Power Check for Engines with Constant Speed Props

[kmccutcheon]

For years I have monitored manifold pressure during run-up to be sure the engine was making power; if more than usual MP was required to get run-up rpm, something was wrong. Now I am trying to figure out where I learned this. At first, I thought it was from Deakin, but I cannot find such a thing in any of his columns. Anyone remember a thread here that discussed this?

Thanks!

[Randy Sohn]

For years I have monitored manifold pressure during run-up

We taught on run-ups (Reese AFB, B-25's) to advance the throttles to an RPM (1700 in that case) where the props were still resting on the stops (Non-governoring) and check the MP required for that, then proceed with the run-up.

best, randy

[Tom Charlton]

For years I have monitored manifold pressure during run-up to be sure the engine was making power; if more than usual MP was required to get run-up rpm, something was wrong. Now I am trying to figure out where I learned this. At first, I thought it was from Deakin, but I cannot find such a thing in any of his columns. Anyone remember a thread here that discussed this?

Thanks!

Hi “Ken”?

I’d ‘ve guessed JD also. (wish he were back on the sig) But, ya that gives a good clue as to how’s all in front of the firewalls.

Most Constant Speed Prop aircraft I flew were also turbocharged which, IIRC, sort’a muddys the waters cause the turbo just crams whatever amount of air in to give the MP you want. Hmm, been awhile since I throttled one.

Regards,
Tom Charlton (Who’s reverted to a non-turbo, fixed-pitch, C-85 for his self now’a days.)

[Scott Dyer HPN/NY]

For years I have monitored manifold pressure during run-up to be sure the engine was making power; if more than usual MP was required to get run-up rpm, something was wrong. Now I am trying to figure out where I learned this. At first, I thought it was from Deakin, but I cannot find such a thing in any of his columns. Anyone remember a thread here that discussed this?

Thanks!

I learned it initially from John on the Martin. And while I learned the technique on radials I have to say that the whys and wherefores baffled me a bit, corrections for altitude, and problems with a wind from behind, etc. as I recall. Perhaps Ray can explain the details from his -3 days.

[Ray Tackett]

Perhaps Ray can explain the details from his -3 days.

My -3 days were 3 (no dash) days. The locals were quite protective of the engines and so did the starting and runup. I just watched the tachs and listened during runup. OTOH, density altitude was a non-issue, so maybe no need for the MP check. I'm not sure what a wind from behind has to do with runup, unless it's howling hard enough to change engine RPM enough to read on the tach. In that case, one had best be doing the runup into the wind for other aerodynamic reasons.

[Reams Goodloe]

https://www.advancedpilot.com/downloads/prep.pdf

A few clues here...

- Reams -

[Bruce Gorrell]

We taught on run-ups (Reese AFB, B-25's) to advance the throttles to an RPM (1700 in that case) where the props were still resting on the stops (Non-governoring) and check the MP required for that, then proceed with the run-up.

best, randy

With our R2800s we set Field Barometric Pressure. Power must be 2300 RPM +50/-0

[Scott Dyer HPN/NY]

I"m hoping that Randy will forgive me for posting his Warbird Notes #8 on this subject (Randy, if you want me to take it down, just say so!):

Warbird Notes #8 (16 Feb 95) FIELD BAROMETRIC POWER CHECK

The idea of discussing this subject has crossed my mind from time to time. We routinely see some fairly distorted ideas of what we're attempting to determine by using this check. A search of the flight manuals of the period seems to indicate that this check wasn't very well understood or used during WW II and for some time thereafter. That's probably been a major contributor to the confusion.

The basic intent here is to measure the power output of the engine against an established standard. A normal aircraft engine is capable of delivering a given amount of horsepower at a given RPM and manifold pressure (MP). This means that with proper precautions the MP can be used as a measurement of power input and the RPM used as a measurement of power output. The propeller blades must be against their low pitch stops, since this is the only blade position at which the blade angle is a known and doesn't vary. In other words, at this point it's the same as a simple wood or ground adjustable propeller. However, once the blades move off their low pitch stops all bets are off, the governor will take over and maintain a constant RPM regardless of power input or engine condition.

