Question on R22 and R44 MAP limits

[WHK4]

I have a question about the MAP limits in Robinson Helicopters.

On looking at the placards for 22's and 44's alike, when considering a fixed air temperature, there is a reduction in the MAP limit as pressure altitude increases.

For example, in the R22 Beta II, at 10 degrees C, the Max Continuous MAP limit at Sea Level is 22.3 inches but at that temperature at 4000 feet it is 21.5 inches.

Why is the ambient pressure affecting the allowable MAP for a given temperature?

Thanks for any help on this.

WHK4

[Grainger]

Take a look at the MAP gauge before starting the engine.

When the engine isn't running, the gauge should read ambient air pressure. (You do check this before every flight, don't you ? ). The reduction from this ambient value is a measure of the airflow through the inlet manifold, and hence a measure of power that the engine is producing.

If the ambient pressure is lower, then you are starting from a lower "non-engine-running" reading, and you will end up with a lower MAP reading for a given power.

[mongoose237]

There seems to be two schools of thought on this.


The first is that there is less exhaust back pressure at altitude so the engine doesn't have to work so hard and becomes more efficient.

Therefore, to maintain a given horsepower requires less manifold pressure.



Others say the "breathing" effect is negligible and the reality is thus:

The MAP gauge is measuring the absolute pressure in the intake manifold, not a pressure differential.

Now lets say you want the MAP gauge to read 23".

At higher pressure altitudes, where the ambient pressure is less, the throttle would have to be open further to obtain 23" of pressure in the intake manifold than it would at sea level.

This results in a greater volume of air entering the cylinders per second, so therefore a greater mass of air per second, and so therefore more horsepower.

So 23" of manifold pressure at altitude generates more horsepower than 23" of manifold pressure at sea level.



In summary, an equivalent MAP reading won't result in an equivalent throttle butterfly position when you change pressure altitude. As you go higher, the throttle butterfly is open considerably more to maintain the same MAP. If the throttle butterfly is open further, the engine will generate more power.

[Johe02]

Grainger is right. .

The first is that there is less exhaust back pressure at altitude so the engine doesn't have to work so hard and becomes more efficient.



Only true of turbo charged motors. . so just skimmed the rest but, the "breathing effect" (volumetric efficiency) is paramount.

[imabell]

manifold air pressure does not generate any horsepower at all.

it is only an indication of how much power you are pulling and more importantly how much is left.

"less exhaust backpressure" i haven't stopped laughing, what book did you get that out of ???, the book of stupid answers i'll bet.

who teaches you guys this crap. go and get a book on aircraft engines and i bet you won't find any reference to back pressure affecting map.:ugh: :ugh:

mongoose, i think you meant at a higher altitude where the pressure is less.

23" of map is 23" of map at any altitude and just tells you that the pressure in the manifold is the same. this has no bearing on the power produced by the motor.

[the coyote]

WHK4, here is a post I made a while ago for the same question:

I am talking about an engine that is not boosted (turbo charged).

Call fuel/air mixture "Stuff".

MAP is the absolute pressure of Stuff measured in the intake manifold downstream of the throttle. The more the throttle restricts the airflow into the intake manifold, the more the manifold pressure will drop (as the engine 'sucks' harder) and vice versa. When the throttle is wide open, the MAP can only get as close to the ambient outside air pressure as possible. When the engine isn’t running, the MAP will be the same as the ambient air pressure.

It is common to have a ‘de-rated’ engine that is capable of producing more horsepower than the drive train can accept at lower altitudes. Horsepower is purely a function of the pressure differential inside the cylinder when it goes bang, compared to the ambient pressure outside the cylinder in the crankcase. In order to control this pressure differential, we must introduce the correct mass of Stuff into the cylinder to burn. We can only do this by varying the pressure of Stuff going into the cylinder, as we have no control over the density or the cylinder volume.

In order to limit horsepower to the maximum that the drive train can accept, the manufacturer (and the pilot) must therefore limit the intake manifold pressure to deliver a pre-determined maximum mass of Stuff to the cylinder under the given conditions.

