibly graduated rpm parameters and measure the differences.
I still don’t see the point of climbing from 200’ to 2000’ feet with a screaming engine, when the obstacle was 10’ high on the ground (another GA plane not vacating the runway quickly enough).
Where is the danger? What does a “safe altitude” mean in that context?
You reach 2000’ with 2700 RPM maybe two seconds earlier, than with 2500 RPM, for a climb that takes very roughly a minute. You stress the engine more, you burn more fuel. Two parameters that introduce new risks, compared to no tangible advantage.