Thorp Air Command - T18.net

Hello,

I have a fixed pitch prop so I do not "need" a MP guage but I have one and I liked it when it worked. I believe it is a ISS like this from AS.

http://www.aircraftspruce.com/catalog/i ... auges8.php

It has been acting up for about 2 years now. When I turn the master on it climbs to 10" then bounces a little and goes up to between 15" and 20". I then contiues to read 10" to 15" low. My question is does this sound like a guage problem or a sender problem? Or could it be something else like a pinched tube? I kind of ruled out the pinched tube because I would think it would just sit a the same place or it would just change very slowly if the tube is restricted.

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Matt Smith
Des Moines, IA
55RC

Matt,
Your MP gauge problem is most likely the sender. Check all the connections and for leaks in the line between the engine and the sender as well.

A MP gauge is always nice, especially when flying at higher altitudes. You can always assess 75% power (roughly) by adding the first 2 RPM and MP numbers( ie:24/24) to equal 48 which is roughly 75% power. Any combination thereof is roughly the same (21/27 etc.)

V/R
Smokey

PS: You can leak check all your intake lines/tubes by hooking a vacuum cleaner or shop air in the "blow" mode to your carb intake. Then spray soapy water around all your intake tubes. No bubbles=No troubles

I don't think it would be a leak in the line or it would read close to 30" with just the master on.

I was leaning toward the sender but the bounce around 10" when powering up has me wondering about the guage.

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Matt Smith
Des Moines, IA
55RC

Matt: Idealy it would be nice to check for leaks 1st and then swap out a gauge with a known quanity.
However, you likely don't have access to a spare serviceable gauge.

The sending unit usually is the culprit as has been stated.
But, just turning the master on you say the needle moves up to 10 inches and then bounces to 15/20 inches to me would indicate a faulty gauge.

I would suggest to check for leaks first as already suggested (No cost).
Then swap out a gauge it is cheeper and easier to replace.

Good Luck.

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Victor J Thompson
C-GIRQ
In memory of Dad

Hi Victor,

Actually when I turn the master on the needle moves smoothly up to 10 inches and then bounces quickly down to about 5" and them quickly back up to 10" and then continues smoothly but slower up to about 20" IIRC. When the engine is running it reads between 10 to 20 inches lower than it should.

Am I mistaken in believing that when the engine is not running that a leak would make no differance in the reading? If the barrometric pressure is 30" and I am at 1000' I shoud get a reading around 29" even if there is no tube connected to the sender, Correct? Or am I missing something?

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Matt Smith
Des Moines, IA
55RC

The manifold should read ambiant air pressure when the engine is not running. In fact, the instrument shops calibrate the steam gage variety by setting ambiant pressure. The difference between ambiant and your reading at full throttle when on take off is a measure of your induction efficiency. In idle, your reading should be ten or below. I suspect dirt in your sender, either in the line to it or in the unit itself. That said, most senders or engine fittings for the steam gage have an orifice in the line to cut down on "pulsing" from the variations in the intake manifold near the intake valve on the engine.

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Bob Highley
N711SH
SN 835
KLAL

I had a similar problem yesterday while flying. Turns out I had a drop of water at the low point of the MP tube, I'll know next time I fly if that fixed it. I'm assuming condensation somewhere along the path caused the water accumulation?. TBA

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Chuck Kincer
Richmond, Me
575CK
KIWI

I went out to the hanger tonight after work and cylcled the master to see notate what the MP guage did. This time it only climbed to 2" and then droped back to 0. If I flip the master off and right back on it would just sit on 0. If I wait a 10 or so seconds to turn it back on it would climb to 2 and then drop to 0. I crawled under the panel and disconnected the tube from the back of the sender. Flipped the master and pretty much get the same thing. So it is not the tube. Still not sure if it is the sender or the guage and since I don't want to pull the panel twice I think I will just buy both.

Thanks for all the input.

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Matt Smith
Des Moines, IA
55RC

I can't inagine a need for an Electric Manifold Pressure Gauge.
Perhaps you could get a mechanical one and route some small tubing thru the firewall to it.
I don't see any "Dangerous Vacuum" leaking into the cockpit if there was a gauge or line failure
I think an electric transducer type gauge wouldn't be as accurate or as light or as reliable as a direct reading gauge?

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Scott Emery
EAA Chapter 326
T-18 N18TE

I already have the tube through the firewall as the sensor is only about 4 inches from the guage. I wondered why the sendor is located where it is? Do you have an example of a mechanical MP guage with out a separate sensor?

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Matt Smith
Des Moines, IA
55RC

I currently have 2 manifold pressure gauges in my panel, a mechanical 3" and an electronic one that is part of the Dynon engine monitor. The mechanical gauge was in the panel when I purchased the airplane and when I installed the Dynon unit I left it in the panel cause it was easier than installing a hole plug. The electric one has a digital readout in 0.1" where the mechanical gauge is marked in 1" increments, I think the digital readout is much easier to use. For sensors, the mechanical has a -4 hose from the manifold to the gauge with a steel bulkhead fitting where it penetrates the firewall. The electronic sensor is on the engine side of the firewall with the wires as part of the wire bundle where it goes through the firewall. I used the SafeAir1 Firewall Pass-Thru to route my wire bundle through the firewall http://www.safeair1.com/averytools/firewallpassthrough.php. I could save some weight if I removed the mechanical gauge, maybe when I redo my panel.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

Hi Bruce, I use my MP Guage offen, even though I have a fixed pitch I find myself using it a lot especially when I'm looking for ram effect. Do you find yourself using it offen? When we fired up Deans plane and we were in the tuning stages we noted that the ram effect was substantial, we were running 30in of MP but The alt setting was just below 29, so we got a little ram effect and it helps out at altitude...........Dan

Matt;
You could always start with aircraft Spruce' online catalog.
they have manifold pressure gauges made by UMA, Westach (cheap) and Falcon gauges listed.
Bruce enjoys a magnitude of accuracy above what I get, but I like the analog gauge all the same.
I do enjoy my Dynon D-10a EFIS, and could easily want to add Dynon's Engine Monitoring System the EMS D-10.

Dan;
I use the manifold pressure gauge when I am not "In the Patern". Just like if I was flying with a constant speed prop. My Ted Hendrickson 68"x68" prop is a high RPM prop, giving close to 2300 static RPM on the ground, and easily 2500 to 2700 in level flight. We set power with the manifold pressure gauge. especially when comming "downhill" to our distination. In a fast cruise decent, the RPM stays high, 2600 -- 2700, even when you pull back the throttle quite a bit. As you get lower, the manifold pressure will creep up, so you can reduce it, to say, 22" Hg at the beginning of your decent, and keep pulling it back to stay at 22", or pull a little more as you get closer to the destination airport, until you are maybe 18" or less, then the RPM is slowing down at the same time you are leveling off to enter the pattern. Then I watch the Tachometer, and fly the pattern based on RPM and airspeed to make the landing.

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Scott Emery
EAA Chapter 326
T-18 N18TE