Say full throttle will let 90% of the ambient pressure into the intake manifold (I don't know the real data). It is physically impossible to get 100%, as there is always some restriction of the air. As the pressure altitude increases, the ambient air pressure decreases and 90% of less ambient pressure is less intake manifold pressure. A lower pressure is required in the cylinder after combustion to push against a lower ambient crankcase pressure to deliver the same horsepower. This means less mass of Stuff to start with, and is why the MAP limit on the chart reduces as you climb.

As the OAT rises the density decreases, but the ambient pressure stays the same. This means that less mass of Stuff will go into the cylinder at a given manifold pressure, and less pressure differential (horsepower) will be produced after combustion. So in order to get back to the limit you must get more Stuff into the cylinder, and the only way to do that is to increase the pressure of the Stuff going in. This is why the MAP limit on the chart increases as the OAT rises, or more correctly the air density reduces.

This is also why the chart allows full throttle at altitude, because up there the engine can’t get enough Stuff to produce enough horsepower at full throttle to exceed the drive train limit. Shovel less coal, get less power.

It is all to do with finding the correct amount of Stuff to burn to create the required pressure differential between the inside and outside of the cylinder, given the ambient pressure outside the cylinder to start with.

[bellfest]

The higher you go up the less gravity there is. Because the helicopter has less tendency to drop Robinson decided less manifold pressure was required for the same result therefore lessening the chance of his new -4 blades delaminating:ok:
To be totally honest I am not 100% sure this is correct, I just thought I might keep in line with some of these other theories:E

[Efirmovich]

Well said IMABELL, I nearly posted the same reply but couldn't see the keyboard for tears !

May I suggest you read Sir Harry Ricardo's books on the internal combustion engine before commenting further.

Ps. He then went on to help Frank Whittle sort out the Gas Turbine !

E.

[headsethair]

The manifold pressure is a figure determined by the density of the material from which the manifold is made. A cast iron manifold exerts more manifold pressure than cast alloy. This in turn has an effect on the studs which hold the manifold to the engine. The opposite reaction to this is the back pressure created by the head gasket being squeezed. Over time this stretches the studs and so the inlet manifold is stressed - increasing the pressure in the system. All of this has a direct effect on the BHP output - where BHP = Bonkers Helicopter Pilots.
:ugh:

[Efirmovich]

So glad to see that HEADSETHAIR has fully grasped the concept of BHP !:D
I will be very interested to read your next post on Thermo dynamics.

Yours, E.

[Gaseous]

Why is the ambient pressure affecting the allowable MAP for a given temperature?

Cos Frank says so.

In fact, as density altitude increases, the power produced for a certain manifold pressure decreases because the mixture gets richer. Restricting the MAP cripples the aircraft even more.

The only possible explanation I can come up with is that engine cooling is less efficient.

Other Lycoming equipped aircraft do not have this limitation.

I suspect this is one of the reasons Robbies generally make TBO and others do not.

[Flingwing207]

Why is the ambient pressure affecting the allowable MAP for a given temperature?
Other Lycoming equipped aircraft do not have this limitation.Well, actually the Hughes/Schweizer 300C and a few others do.
Fact is, that for a given MAP, a piston engine will make more and more power (assuming mixture is not excessively rich) as ambient pressure goes down, up to the point where ambient pressure prevents that given MAP from being made.
To be honest, I've never seen any textbook which explains this, but on the surface, the battle line between the expanding /cooling exhaust gases and the relative density of the outside air, combined with the fact that to achieve a given MAP, the throttle plate will be opened wider as altitude increases, would explain the increased power. It's not that the engine is making more power in the combustion process, it just expends less energy to sustain that process - getting the "stuff" in and out.

[Gaseous]

Fling, Yes, I phrased it badly. I should have said 'Many other'. Thanks

Your second statement is true providing the engine is operating at the optimum mixture,and that is the key thing. The problem with the Robbie is that leaning is not an option and the increase in power due to the reduced exhaust back pressure is offset by the increasingly rich mixture. Exhaust back pressure is a real issue despite some of the sniggering above.

