Limit MP

[Jcooper]

Im sorry if this has been asked but on the R22 why does your limit MP increase with temperature and decrease with altitude? Both situations would lower the density of the air so I guess I am just confused.

[CJ Eliassen]

JC,

The higher the temperature gets, the less dense the air which means that the engine will produce less power for a given MP. However, as you climb and the pressure decreases, you do not lose any MP or intake air density because the engine is derated. If you pull 24 inches at sea level and 24 inches at 5,000 feet at the same temps, the engine will produce more power at 5,000 feet because you have less exhaust backpressure. Therefore, to limit the horsepower to 131TO or 124MC, you can pull less MP at altitude for a given temperature.

[Jcooper]

Quote "The higher the temperature gets, the less dense the air which means that the engine will produce less power for a given MP. However, as you climb and the pressure decreases, you do not lose any MP or intake air density because the engine is derated"

Well you just answer my question with the same words and it still doesnt make since. "the higher the temp gets the less dense the air which means the engine will produce less power for a given MP" AGREED

"However, as you climb and the pressure decreases *less dense air* you do not lose any MP or intake air density (well yes you do because the air is less dense) because the engine is derated * well if they are both making the intake air less dense then why does the chart run opposite ways???

Some one has to have a good explaination for this...please

[helimatt]

G'day Jcooper
M.P is limited at altitude because due to thinner air up high, the throttle needs to open wider to draw the same amount of air molocules than for lower altitude denser air. The M.P is limited plainly because it comes to a point where throttle cannot open any further, so therefore no more M.P will be available, and RRPM will begin to decay. If you look at the M.P limit placard it will read wide open throttle at high temp alt combinations.
Hope this makes some sense:ok:

[CJ Eliassen]

JCooper,

I guess I have to take a few steps back.

You have to understand that if you are at seal level with 30 inches of ambiant air pressure and your MP is 24 inches, the vacuum created by the intake stroke of the piston against the throttle plate reduces the air pressure to 24 inches. This is a reduction in air density. As you increase in altitude to say 5000 feet and you don't change the throttle setting, you will see something like 20 inches of MP. If you open the throttle more to get 24 inches, you still have the same pressure in the intake manifold as you do at 24 inches at sea level. However, because the exhaust back pressure is less, the horsepower output is higher. 24 inches of MP at sea level has the exact same density as 24 inches of MP at 5,000 feet when at te same temperature. Only two things can change density, pressure & temperature.

Make sense?

[ShyTorque]

Humidity has a minor effect but not nearly as much as T & P do. It is usually ignored for our purposes.

[CJ Eliassen]

Humidity doesn't effect the density of the air, however it does reduce engine performance since the water molecules displace some air. Humidity doesn't effect the efficiency of an airfoil, so the aircraft will still fly as if it is at the density altitude determined by the pressure and temp.

Humidity only increases the DA to the engine, but it does not change the density of the air. Other things can increase the DA to the engine such as pollution or fires.

[Rich Lee]

Don't stop now Charlie s charlie. I was just beginning to feel the 'love' in the room.

[Jcooper]

Alright, so if you have a cool temperature of 20 degrees you have a MP of say 20". Now you move into air which is 100 degrees. Why wouldnt the MP do the same thing? The air density is the contributing factor to the MP. So if a rise in temp has a decrease in density, and a rise in altitude has a decrease in density, why are their opposite effects on the MP limit chart?

Don't worry about hijacking my thread but if you can help on this I would appreciate it.

PS I agree, humidity does effect air density. Water vapor is less dense than O2 and N2 so if you have more water vapor in the air the overall density will be decreased.

One more post script, has anyone else noticed there are a lot of poetic responses on here? I almost feel guilty for not using metaphores or similes.

[CJ Eliassen]

The effects of humidity on air density are around 1%, The effects of humidity on engine performance can be as much as 10%.

Quote: Alright, so if you have a cool temperature of 20 degrees you have a MP of say 20". Now you move into air which is 100 degrees. Why wouldnt the MP do the same thing? The air density is the contributing factor to the MP. So if a rise in temp has a decrease in density, and a rise in altitude has a decrease in density, why are their opposite effects on the MP limit chart?

The air density is not a contributing factor to MP. Only the air pressure is. If the ambiant pressure is the same, the MP will be the the same regardless of temperature. However, the higher the temp, the lower the power because the air density is lower. As you climb, you lose available horsepower because of the reduction of air pressure available to give you the same MP. But for a given MP, the horsepower increases as you climb because of the reducton of back pressure.

