I read some responses on the forum last week about Engine RPMs that got my wheels to spinning. I, like most people out there, am always interested in running my engine using techniques aimed toward longevity, i.e. adequate warmups, smooth power changes, small reductions for let down, etc. I understand that to "baby" most engines requires running at the highest recommended RPM for that stage of flight as opposed to a lower RPM. My 170 operation manual recommends 2450 for cruise yet the Continental manual just gives 2700 for max continuous, no intermediate recommended RPM limitations. There were several responses on the forum alluding to running your engines at 2700 for cruise. George, you wrote a long paragraph about this yet I vaguely remember you talking about climbing and I thought cruising at 2450. I like the idea of being able to cruise at 2700 without problems but the acronym WFO (wide ---- open) pops into my mind, that usually isn't good. Perhaps more info, or at least a couple of pennies worth of opinions?
By the way my engine is an O-300A.
David
RE: Engine Cruise RPM: Again
Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
David, I don't know which hurts my feeling more; Your accusation that I write long paragraphs... 
...or the fact that you don't consider it important to commit to memory every single word I ever write. 
The recommended cruising RPM is 2450 but that doesn't mean thats your only option. The performance charts give plenty of other options (see the owner's manual) and TCM allows up to 2700 RPM continuously,...so you shouldn't find yourself limited as to choices.
I personally cruise as much as possible at altitudes that allow full throttle, 2450 RPM so long as it results in 65% power or less. (I don't like to run my engine at more than 65% for more than necessary in cruise.)
This usually finds me in a cruising altitude of 6500-7500 ASL, full throttle, and results in 55% power or less. (2450 only equals 65% or greater power at 2500' pressure altitudes and below, with the standard 7653 propeller.)
In other words, if I can justify climbing above 5K', then I can justify running full throttle, otherwise I reduce throttle not to exceed 2450.
If you run full throttle at low altitudes, you may not like your fuel burn costs vs your TAS. IF your prop will allow you to achieve 2700 RPM at low altitudes, ...your fuel burn may exceed 12 g.p.h.
Will any of this hurt your engine? Not likely, according to TCM.
...or the fact that you don't consider it important to commit to memory every single word I ever write. The recommended cruising RPM is 2450 but that doesn't mean thats your only option. The performance charts give plenty of other options (see the owner's manual) and TCM allows up to 2700 RPM continuously,...so you shouldn't find yourself limited as to choices.
I personally cruise as much as possible at altitudes that allow full throttle, 2450 RPM so long as it results in 65% power or less. (I don't like to run my engine at more than 65% for more than necessary in cruise.)
This usually finds me in a cruising altitude of 6500-7500 ASL, full throttle, and results in 55% power or less. (2450 only equals 65% or greater power at 2500' pressure altitudes and below, with the standard 7653 propeller.)
In other words, if I can justify climbing above 5K', then I can justify running full throttle, otherwise I reduce throttle not to exceed 2450.
If you run full throttle at low altitudes, you may not like your fuel burn costs vs your TAS. IF your prop will allow you to achieve 2700 RPM at low altitudes, ...your fuel burn may exceed 12 g.p.h.
Will any of this hurt your engine? Not likely, according to TCM.
Wow, this is why I seem to constantly be in the dog house with my wife and girls! What I meant to say was you "wrote a complete and informative paragraph on this subject some while back, but my addled brain doesn't allow me to recall the entire content" (this works at home).
You brought up another issue, I am not sure if I could reach 2700 in straight and level at the altitudes I fly at anyhow, so it may be a mute point. I have generally always used 2450 for cruise, and I seem to average about 8 to 8.5 gph at 117 kias, give or take. I could see 2700 sucking down the petro.
Keep up the "complete and informative" posts!
David
You brought up another issue, I am not sure if I could reach 2700 in straight and level at the altitudes I fly at anyhow, so it may be a mute point. I have generally always used 2450 for cruise, and I seem to average about 8 to 8.5 gph at 117 kias, give or take. I could see 2700 sucking down the petro.
Keep up the "complete and informative" posts!
David
117 Kts IAS @ 2450 out of a O300-A, sucking 8 to 8.5 GPH???
That's almost 135 MPH! Holy Moly! All I get with mine is about 115 MPH ground speed (GPS) @ 2450 under 5000' and burn around 7.2 to 7.5 GPH (US).
My girl sports the 800X6's, no pants, droop tips, 76X53 prop. From what I'd seen so far, other B's with 600X6, wheel pants, regular tips, same prop get maybe 5 or 10 MPH more. Wazzup??
JD
That's almost 135 MPH! Holy Moly! All I get with mine is about 115 MPH ground speed (GPS) @ 2450 under 5000' and burn around 7.2 to 7.5 GPH (US).
My girl sports the 800X6's, no pants, droop tips, 76X53 prop. From what I'd seen so far, other B's with 600X6, wheel pants, regular tips, same prop get maybe 5 or 10 MPH more. Wazzup??
JD
Yeah, how you gettin' a B to go that fast? Bigger engine?
Mine (C-145-2, 850 tires, Sportsman STOL kit, Metco
wing tips and aileron gap seals) goes 112mph at 2475rpm
with a 7651 prop (tach was checked with a hand-held
strobe and found to be very accurate). When I had
a 53 pitch prop on there, it went 116mph. If I put
itty-bitty tires on there and add panties, it might go
120mph....?
