Leaning and Engine Performance (split topic)

[mvivion]

George,

Well, you're finally onto me. I just bought stock in JP Instruments.

I really don't care if anyone uses these techniques, but.....

I was recently in a situation that required me to orbit above some folks on a mountain top, awaiting a helicopter's arrival. I was pushing my 1 hour reserve for fuel, and continued to experiment with fuel flows, using the (in this case) VM 1000 engine instrumentation in the Husky I was flying.

By playing with all the parameters, including carb heat, I was able to get the fuel flow down to 4.6 gph, in orbit at 80 mph or so. These were cool temperatures.

I started the orbit with enough fuel to get home with about 1 hour fuel remaining, at normal leaned power. I arrived home, after 30 minutes orbiting, with a good solid hour's fuel at normal cruise power settings.

That's the kind of situation where these things can make a difference. '

I try never, ever to push fuel, but there are always instances where wind aloft is higher than forecast, or a different direction, or I have to divert around weather or terrain, etc.

In those cases, full instrumentation of the airplane can be a REAL comfort factor, as well as allowing you to run your airplane more efficiently.

Frankly, any aircraft I own in the future will have a full featured engine monitor on it, like the EDM 700 that's currently installed in my 170.

Oh, yeah, the EDM 700 is stc'd in the 170, but not in the 180 hp 170, like mine. I got a field approval, but JPI is working now on an stc for the 180 in the 170.

Mike

[GAHorn]

For pure endurance it's interesting to note the factory performance figures claims that at 10K' and 2100 rpm the fuel burn for the standard airplane will be 4.8 gph and endurance of 7.7 hours for a no-wind range of 684 sm. TAS is 89.

[N170BP]

I escorted a Champ across my state awhile back (it was being
ferried) and was amazed at how low my fuel-burn was (it
seemed to take forever for the needles to quiver away from
"F" (full). As I recall, I ran about 1950/1975 RPM to stay in
loose formation with the Champ. If you have the time, and
are in no particular hurry, the 170 has some long legs indeed
(way longer than my bladder and/or patience allows....).

[N1478D]

Wonder how the dollars per mile work out at something like 2000 RPM. Not burning as many gallons per hour, but it takes longer to get there so the engine has to run longer?

[lowNslow]

Wonder how the dollars per mile work out at something like 2000 RPM. Not burning as many gallons per hour, but it takes longer to get there so the engine has to run longer?

The B model performance charts in the owners manual actually list the MILES per gallon. I don't have mine handy or I would look it up for ya. Remember that the tach time (engine time) varies with RPM, so at the slower revs your putting less wear on the engine.

[GAHorn]

Joe, you're going to have to give it up and start referencing the performance charts for this airplane.
2000 rpm is not a recommended power setting for cruise.
The recommended rpm range for cruise is 2100-2700 and the mpg varies from 18.7-10.9 mpg respectively (depending upon altitude of course.)

Whether or not it is more economical to cruise with less rpm to save fuel VS more rpm to gain speed will depend on other factors. What is the wind aloft?

Generally speaking however, it's better to use lower/longer-range power settings with tailwinds, and higher/faster/shorter-range power settings when confronted with headwinds.

(In some aircraft, one must land for fuel on certain long-range flights if long-range power settings are used, while a high-speed power setting (burning more fuel per hour) will result in a non-stop flight! (Not the 170,....but this is an illustration that can apply to certain jet aircraft over certain routes, and is an illustration of the headwinds VS tailwinds situation.)
An extreme example to illustrate this is the 100 kt airplane flying into a 100 kt headwind: What is the groundspeed? (Answer: Zero) What is the miles per gallon? (Answer: Zero)
In that particular example, ANY increase in power (which also increases fuel burn of course) that results in an increase of groundspeed is an IMPROVEMENT in fuel mileage.

The obverse of that example is the aircraft flying with a tailwind: The lower the hourly fuel consumption, the greater the miles per gallon. The tradeoff in the consideration is the flight time VS total hourly operating expenses. (Few of us actually operate an engine from Zero SMOH to TBO, but that is the consideration being asked.) $10-$12/hr is a common hourly cost attributed to engine overhaul costs.

Aircraft performance charts are not fiction. They are useful tools.

[N170BP]

My owners manual (an original) says the following:

"Cruising: Any cruising r.p.m. between 2200-2450 (green arc on
the tachometer) may be selected. The recommended cruising
r.p.m. is 2450...."

[GAHorn]

Thanks, Bela, that is correct, however since this thread has centered around the performance charts.... my previous comment was in reference to the Cruise Performance Power Charts which chart rpms from 2100-2700.

[jrenwick]

My owners manual (an original) says the following:

"Cruising: Any cruising r.p.m. between 2200-2450 (green arc on
the tachometer) may be selected. The recommended cruising
r.p.m. is 2450...."

George,

Do you understand Cessna's or Continental's reasons for specifying these limits, and why they recommend cruising at the upper limit? The reason I ask is that my flying friends and I often make trips as a flight of aircraft, and I throttle back to 1950 or so to keep pace with them. Is there any harm in that, that you can document?

Apropos of this, I had an experience following the Alaska Highway in my J3, where I really had to go for maximum range -- without knowing exactly how to achieve it, because there were no performance charts available for a J3C-65 with a C90 installed. I knew the leg would take four hours at 60 knots ground speed, and I knew I had four hours of fuel at my normal cruise setting. So I just pulled the throttle back until I saw 60 on the GPS, knowing that I was burning less per hour than my estimate. In the end my range at that speed turned out to be nearly five hours, so that's now an important data point that I didn't have while planning the trip. I normally get cross with myself whenever I land with less than an hour in the tanks.

Best Regards,

John

[N170BP]

Yeah, why would they say cruise between 2200-2450
and then have the performance chart go all the way
down to 2100rpm? (just curious....).

[pdb]

Yeah, why would they say cruise between 2200-2450
and then have the performance chart go all the way
down to 2100rpm? (just curious....).

I think that some of the answer is found in Stick and Rudder under the section The Working Speeds of an Airplane. I highly recommend it for any interested in knowing how to fly.

Best Range (still air) is dictated by a combination the engine characteristics and the drag profile of the aircraft. Normally, best range in still air is going to be acheived at a speed that is just a few miles an hour faster than best glide.

In a headwind we are going to fly a bit faster, in a tail wind a bit slower. This should come as no surprise to the glider pilots among us.

Its tough to find out what best glide speed is in our 170 because the data is not in the manual. (I don't have my manual in front of me, but I think that the term of art in the manual is "normal glide" whatever that means.)

I have done pretty extensive glide tests in my 170 and found that the polar curve (speed vs sink rate) for the 170 is pretty flat between 76 and 88 mph. This means that, within the limits of my crude testing, I can't see a significant difference in glide ratio between those speeds. So, if the engine quits and the prop stops, I will be flying around 85 looking for a field.

However, if I am flying up the Trench and I am getting concerned about gas (I always am in the Trench, just over 400 sm) I am going to flying at 88 (no wind), leaned, and all my gear pushed to the back to get my CG as far to the rear as possible.

Again, I don't have my manual in front of me but I suspect this is generally consistent with the trend of range/power settings charts. But it may also be off the end because no one wants to fly that slowly unless they really have to. But, if you have to, its best to know how to do it.

[GAHorn]

My owners manual (an original) says the following:

"Cruising: Any cruising r.p.m. between 2200-2450 (green arc on
the tachometer) may be selected. The recommended cruising
r.p.m. is 2450...."

George,

Do you understand Cessna's or Continental's reasons for specifying these limits, and why they recommend cruising at the upper limit? The reason I ask is that my flying friends and I often make trips as a flight of aircraft, and I throttle back to 1950 or so to keep pace with them. Is there any harm in that, that you can document?

Apropos of this, I had an experience following the Alaska Highway in my J3, where I really had to go for maximum range -- without knowing exactly how to achieve it, because there were no performance charts available for a J3C-65 with a C90 installed. I knew the leg would take four hours at 60 knots ground speed, and I knew I had four hours of fuel at my normal cruise setting. So I just pulled the throttle back until I saw 60 on the GPS, knowing that I was burning less per hour than my estimate. In the end my range at that speed turned out to be nearly five hours, so that's now an important data point that I didn't have while planning the trip. I normally get cross with myself whenever I land with less than an hour in the tanks.

Best Regards,

John

Well, John, firstly they are not "limits"....they are merely recommendations, and if one examines the charts those rpms cover the range from approx. 55% to 75% power settings at sea level....the settings where most aircraft find their most useful/economical settings. (This is because most aircraft have already compromised in the selection of powerplants with regard to engine weight/horsepower/fuel consumption to achieve power settings that will run the powerplant in that power range for reasons of economy/longevity.) As for operating at the upper recommendation, it's most likely due to Cessna's desire to offer the most speed for the expenditure of fuel VS hourly ops costs. There is no reason to expect engine damage from operating within any approved rpm range as long as sufficient cooling is maintained. (The only rpm range the engine mfr cautions against is idling below 600 due to lack of upper cylinder/piston lubrication.) (The crankshaft slings oil to the upper cylinders/lower piston surfaces except at very low rpm.)

[blueldr]

A question for pdb. ----- If you were flying the trench which at 400 sm is well within the normal range of a C-170 (A or B) in still air, you would have no reason to slow to 88 mph for fuel conservation. If you were bucking a headwind that was eating into your fuel mileage, 88 mph would surely be the wrong thing to do.
I don't see what advantage 88 mph would be other than to save fuel in a tailwind. Enlighten me!

[R COLLINS]

BL, I live in Texas and probably will never fly the Trench but I would think that you would want to conserve every ounce of fuel possible. What if you got 250 to 300 miles up the trench and ran into IMC weather and had to turn back You had better know exactly how far your fuel can take you or your in the bush. Correct me if I'm wrong.
Randal

[GAHorn]

I believe that Stick and Rudder reference is in regard to best L/D ...which is not necessarily the best procedure in actual operations.

Blueldr is correct. Except for "loitering" purposes (or "loitering" in tailwinds) it makes very little sense to attempt the extreme ends of fuel conservation efforts.
Example: In order to "stretch" fuel consumption by flying at extremely low power settings, I.E., 88 mph (89 actually according to the book @ 10K feet) one would fly using 2100 rpm.
Notice that this power setting gives 18.5 miles per gallon. But notice also that power setting is actually not the most efficient and may actually expose you to danger for a longer period of time. Notice that 2200 rpm actually gives better speed (97 mph) and that altough it increases fuel consumption from 4.8 to 5.2 gph, that it also reduces your exposure to the headwinds and the local terrain by over 35 minutes time savings, and actually increases your range from 684 to 692 sm. (And if one were willing to give up 10 miles of range, one could reduce the time exposure to the elements and terrain by well over an hour!...by increasing power to 2300 rpm!) In other words for a sacrifice of 10 miles range, one either loses or gains an hour endurance/exposure, depending upon how one looks at it. (You wanna spend another hour in the soup/ice/turbulence for 10 miles range? Or no?)

But if not in high terrain, and can fly at altitudes below 7500 then the lower power settings give better consumption/endurance/range without nearly the penalty in speed/time.

How would one figure these relationships out without Cruse Performance Charts? They're good to know. But they're only accurate if one follows the instructions in the small print: Lean for maximum RPM.