You might want to study a private pilots book meteorology section.

[Jcooper]

Quote "The air density is not a contributing factor to MP. Only the air pressure is."

and what by god, causes the air pressure to change? If you have less dense air there is going to be less pressure in that air. If you have more dense air there is going to be more pressure in that air. The two are hand and hand you cannot just discredit one.

Can anyone else help me out with this, I know where this thread is going with CJ's advise and itd be better to get someone elses opinion

[Buitenzorg]

JCooper

"If you have less dense air there is going to be less pressure in that air. If you have more dense air there is going to be more pressure in that air. The two are hand and hand you cannot just discredit one."

Not correct. Density = weight / volume. Take a sealed container (such as a scuba air bottle) and heat it up. The pressure will increase, but there will not be a single molecule more in that bottle, and its volume will remain equal too (for the purposes of this discussion anyway), so the density will remain equal. Now put the bottle in a freezer: pressure will drop with temperature, weight and volume remain the same, therefore so will density.

The limit that we try and set with the limit MP is really the maximum horsepower we want the engine to put into the drive train. The more WEIGHT of air goes into the engine, the more oxygen it has to burn fuel with, thus the more horsepower delivered. Higher OAT at equal ambient pressure means lower air density, so to get the same weight of air into the engine, more volume has to be delivered per second, which requires a higher (manifold) pressure.

Lower ambient pressure (higher PA) at equal OAT means lower density, but here the plot thickens somewhat. The pressure read by the MP gauge is the absolute pressure in the intake manifold (rather than a pressure differential). So to obtain an equal MP reading at higher altitude (lower pressure) and equal OAT, the throttle butterfly will have to be open quite a bit further than at the lower altitude. This means a larger volume of air will enter the engine per second than at the lower altitude, which means more weight per second, which means more horsepower. So to maintain the same HORESEPOWER output at higher pressure altitudes (lower ambient pressure), the manifold pressure will actually have to be decreased slightly.

[Pat Cox]

The engine is limited in power output primarily by pressure altitude, not temperature. For example:
a. At 1000 feet pressure altitude & +20C OAT the density altitude is ~1800 feet and the 5-minute MAP limit is ~24.4 inches Hg per the R22 Beta's limit MAP chart.
b. At 2800 feet pressure altitude & +20C OAT the density altitude is ~4000 feet and the 5-minute MAP limit is ~24.0 inches Hg; your density altitude is higher but your limit MAP is lower.
c. At 1000 feet pressure altitude & +40C OAT the density altitude is again ~4000 feet but the 5-minute MAP limit is ~25.0 inches Hg; your density altitude is higher (than example a.) and your limit MAP is higher.

Due to decreasing atmospheric pressure with increasing altitude, the engine does not have to work as hard to push out the exhaust gases (it "breathes" easier). In effect, the engine's volumetric efficiency increases so less MAP is required to maintain a given horsepower.

[For the density altitude numbers above, all I did was refer to the density altitude chart on R22 POH page 5-3 and then plot & overlay the appropriate density altitude lines on Beta Limit MAP chart on page 2-10 (actually, I enlarged said charts by a factor of 2 on our copy machine to avoid eyestrain). When a density altitude line is superimposed on the Limit MAP chart it becomes readily apparent (due to the line's slope) what the major variable is. One could get really fancy and perform a similar feat with a computer program, but a pencil is faster.]
:D

[The Nr Fairy]

Say, Pat - who do you work for ? :D

[Robbo Jock]

I don't think it has so much to do with how many AIR molecules are going into the engine per se, more to do with how many FUEL molecules are. The power generated by an engine is based (all other things being equal) on the _mass_ of fuel burnt. The ratio of fuel to air should (if the carb's doing its job) be fixed, so as the air density increases (i.e. the temp decreases) more fuel is sucked into the bang box and more power is produced. So, as the engine is derated, the power limit is reached at a lower MP. As temp increases, fewer fuel molecules are sucked in, so you need a higher MP to generate the same amount of power.

Or something like that.

[[email protected]]

The reduction in effort required for the engine to push the exhaust gases out is nothing like as significant a factor as the lack of atmospheric pressure at altitude to charge the cylinder with the required weight of fuel air mixture to produce the power.
See Buitzenorgs explanation, it's spot on.