At a standard air density this power can be measured pretty accurately, it will always require the same RPM to absorb the same horsepower from the engine, day in and day out. If density changes, that's okay, the prop will still require the same power to furnish the same RPM if the relationship between power output and air density is kept constant. This constant relationship is maintained simply by noting the reading on the MP gauge during the pre-starting checklist and then setting the throttle to that reading when accomplishing the field barometric power check. After all this discussion some still try to make this difficult, introducing exotic discussions of density and other exotica. Just use what the MP gauge says, period! Okay?

When you set this MP you should obtain a specified RPM on the tachometer. The later the date of your pilot's manual the greater the chances that this specified RPM will be mentioned. Earlier ones didn't seem to mention it, apparently the importance of it wasn't too well accepted back then. In fact, a wide variety of methods seemed to be advocated. Whatever unit or group that wrote the manual or set up the program seemed to do their own thing, along with a lot of the other procedures. Some advocated what would be considered backhanded by the present method, but achieving the same results, setting a certain RPM and then looking at the MP it required. Others just said to run the throttle up and see if the engine seemed to respond well and felt like it was putting out good power. I get the feeling that standardization didn't seem to occur until well past the post war period, probably in the middle fifties.

Looking at the "J" model B-25 manual indicates this power check was included as a revision sometime after the basic flight manual was published in 1949, probably in 1953. The required RPM was 2200 ( 50). By the time we started operating the "L"s and "N"s it had been accepted as a standard procedure and was included when that manual was written. By the time I came to North Central Airlines in 1960 it had been well accepted, 2125 (50) was prescribed for the Wright R-1820-G202As on those DC-3s. A headwind or tailwind will have an effect on this number, i.e. any appreciable headwind will have a tendency to increase your RPM due to the change in air load. If the cylinder or carb air temperature is high because of factors other than atmospheric conditions, this will tend to give a low RPM. Also, high viscosity caused by low oil temperature will cause a lower RPM due to friction loss.

As a indication of what can be found with this check the following three items provide anecdotal examples. After reinstalling an overhauled R-1340 on a T-6 some problems were encountered in getting it to run right and I was asked to take a look at it. After completing a runup I offered a few ideas and I wondered if the prop had also been worked on. I was told the engine was putting out exceptionally good power, probably due to an excellent job of overhauling. As evidence they said that it was turning up about 2200 RPM at field barometric. My reply was that was the reason I wanted to know about the prop, I suspected that the pitch stops were at something other than normal for a T-6. That turned out to be the exact problem, the high RPM had nothing to do with power output, a worn out engine would have done the same. Apparently the stops were indexed at a lower setting more commonly used for a crop duster or a seaplane application in which some operators consider an initially high RPM more desirable (albeit risking some initial overspeeding at first throttle application). When subsequently re-indexed to a setting used on a normal T-6 the engine turned 2000 RPM and, incidentally, the engine has subsequently turned out to be a very good one. But, the RPM was lying when the prop was first installed, it did not indicate a surplus of horsepower.

The second case involved a PBY Catalina (R-1830) that turned about 2450 RPM when set to field barometric. The manual for this particular aircraft doesn't prescribe a RPM, being from the "dark ages" but I've used about 2200 (50) as a benchmark. At any rate, suspects included short blades, improper blade numbers or low pitch stops set for another application. Further research disclosed the blades were indexed at about 16, after reindexing to 19 the field barometric check resulted in a more normal RPM for that engine. This would probably be a good spot to digress slightly and discuss a couple of flight characteristics exhibited by this particular aircraft. On takeoff it required a rather slow throttle application at first to avoid an initial RPM overspeed. Also, at a normal 80 KT final approach speed this aircraft would become extremely nose heavy when the throttles were closed. I had to advise my students on checkout in this particular Cat that they might find the use of two hands necessary to raise the nose for landing, otherwise an inordinate amount of nose up trim on short final might be necessary. This is not a normal characteristic of the other Cats I've flown. What's interesting here is that after reindexing the aircraft flies completely normally, it's easily controllable with one hand and doesn't want to overspeed on initial throttle application. All of which demonstrates what a knowledgeable use of these checks can tell you. I would strongly suspect any aircraft with which I'm unacquainted if I find a high RPM on the field baro check during initial runup. It very likely will exhibit a nose heavy tendency when the throttles are closed on short final due to a "disking" effect. The third example involved a B-25 that had newly overhauled engines installed. The cores used for overhaul were of indeterminate age and heredity. The supercharger controls were re-installed duplicating their installation as removed from the old cores. Upon initial runup the right engine delivered less than 1900 RPM on the baro check. Although the complete story is too long to include here investigation revealed that the right supercharger control was reversed with the blower in HIGH when LOW was selected. Loss due to the increased horsepower required to turn the HIGH blower absorbed between 200-300 RPM worth of power.