Many years ago I was involved in the design of competition exhaust systems. Part of the research was to measure the effect of back pressure. The effect on power was found significant. Decreased exhaust pressure did increase the power for a given manifold pressure. However the gain was easily lost by having incorrect mixture. Mixture is critical. The mixture as set on a Robbie is already way rich of maximum power, and increasing altitude makes it worse.

Which has more effect on a Robbie engine as it climbs? Decreased back pressure or richer mixture?
I think the mixture has it. (edit:but I'm not so sure now)
The true answer to this will probably only be confirmed by running a Lycoming on a brake with an altitude simulator. Anyone done it?

[Efirmovich]

Sorry Gaseous, but you have to agree some of the posts are amusing ! !

I fully understand the effects of exhaust gas scavenging in high performance engines, however it is not really the same as the old plodder fitted to the 22.
Were the racing engines you designed the exhaust systems for running at 2600rpm at any other time than idle, I doubt it.

E.

[Gaseous]

Cant deny I sniggered too:O You are right about the old plodder which has an exhaust system which appears to have been designed simply as a dumping mechanism. None of the finer points apply. However, bear in mind that at 10000ft the back pressure is about half what it is at 0ft. My guess is thats got to have some effect but there is a hell of a lot more to it than just atmospheric pressure.

There is no denying that mixture has a major effect on power . For fixed wing aircraft Lycoming recomend leaning at altitude to restore maximum power for a given throttle setting.The implication of that is the richer mixture causes a loss of efficiency.

[WHK4]

Thanks to everyone for posting replies. I am enjoying the mix of technical and humorous viewpoints.

WHK4

[Efirmovich]

Gaseous, you are correct in that the air/fuel ratio will have a much bigger effect than the back pressure, however all this is irrelevant for me, as you must be a far braver pilot than I to go to 10,000ft in a R22 on those -4 blades whatever the MAP, Back Pressure ,Ambient, bla bla bla ...............!!!!!!!!! :uhoh:

E.

[Gaseous]

E. I'm with you on that. I've not been in a death egg for some time and hope not to again.

Edit: I dug out my old books to look into this further. I measured my Enstrom exhaust and all calculations are based on that engine as I have the details to hand. It is a 360 running at 2900 rpm.

Basically the exhaust is too short to have any bearing on gas scavenging. All major resonance, extractor and gas slug effects are over by 18 degrees of crank rotation after the exhaust valve opens. By BDC the pressure in the exhaust will be just above ambient. This means that for the rest of the exhaust stroke the cylinder is acting simply as a compressor. By calculation, to empty the cylinders at sea level will require about 25 BHP. At 5000 feet density altitude 6 BHP less is required. so 6 BHP more is available for the same manifold pressure assuming the combustion event is the same. I do not know for sure how much power is lost due to enrichening. My guess is more than 6BHP. Draw your own conclusion.

[mongoose237]

Imabell, no I did mean higher pressure altitudes. Higher pressure altitudes = higher altitudes. Seems a regular gotcha in exams too that one. Probably would be simpler if it was written "higher pressure-altitudes" but hey-ho such is life.

As I said, there are two schools of thought and you will notice that although I mention them both my (probably unclear) conclusion mentions the latter not the former. I made reference to the "back pressure / breathing easily" argument because that is the explanation voiced by Robinson technical refs ... the manufacturer of the helicopter in question. Although I don't subscribe to said theory, I equally felt it merits a mention as the company line.

My apologies if my explanation was oversimplified, I was merely trying to explain the pressure differential between (a) 23" and ambient pressure at sea level, and (b) between 23" and ambient pressure at 3,000ft is significantly different and will result in more horsepower being generated in situation (b) ... which the coyote explained far better than I did.

Efirmovich - Constructive replies, thanks for the history lesson and suggested reading material to compliment your commentary :hmm: