Is there a “rule of thumb” that indicates what MP should likely be at any gven altitude? I am thinking only of a normally aspirated engines.
The rule of thumb is to read the manual :-)
Otherwise a lot of people think that you should remain “under square”, i.e. the MP shall not exceed 1/100th of the RPM.
The rule of thumb is to read the manual
More specifically, the engine manual (for the Lycoming O-360 series, PN 60297-12), because the POH is usually very short on engine operating details
you should remain “under square”
This rule is very conservative for most common engines. For the IO-360, for example, Lycoming lists the max MP for 1800RPM as 25 inch Hg (chart 3-36 of the above document)
Unfortunately, there’s no graph that gives MP at altitude, but from 3-36, MP is around 1.5 inch Hg less than ambient pressure at WOT / full power and sea level on a standard day.
With every Lycoming engine I have ever heard of, you climb at full throttle all the way to the top of the climb.
It is absolutely wrong to reduce power during the climb. The carb or (if injected) the fuel servo is factory set to deliver a certain rich mixture (about 150F ROP – rich of peak) when the throttle and the mixture are fully forward and this allows the engine to deliver max rated power, without exceeding the CHT limits.
This assumes adequate airflow however which is why many/most planes cannot climb at Vx or Vy all the way up, and need trimming for a higher speed ASAP. Actually I suspect they can all climb at Vx all the way up without exceeding the “Lycoming max” CHT of 500F but everybody who knows about this stuff thinks that is a crazy figure and 400F is a much better one to work with, and 400F definitely needs more airflow.
The problem is that with a fully rich mixture, as you climb and the air gets thinner, you eventually reach a point where the engine is not developing much power and starts to get rough, from being too rich. Normally one can climb with everything forward to about 8000ft, though 5000ft is a better point at which one should start leaning.
The question is how to lean during climb…. the best known method is the “constant EGT climb” but that needs an EGT indicator. Look here and search for
“constant-EGT method”.
This is the very best way which is still quite simple to do. There is a more clever method involving Vz – see the paper here
I actually interpreted the question as being what MP to use for cruise at different cruise levels.
I choose my desired power setting (normally 65%) , and then pick RPM/MP from the POH according to the altitude. I will normally use the lowest RPM in the table provided the engine runs smoothly at that setting. I subsequently lean to peak EGT, again provided the engine runs smoothly (all cylinders peak at about the same time)
Yes – that’s possible.
Then the answer should probably be: set 65% of max rated power, or full throttle and peak EGT if there is not enough air to make 65%.
I think one can get 65% up to about 8000ft. However this suggests an IO540 can do it (at max revs) up to about 12000ft, which puzzles me. I thought all petrol engines were the same in this respect.
To be fair my question was a little ambiguous. I was interested in MAX manifold pressure indications at any given altitude. Of course we all understand that this will drop as one climbs but I thought it would be of interest to know what others experience. As Peter says climb at full throttle etc., so what happens to MP when you do this?
With normally aspirated engines you normally climb at full throttle and max continuous RPM. Beware that some engines call for a slight reduction of RPM after takeoff, while maintaining WOT (wide open throttle).
Max MP will drop proportionally with altitude unless you have a turbo.
The natural drop in MP when climbing is about 1 inch per 1000ft.
I was interested in MAX manifold pressure indications at any given altitude
Maximum manifold pressure will be ambient pressure at a given altitude. Subtract from that the specific losses of your aircrafts air filter and induction system (typically 2-3 inHg) and you will get the maximum MP indication.