Bela P. Havasreti
'54 C-170B N170BP
Mine (C-145-2, 850 tires, Sportsman STOL kit, Metco
wing tips and aileron gap seals) goes 112mph at 2475rpm
with a 7651 prop (tach was checked with a hand-held
strobe and found to be very accurate). When I had
a 53 pitch prop on there, it went 116mph. If I put
itty-bitty tires on there and add panties, it might go
120mph....?
Bela P. Havasreti
'54 C-170B N170BP
KIAS vs TAS
I believe KIAS was used instead of True AS. I fly a Model A with a 7649 prop at altitudes over 7000' DA on a regular basis and see ~116 mph GPS @ 2450 RPM and ~ 8 gph. I can just about reach 2700RPM WFO in level flight.
Pete 1320D
Lewistown, MT
Pete 1320D
Lewistown, MT
Yeah, I meant 117 mph indicated. After I signed out the other night I realized what I had said - was hoping no one caught my goof. Maye it was just wishful thinking. You've got to remember most of the stuff I have flown in my limited experience indicated knots and nautical miles and was mostly done below 500'.My indicated airspeed is usually between 115 and 117 at 2450 RPM indicated, betwixt all of the "indicateds" (considering guage error) lies the truth.
David
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Dave Clark
- Posts: 894
- Joined: Mon Apr 22, 2002 6:25 pm
IAS vs True
I flew formation a few weeks ago with a (claimed to be) very fast 85 Ercoupe. I throttled back a wee bit (2400 rpm) and was indicating ~ 100 mph @(6000'DA) and he was indicating ~ 120 mph at a high power setting. Our true airspeed was more like 112 mph. When speaking about performance, especially between two machines, my feeling is that IAS without a lot more data is meaning less and quite often very misleading .
Pete 1320D
51 Model A
Pete 1320D
51 Model A
I've been flying jets every day for 20 years using "knots". When I did my CFI cert a while back I had a hard time with the correct terms while in the Arrow. I started my check ride brief by giving the DFE a dislcaimer stating that fothe purposes of our flight, knots=mph=knots and that I would reference our velocity as "airspeed units". Hard to go wrong there - just fly the right number on the guage...he chuckled...I passed..life is good.
Bruce
David...reference the A-10. It couldn't have been the Hog - their ASI uses niether KIAS or MPH...they use days!
Bruce
David...reference the A-10. It couldn't have been the Hog - their ASI uses niether KIAS or MPH...they use days!
Bruce Shipp
former owners of N49CP, '53 C170B
former owners of N49CP, '53 C170B
I flew the H-3 Sea King, also known affectionately as the Sea Pig (not HOG). I flew SAR, and utility missions including recovery of unmanned drones and torpedoes, hauling just about anything you can imagine internally or externally. Lotsa fun, no glamour!
If an A-10's ASI is in days then an H-3's is in weeks. We were limited to 120 kias but I have on occasion seen 140-145 kias (not mph this time).
Guys, thanks for asking, as anyone who knows me, knows I will talk about helicopters for hours at a time.
I was with a composite squadron that had A-4s as well as the helos, we (the jet jocks and helo bubbas) would torture ourselves by giving each other rides in the other's aircraft. Lots of embarrasment all around!
Then I started flying taildraggers, not quite as much fun but still challenging.
As far as indicated airspeeds go, I have to agree. In general, an analog guage of anything is rarely accurate to within 5%. If you take in to account the disturbed air at our pitot, dirt and leaks in the system, age, reading error, and any other factor (it is all cumulative), then you will see that our ASIs may not be that accurate. GPS Ground speed you can take to the bank but it still doesn't tell you the true performance of your aircraft unless you accurately know the wind component. As I said before, mine indicates 117 at 2450, but what I am really saying is mine is flying at 117 mph +- 6mph at 2450 rpm +- 125rpm. I do honestly believe that it is a little more accurate but I wouldn't bet the farm.
David
If an A-10's ASI is in days then an H-3's is in weeks. We were limited to 120 kias but I have on occasion seen 140-145 kias (not mph this time).
Guys, thanks for asking, as anyone who knows me, knows I will talk about helicopters for hours at a time.
I was with a composite squadron that had A-4s as well as the helos, we (the jet jocks and helo bubbas) would torture ourselves by giving each other rides in the other's aircraft. Lots of embarrasment all around!
Then I started flying taildraggers, not quite as much fun but still challenging.
As far as indicated airspeeds go, I have to agree. In general, an analog guage of anything is rarely accurate to within 5%. If you take in to account the disturbed air at our pitot, dirt and leaks in the system, age, reading error, and any other factor (it is all cumulative), then you will see that our ASIs may not be that accurate. GPS Ground speed you can take to the bank but it still doesn't tell you the true performance of your aircraft unless you accurately know the wind component. As I said before, mine indicates 117 at 2450, but what I am really saying is mine is flying at 117 mph +- 6mph at 2450 rpm +- 125rpm. I do honestly believe that it is a little more accurate but I wouldn't bet the farm.
David
A triangular course flown with the middle portions of each leg timed and averaged should give an accurate comparison to the AS readings. How 'bout some of you math whiz's coming up with a good leg-length for each leg so we can time the middle portion of each leg and come up with an easy way to compare our AS indicators with a GPS groundspeed readout? For instance: Suppose we fly at 2450 and get an IAS of 104 kts (120 mph) but the GPS gives readouts on each leg of 101, 103, 112, yadda-yadda-yadda. Any Einsteins out there? (No, Harris!! I'm not gonna believe your family changed their moniker from Harrisberg-stein-wich-man-levy- or anything else! You can't add anything but girlfriends!)