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I had recently had a look at the Ford Trimotor NC9645 (link), currently on an EAA-sponsored tour. The propellers were fixed pitch (possibly adjustable by mechanic on ground), and the engines were not supercharged. On the instrument panel directly in front of the pilot were manifold pressure gauges for all three engines. Oil temperature, oil pressure, and RPM gauges for the central engine were nearby on the panel. The same three gauges for the wing-mounted engines were out on the engine nacelles-- rather difficult to read precisely from the cockpit.

The pilot seemed to fly mainly by reference to the gauges on the panel, only occasionally (at least after warm-up and run-up was completed) checking the gauges on the nacelles.

So, why did this aircraft have manifold pressure gauges for all three engines on the instrument panel in front of the pilot, while other gauges for the wing-mounted engines were mounted on the nacelles? Is it simply easier for some reason to transmit the manifold pressure over the distance from the engines to the instrument panel, than is the case for the other parameters? Or is the manifold pressure significantly more useful to the pilot than are the other parameters?

In a non-supercharged aircraft with fixed-pitch props, does the manifold pressure tell the pilot anything useful that an RPM gauge would not?

Am I correct in believing that it's rather uncommon today to have a manifold pressure gauge for an engine with no supercharger and a fixed-pitch prop? Is this something that used to be more common on aircraft from the earlier days of flight?

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    $\begingroup$ A manifold pressure gauge is useful even on an engine with a fixed pitch propeller. It is really just a vacuum gauge as used in many other piston engines. $\endgroup$ May 5 at 15:45

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The MP gauges would be in the cockpit because they are referred to frequently and it's easy to install them remotely with just a hose connecting them to the engines. Gauges mounted on the nacelles were done to shorten the connections for various reasons, such as to minimize the length of an oil pressure gauge capillary line or a thermocouple wire, as well as save instrument panel space in the cockpit. This practice was done on airplanes like the Me 110 twin engine fighter as well.

As far as using an MP gauge with fixed pitch props, since horsepower is a calculation involving torque and RPM, if you don't have a torque measurement device on the engine like you do with a turboprop, you can get a decent proxy for torque by measuring manifold pressure.

So on piston aircraft engines, manifold pressure vs RPM is used to calculate horsepower. Where RPM is controllable independently of torque because of a constant speed propeller, you must have both RPM and a torque or torque proxy measurement to be able to compute horsepower at all.

With a fixed pitch propeller, you can get away without the torque proxy (MP gauge) because RPM varies directly with torque in a fixed relationship and you can estimate horsepower based on RPM in flight. However, if you want to be more precise, it's still nice to have the torque proxy - manifold pressure - to know the true horsepower being produced at any one time, even though you don't have independent control over RPM.

This is not widely done on GA airplanes with fixed pitch propellers, probably due to cost/complexity reasons, but MP gauges are still nice to have. My own fixed pitch prop homebuilt airplane has an MP gauge, and it is very handy, as I can know the precise horsepower setting (and therefore fuel burn) at any one time from the MP/RPM being presented, even though I only have control over the MP being set with the throttle. If I was King Of Aviation, I'd decree that all airplanes have MP guages.

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RPM tells how fast the engine is turning, manifold pressure (indirectly) how much power it's producing.

If you're in a steep climb, for instance, RPM will drop a little due to loading of the propeller, while in a dive, RPM will rise -- but if the throttle setting is the same for both conditions, manifold pressure will be nearly constant (high RPM will slightly reduce manifold pressure, other factors equal). Looking at the throttle levers won't quite tell you the same, either -- a number of conditions, notably carburetor icing, can reduce manifold pressure without any change in throttle lever position.

The interaction of RPM and manifold pressure is an important piece of information, especially without head temperature and exhaust gas temperature readings available.

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  • $\begingroup$ OK, but as the RPM drops in the climb, with fixed throttle position and fixed-pitch prop, surely engine power output decreases as well-- isn't it pretty uncommon to have a manifold pressure gauge on a non-supercharged engine with a fixed-pitch prop? (Maybe I should make that last sentence part of the question-- ) $\endgroup$ May 5 at 13:47
  • $\begingroup$ Re "OK, but as the RPM drops in the climb, with fixed throttle position and fixed-pitch prop, surely engine power output decreases as well"-- we know this even from the simple experience of driving a car up a steep hill; once RPMs start to fall off, you'll probably need to downwshift to keep going-- (did add a couple of sentences to end of question) $\endgroup$ May 5 at 13:58
  • $\begingroup$ The RPM change at reasonable climb and dive angles is pretty small -- a couple hundred RPM out of a couple thousand. If you're diving steeply enough for RPM to climb a lot, you've got other issues (like overspeeding the engines). $\endgroup$
    – Zeiss Ikon
    May 5 at 14:03
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The MP gauge will tell you at a glance if your carb is icing up, by reading greater suction in the manifold at constant throttle setting- telling you that something other than the throttle was restricting airflow through the carb. I wish my old 1973 Subaru had an MP because the induction air preheater on the engine was broken and the carb would ice up regularly after I moved to Oregon and routinely drove through freezing fog conditions.

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  • $\begingroup$ Interesting info, but in an airplane-- at least if the icing happened during an interval where you were not changing the throttle position, and also were not transitioning from level flight to climb or dive or vice versa-- wouldn't you likely also notice a dropoff of RPMs? $\endgroup$ May 5 at 17:58
  • $\begingroup$ Yes you would, as did I in the subaru. RPM going down, suction getting stronger = carb getting choked. $\endgroup$ May 6 at 0:52
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Everything I'm about to write is pure speculation, as I have no expertise in aviation and more specifically the Ford Trimotor, other than having visited the KingsfordSmith museum at Brisbane airport a number of times :-). Anyhow, my guess would be on the Trimotor, there's probably two things:

  1. As mentioned by other answers, there's technical issues to do with remote mounting the guages due to 1920s tech.
  2. Maybe the engines have manual mixture control and for that reason, manifold pressure is relatively important for adjusting engine operation. I would also speculate that given you have a tachometer etc for the front engine, the pilot can easily tell if throttles need adjusting for the wing mounted engines, once the mixtures have been adjusted due to the audible beat frequencies that occur if the engines are "out of sync".
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