One of the best "peace of mind" items derived from everyday use of this check is an awareness of long term performance or "health" of the engine(s). Let's say you're operating a B-25 and every time you've flown it both engines have given you a nice 2200 RPM on runup. One day you're sick or busy or out-of-town or something, whatever. A friend of yours is operating it and he sees that, while it's running as smoothly as any B-25 could ever be accused of, one engine turns up only 2100 at field barometric. If both of you are routinely using this check, the machine is trying to tell you something. The mag check may be O.K. but 100 RPM has gone somewhere from the last time it was flown. It at least provides a starting point, the base line leading up to that point has been constant and now something's wrong. Without both of you using this check the second guy wouldn't really have a clue, this is the only common point of reference between the two of you. Another situation would be if you're the only one that flies this aircraft, in this case you'll be in the position of easily noticing that something's gone wrong since the last time you flew it.

R. L. Sohn C

[Randy Sohn]

With our R2800s we set Field Barometric Pressure. Power must be 2300 RPM +50/-0

Yes, I'd forgotten to totally finish that, with the B-25's R-2600s we expected 2250 (+ -50) at Field Barometric. Did the mag checks there aslo. Can't recall exactly the numbers on our R-2800 CB-16s on the 340/440 airframes, if I could find one of the old piston CV manuals that I'd written in those old days I could find out. Can still recall briefing JD on that procedure for their "China Doll" C-46 and he did an EXCELLENT job of writing and especially illustrating that subsequent flight article/manual article. He really did a great job there, really impressed me!

[Randy Sohn]

(Randy, if you want me to take it down, just say so!

Nah, that's fine Scott, to tell'ya the truth I'd sorta forgotten about writing that, didn't know that they were still around somewheres but anyhow. that's why I wrote those things, so people could see how we did it and why, that's what they were for.

best, randy

[Scott Dyer HPN/NY]

Nah, that's fine Scott, to tell'ya the truth I'd sorta forgotten about writing that, didn't know that they were still around somewheres but anyhow. that's why I wrote those things, so people could see how we did it and why, that's what they were for.

best, randy

Well, I'd gotten a copy of them all over the transom years ago......

And studying this one greatly helped my understanding of the process! So thank you.

[Randy Sohn]

And studying this one greatly helped my understanding of the process!

Ah-so, great! Reminded me about why I'd written them. Prolly should'a written them w-a-y back at Reese, wonder if it would'a helped my cadets/student officers?

Also made me realize once again why Deakin had always impressed me! Gotta give him a call when I quit this.

best, randy

[kmccutcheon]

Many thanks to all of you for your help with this. I've read all of Deakin's columns, along with all of Randy's Warbird Notes. None of these appear to mention normally aspirated engines with constant speed props, but the principles still apply.

I wasn't taught to cross-check manifold pressure during run-up, and learned the technique after my Mooney M-20F's performance started to suffer. The engine ran great; good mag checks, redline on takeoff, smooth. But the rate of climb was suffering. I blamed it on the heat, but when fall came and I began counting grass leaves when climbing out of short fields, I looked deeper and discovered the cam lobes were nearly worn off. It was at that time I learned the MP cross-check, which I applied not only to my aircraft, but to those I maintained also. I know the technique works, but don't remember how I learned of it.

Thanks again!

Kim

[Bill Bridges]

Is the "Field Barometric Power Check" applicable to supercharged engines like the R-2800 Twin Wasp on the DC-6?

Grace and Peace,

[Bruce Gorrell]

Is the "Field Barometric Power Check" applicable to supercharged engines like the R-2800 Twin Wasp on the DC-6?

Grace and Peace,

Yes. See my post above. I doubt that being supercharged has anything to do with it. What's critical is to ensure that the blade pitch is still flat (fully fine, as the Brits would say). If not, you introduce too many variables. The other thing missing, of course, is just what RPM you should looking for for any particular engine.