SE Performance Question

[VH-XXX]

This is a little long-winded sorry......

Scenario:

Single Engine
To make it easy, let's say it is a Cirrus with a FIXED pitch prop and automatic mixture control with fuel injection.

At 500ft AMSL (close enough to sea level for this discussion), straight and level, the engine monitoring reports 90% POWER at Wide Open Throttle, RPM 2800 (redline).

At 8,500 ft, the engine monitoring states 60% power and 2800 rpm.

So the question is:

a. Is it the case that the engine is revving to 2800rpm but only developing 60% power.

or

b. Is it that 60% power gives 2800rpm because the air is thinner etc.

Are they the same thing?

So, if a. is correct, if you were to pull up into a climb, RPM would quickly decay because there is no power behind those RPM, like running a car on a dyno at 100kmh with no resistance, you would only need 10kw of power to achieve this?


Point 2:

All that aside, it leads into the question, do you run the engine at 2800rpm (or close to it) knowing that is near red-line when operating at 8,500ft or would you back off to your 2,500rpm cruise revs so as to not damage the engine, thereby running the engine at 50% power, or would you not damage the engine by running it near red-line because there's no "power" behind it / aka it's not under as much load as at sea level.

:ugh:

[The Green Goblin]

I suppose A is your answer. Think of a CSU setup. The higher you go the more manifold pressure you lose at a given RPM until you go through your full throttle height.

If you look in the POH it will give you the required power settings for % powers for every height. Use them :ok:

[VH-XXX]

Limited info in the POH, will have to search for engine info instead I suspect.

[ForkTailedDrKiller]

Oh dear! My head hurts.

Interesting question!

I assume that if you go high enough, you will evenually get to the point where the engine cannot make enough power (as reflected in manifold pressure) to maintain 2800 rpm.

Has anyone in here been there? I have had the FTDK to 18,000' DA and it was still doing 2700 rpm. MP was down around 16', if I recall correctly. I figure the engine was making maybe 50% of the 285 hp it is capable of, but somebody in here who is smarter than me can probably come up with a more accurate figure.

If the engine manufacturer has certified the engine for 2800 rpm maximum continuous, then there is no reason not to run it at 2800 rpm all day - other than noise and burning up $$$ in fuel, unless you go LOP.

I generally run the FTDK at 8-10,000' on WOT and 2300 rpm cause it gives me a good compromise of speed vs fuel consumption vs noise.

Dr :8

PS: Interestingly, I need to run the IO500 powered A36 at 2500 rpm/WOT to get the same speed out of it as I do the IO520 powered V-tail at 2300 rpm/WOT.

[Fred Gassit]

A fixed pitch prop is a bit like driving in one gear. The only way to maintain your hypothetical 60% power while reducing rpm is by changing the propeller pitch.

If you pull up and reduce airspeed your rpm would drop, perhaps even down as low as your static rpm speed (usually 2-300rpm) but a propeller doesn't "coast" the way a car can on a flat, at least not in steady level flight.

If 2800rpm at 60% power was uncomfortably high the solution in a fixed pitch situation is to fit a cruise prop, one with a bit more twist.

Static RPM (Powerplant speed on runup) will be lower again, acceleration is slower, (more runway needed). But you would have lower cruise rpm and sometimes a bit more speed. Analogous to driving in a higher gear.

In any event if the POH approves the setting it is fine. The higher/lower engine rpm is one of the compromises that needs to be accepted with a fixed pitch installation.

[ForkTailedDrKiller]

Fixed pitch ??

OK, OK, ignore my post above - I shouldda read the question more carefully!

Dr :8

[VH-XXX]

Dr. in the SR20 from memory it develops around 69% maximum power at 8,500ft so you'd be struggling to have much more than 50% at 18,000ft you'd think.

[Old Akro]

XXX's post actually raises a number of questions. Starting at the end. Redline is a limit. Unless the POH limits the time at max power, anything below that is OK indefinitely. It should be noted that the easiest way to derate an engine to accommodate an airframe is to lower the redline, so potentially redline is a dawdle for the engine.

Secondly, an engine can only burn as much fuel as it has oxygen to make the correct air/fuel ratio. So higher density altitude = less oxygen = less fuel = less power.

Most non-turbocharged engines hit WOT at 75% power at around 6,000ft. That is why it is generally the fastest altitude for non turbocharged aircraft.

Unlike car engines that might hit max power at 2/3 to 3/4 the way to redline, most aircraft engines develop max power at redline. So, the more RPM the more available power.

Fixed pitch requires compromise. A cruise optimised propeller is unlikely to allow the engine to achieve redline on take-off. But aerobatic aircraft generally want maximum power to extend vertical penetration, so they often set them to achieve redline at airspeeds below the 1g stall speed. The airshow guys might use props that give them max power at zero airspeed for toque rolls and low speed push overs.

If at your cruise altitude you put extra load on the engine (ie pulling up) and you do not or cannot increase throttle opening, then the engine can only react in one way - reduce RPM. This will happen if the engine is producing 100% power or 50%, although it will be less pronounced at higher airspeeds and the major effect is likely to be that airspeed reduces to match the available power which will mitigate the airspeed loss.

[Obidiah]

Good Question.

I'm not an instructor but I do flog about in an aircraft at altitude with a fixed pitch prop trying to go as fast as I can so I have a bit of operational experience I guess.

a. and b. I guess could be classed as much the same.

2nd question is a bit tricky, you probably need to know the particular engines build tolerance for high RPM low MaP.

Some don't seem to mind it too much ie. TCM O-200 in fact they're often operated well over OEM red line speeds, that is not to say it's ideal for the engine.

And others don't handle it as well, ie. PW 1340 radial.

Often one of the issues is ring land wear due lack of combustion pressure. (In fact there is some evidence to suggest that the over square wives tale started out as an under square rule for radials).

Balancing reciprocating loads are often sighted as another reason against high RPM low MaP.

But really it something which is likely to be somewhat engine specific.

Hope that offers a bit of insight.

[Andy_RR]

If you make 2800rpm at SL, S+L with 90% power, then at 8500' you'll probably be needing more than 60% power to acheive 2800rpm when S+L. You'll need to be in a dive to achieve it, in which case both A and B are correct.

The other thing that compounds this question is that propellor geometry is optimised for a fixed TAS/RPM ratio, but combined with the airframe drag power-v-TAS requirements, this yields only one optimum point (at least when S+L). If you move away from this point, efficiency is degraded. It might be that at SL, the efficiency is worse than at 8500'

[43Inches]

a. Is it the case that the engine is revving to 2800rpm but only developing 60% power.

b. Is it that 60% power gives 2800rpm because the air is thinner etc.

Both a. & b. are correct.

Air is thinner (less dense) so a. power is reduced due to less dense fuel/air mixture available for combustion and resultant less power being delivered to the prop. And b. the less dense air offers less drag componant so therefore it can still maintain the higher RPM for the power being delivered. However the thrust being delivered from the prop is also reduced due to the less dense air, to achieve the same percentage power output at altitude a fixed pitch prop must be operated at a higher RPM.

Use power settings the manufacturer recommends for the engine. All settings below redline are not guaranteed to be engine friendly as some may produce unacceptable vibration even at low power settings.