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Carb heat

Carb Heat - RPM Increased... Why? Plus odd fuel flow figures.......

Hi All,

So had a great bimble today, apart from the bloody restaurant was closed where I flew, but that's for another post :D

I did a couple of experiments today on leaning and the results confused me. I flew a PA28, and followed the leaning of the hand book. The first run was to follow power procedure, basically set about 50 RPM below required, then lean for peak RPM and fine tune for desired power setting, this worked well. The oddity here is that I selected 75% Power, which should have given me 112 knots at 38 litres/hour. I got about 118 knots at 27 litres per hour. The carb heat worked ok.

Then for the second run I used Economy setting, I.e. full throttle then lean for desired RPM. I selected 75% Economy which should have given me 103 knots at 34 litres/hour. I got 103 knots but at 30 litres per hour, and during FREDA check the RPM increased when Carb Heat was selected on.

I am confused why the fuel flow figures are so out from the book, and why the selection of carb heat increased the rpm...

For both flights the pressure altitude was 4500 ft, and setting rpm was as per operating hand book for pressure altitude and OAT.

What gives? Ideas?

Depending upon exactly which operating handbook you were referring to, some of Piper's "POH's" can be extremely optimistic. Before you take that book too seriously, have a look inside the front cover. Is there anything written which causes you to understand that it is the FAA approved Flight Manual or it's equivalent? Depending upon the year of the PA-28, it might predate "Flight Manuals", but that is your starting point.

In doing flight testing of a late '60's Twin Comanche, and being provided the POH, I was surprised to find that it was not FAA approved at all - thank goodness, 'cause it was quite wrong in performance. There actually was a separate FAA approved Flight Manual, which was identified as such, and FAA signed. It was typed out on someone's courier typewriter, but it was the authoritative document. The POH was bright and flashy, but it was a marketing guy's dream document - the plane would not do what it presented, unless in tow behind a more powerful, faster aircraft!

The condition of the aircraft (clean, smooth paint, waxed, and rigging) can affect performance measurably. Also, props get changed and repitched over the years, and the POH data might not account for that. A 2' pitch change can noticeably affect performance.

As for the carb heat increasing RPM, is it possible that you had leaned to lean of peak, and the carb heat was actually richening it somewhat, and restoring power? I have not thought that all the way through, but that's what I'd look for first. Unless you actually had carb ice, and melted it away, resoring power that you did not know that you'd lost.

Home runway, in central Ontario, Canada

some of Piper's "POH's" can be extremely optimistic.

This is what I thought, and expected, so I thought for a new "out of the box" I could have archived these figures, but for a 1960's PA28 Cadet with the 160hp (SEP) up front that does have knocks in the body work, areas of chipped paint, touched up paint etc, all spoiling the airflow; then I would have thought I would have achieved worse than book performance and not exceeded it.

FAA Approved

Good question, I don't think so, I will check again carefully but it seems to be a DFS authorised translation of the US manual. The way it is written though does imply (to me anyway) that there may have been some form of flight test, or at least verification of the figures by DFS on import.

Prop change/re-pitch

Interesting point at the moment I have no idea. But next time I have my club desk duty day in a month or so, I will have all the paperwork to hand and could look through see if anything has changed.

Carb Heat

Interesting point, and I am guessing that if it is operated at full throttle and then leaned until the rpm drops to the desired level, then you have to be lean of peak. The affect of carb heat definitely had the effect of raising the rpm when selected hot, and returning to previous rpm values when selected cold, so I do not think I had carb ice, because there was no change in the before and after rpm value.

The bit that has really confused me on the power vs. economy leaning exercise is that the fuel consumption was higher with economy when I would have expected that to be less. The only thing that could have "skewed" the figures is taxi time time, but again the effect is the other way around. On power sortie, the taxi time was 16 minutes because I had to wait for a couple of circuit bashers to land before I could depart, but on the economy run the total taxi time was 3 minutes... In that I would have expected the power to have been a litre or so higher... Averaging out in terms of use per minute I got 0.44 litres per minute with power leaning and 0.47 litres per minute using economy... Both figures well under the book value.

First of all, there is no such thing as a 1960s Piper Cadet (the Cadet existed only for a few years in the late 80s). However, I take it that you are referring to some other type of vintage PA28.

I think the most obvious and plausible explanation would be in the accuracy (or lack thereof) of the instruments. I mean... these instruments are probably 50 years old... RPM gauges are famous for being way off (usually on the low side, which would explain the point that you were probably pulling more power, thus flying faster). Airspeed indicators are also sometimes way off and nobody ever notices or checks. Finally, also the fuel flow meter (what type, BTW?) may be completely off if it hasn't been calibrated well.

So, I wouldn't worry too much about these things and just enjoy the flying. It's a matter of fact that these old aircraft camnot be flown with such degree of accuracy in terms of performance. That's why it is important to plan conservatively.

Oh, (and sorry in advance for asking...) given that you are talking about a 1960s PA28, you didn't by any chance make the classic mistake of misreading an ASI indicating MPH for one indicating knots...? ;-)

Frankfurt (EDFZ, EDFE), Germany

there is no such thing as a 1960s Piper Cadet

yeah you are right. It's a Piper Cadet for sure, but not sure of the exact year of manufacture.

There is no fuel flow gauge, what I did was to carefully measure the fuel level using a calibrated dip stick before and after the flight. And used only 1 tank for each of the legs, so i had a clear idea of the fuel use for the Power leg and the Economy leg.

On the IAS, your comment made me think a little bit, and what i can say is that the wind forecast was about 260/10 to 15 knots and on the leg towards NKR I would have expected a GS of 97 to 102, but i was down around 90 by GPS However the leg NKR to WUR my GS was up at about 130, so that was higher that an expected 122 to 127. So I think the wind was more than forecast. But I would need to do more work on the Wind Vectors to see... but this is only for accuracy, but as you say instruments are often well out.

My aim was to understand the aircraft better to try to attempt some of the longer trips planned. I am not sure I can afford La Rochelle, but i am thinking of trying to get to Calais; and I wanted to have much better understanding of the fuel use for long trips like that. Most of my legs have been 2 hours tops and so i have never had to be that sure of the fuel, as i have always had lots of margin and just assumed 38 litres/hour.

But what this has showed me is that I can use 30 litres per hour and have a small margin built in before I even add contingency.

And no one seems to be shouting that I am doing something that could whack the engine, so I guess that is a good sign :D

And the last point... definitely knots ;) I even thought of that one... lol!

Check your air inlet hoses. We had one crushed on one our PA28s so it was getting starved somewhat, when you applied carb heat the better airflow actually allowed an increase in RPM (the better airflow outweighing the normal loss due to the increased temp)

FI - FE - FICI
Oxfordshire / Glocs

Yes; definitely check the instrument accuracy.

The way to check the speedo is the GPS three-headings method. If you google on tas from gps ground speed rodgers
you will find the definitive treatise from Prof Rodgers (google mangles the URL to the PDF so it can't be copied, AFAICT) but, on a smooth day with winds aloft of less than about 10kt, you just fly three headings 120deg apart, note the GS for each one, and take the average of the three. That gives you the TAS which you correct back to IAS using the usual methods (there are also several online calculators).

I don't know about your POH but on some I have seen which show two columns, marked "best economy" and "best power" (or similar) these are believed to refer to "peak-EGT" and "120F ROP" respectively. There is about 10% loss of MPG on the 2nd one.

On a carb engine, no EGT instrument, the way to get roughly peak EGT is to lean until the power (as evident in the IAS) suddenly drops, then enrich a little so you get back to just before it dropped.

Administrator
Shoreham EGKA, United Kingdom

Check your air inlet hoses. We had one crushed on one our PA28s so it was getting starved somewhat, when you applied carb heat the better airflow actually allowed an increase in RPM (the better airflow outweighing the normal loss due to the increased temp)

I see your point, but if a richened the mixture a little, even like say 1cm, it increased by rpm and then behaved normally. (Remember I was operating in the "Economy" procedure of full throttle then lean for desired rpm)... It was only when leaned to the 75% or below economy power settings that this reverse carb heat behaviour was observed.

On the power procedure this behaviour was never seen.

@Peter, thanks for the GPS speed method I will try that... but i want to understand, more, your lean comment. If you lean for peak RPM, then if I understand from some other posts and the wonderful online graphs, I think I am in the range of 0 to 50F ROP. But what is confusing me, is on the PA28 handbook when you follow Economy, of full throttle and lean for desired RPM, where the hell are you in the lean curve, surely you are LOP by some margin.

What I noticed however on the club aircraft, when using power procedure, is that the rpm gradually, very gradually increased, then suddenly dropped, during a slow lean. I then nudged the level back just enough to restore peak rpm.... This is just ROP isn't it?

but i want to understand, more, your lean comment.

If you look at the HP curve in the first diagram here you get the idea.

Unfortunately I didn't draw the HP curve too well and actually the HP drops off quite rapidly once you go lean of peak.

That is why if you simply start full rich and lean, you will first see a point where the RPM hits a peak (which is the best power point) and then as you keep leaning you will see the RPM drop a little, and then when you go LOP it drops off quite a lot.

So if at that point you enrich just a little, you hopefully get back to the "peak EGT" point. Without instrumentation it won't be accurate but the SFC curve (the MPG, in effect) is pretty flat around there so it doesn't matter. My plane, for example, does almost the same IAS for various flows around peak EGT: 11.3 to 11.8 USG/hr all give about 138 IAS.

I doubt you will achieve "deep LOP" on a carb engine. It's possible but it's very likely it will just run really rough.

the rpm gradually, very gradually increased, then suddenly dropped, during a slow lean. I then nudged the level back just enough to restore peak rpm.... This is just ROP isn't it?

If you take out the "peak" word, it's somewhere between (at a guess) 50F ROP and peak EGT. It's good enough for cruising

If you lean for max RPM, you will be about 120F ROP, on any petrol engine. You will achieve the max speed but the MPG will be roughly 10% worse than at peak EGT.

I have flown 100% at peak EGT since I got my TB20 in 2002. Well, except during climb (obviously) and during high altitude flight (above FL170 or so) when max power is simply necessary to stay up there.

Peak EGT is great for the engine for cruise, costs you the least money, and if you pick about 65% of max rated power then you get the best engine life (supposedly).

Administrator
Shoreham EGKA, United Kingdom

I have flown 100% at peak EGT since I got my TB20 in 2002. Well, except during climb (obviously) and during high altitude flight (above FL170 or so) when max power is simply necessary to stay up there.

I do the very same, however at high ambient temperatures and 65%, TIT (turbine inlet temperature) becomes a challenge. The redline is 1650°F.

Having a TIT sensor (in front of the turbocharger but every engine could have it, just at the point in the exhaust stack where all cylinders flow together) is very practical for leaning. One just has to lean according to one single temperature.

Sometimes I fly LOP with my carbureted engine but only around 20-30°F and it does get a bit rough. A lot of pilots don't feel comfortable but it's kind of pointless as one does not save any fuel as opposed to operating at Peak EGT, just the cylinders stay cooler which -- depending on the installation -- can be a significant advantage.

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