The International Cessna^® 170 Association

I know that there is probably a topic already listed on this subject, but here goes. I have a 1955 Cessna 170B with the 0-300 continental with the Marvel -Schebler MA-3SPA carburetor. It has a fresh annual. I have only had it for about a year and have only about 30 hours flying experience with this aircraft. Here is the problem. I can't get 2700 rpm on take-off or in straight and level flight. I had a full load yesterday and on take off with a 20 knot wind and it did not feel real comfortable. I can only get 2450 rpm at sea level. I have the McCauley Prop. DM 76-53. My A&P says it is the maximum rpm that prop will allow. If that were the case then a cruise prop 76-55 might only turn up 2200 rpm at full thottle. Any ideas would be appreciated. koop Member 7435

Interesting, I have a C170A with the C145-2, at 5500 MSL it will turn a max of 2650 rpm with the 53 inch prop.

The C145/O300 engine and the 7653 prop is specified on the Type Certificate Data Sheet to turn "not more than 2330 nor less than 2230 rpm". This is the correct rpm range for static conditions. (Brakes held, max throttle, no wind, sea level std.)
If you indicate anything different, something is wrong. (Most likely candidate is an incorrect tachometer.)
All performance is predicated upon that static rpm of 2230-2330, including takeoff, climb, range, fuel burn etc. etc. (By the way, that rpm means you're getting about 120 hp on takeoff. The engine doesn't achieve 145hp except at 2700 rpm.)
As an aside, ... I have a 7655 prop (EM series) that shoud be equivalent to the MDM7653. I see 2230 static, and full throttle, level flight low altitude rpm of around 2650, running full rich, and indicating about 125-130mph. (I said "about" for national security reasons. The actual top speed of my B-model is classified. I could tell you, but then I'd have to kill Joe.)

gahorn wrote:The C145/O300 engine and the 7653 prop is specified on the Type Certificate Data Sheet to turn "not more than 2330 nor less than 2230 rpm". This is the correct rpm range for static conditions. (Brakes held, max throttle, no wind, sea level std.)
If you indicate anything different, something is wrong. (Most likely candidate is an incorrect tachometer.)
All performance is predicated upon that static rpm of 2230-2330, including takeoff, climb, range, fuel burn etc. etc. (By the way, that rpm means you're getting about 120 hp on takeoff. The engine doesn't achieve 145hp except at 2700 rpm.)
As an aside, ... I have a 7655 prop (EM series) that shoud be equivalent to the MDM7653. I see 2230 static, and full throttle, level flight low altitude rpm of around 2650, running full rich, and indicating about 125-130mph. (I said "about" for national security reasons. The actual top speed of my B-model is classified. I could tell you, but then I'd have to kill Joe.)

You have me dying laughing anyway!

Koop,my advice/opinion,for what it's worth:
1) check tach with a digital handheld (strobe) tach,or preferably,more than one of them--they're pretty inexpensive (cheap) & may be just as inaccurate as your panel tach.
2) have your prop checked by a propeller shop as to actual pitch. You might want to repitch it to 51,the so-called "climb prop" pitch.
You'll generally see around 115 mph cruise with a 76-51,but the increase in climb will be worth the loss in speed,if you operate off shorter than norm &/or grass strips.
You should be able to turn redline or just over in level flight at low altitude with a 76-51.

Eric

I have a new tach and cable. The other tach I took out was checked by the hand held and it was right on. I understand how to get a static reading but what I don't understand is the significants of static rpm vs redline rpm. Will I ever get 2700 rpm? I have been cruising at around 100 mph ground speed at 2250 rpm. My Cessna 150 got 2700 rpm. Like I said, the take off a couple of days ago did not feel comfortable. Two adults in front, two children in the back seat at about 120 lbs a piece and 40 lbs luggage and full fuel. Yes, I checked both mags. Carb heat was off.
I just feel I should have had more power on takeoff. I have been just flying it solo so there would be quite a difference. AM I SUPPOSED TO GET 2700 RPM IN STRAIGHT AND LEVEL FLIGHT?

Think as a fixed pitch prop as being a car with only one gear; in this case, second. A climb prop is more like being in first, and a cruise prop is third.
(I know, it's a crude analogy.)

A variable pitch (and except for some old Bonanzas and Bellancas, constant speed) propellor is like having all the gears available. It allows you to obtain maximum power on take-off in fine pitch. There's a reason a Cessna 180 leaps off the ground, compared to the 170 - it can develop the full 230 hp, at least at sea level! You'll never see 2700 RPM on the ground in a 170, or any fixed pitch airplane for that matter.

If you aren't making rated static RPM on the ground, 2230 as George sez, I would check the obvious. Is your air filter in good shape? Both mags timed correctly? A muffler with a blockage would cause an increase in back pressure and a power loss. So would an induction leak. Does the throttle have full travel forward? Has the carb been inspected in recent memory? Badly worn plugs might also be a suspect. Or it could be a combination of any of these things.

2450 wide open is low, that's for sure. Mine with a 7653 prop turns 2230
EXACTLY static, and winds up to 2700 in level flight with no problem. Have it checked out, sounds like your concerned enough to get to the bottom of it, and let us know what you find, so we all can learn...Russ Farris

Thanks Russ:
All the things you said to check have just been done, maybe something was over looked. Thanks for the explanation. koop

Koop, ...a new tach (that you mentioned you have) can be very inaccurate.
For one thing, tachs are calibrated to the median range of their anticipated use. A tach that is anticipated to spend it's life cruising around at 2300 will use that speed as a calibration point, but it will be wrong at any other speed. (Most likely reads high below 2300, and reads low above 2300.) So,...still have your new tach confirmed accurate at static with a digital tach-checker. If you're getting accurate static rpm, then quit worrying about it. (Does it matter if you can reach 2700 rpm in flight? Do you really intend on cruising around at 100% power? Not likely.)
What would be ideal is to have a wide-open throttle at 7500' equal 2450 rpm. That would be a perfect match for cruising.
But if you use short runways, or need climb capability for mountains/obstacles you'd prefer a climb prop. A climb prop would result in higher static rpm, higher cruising rpm (more hp) and higher fuel consumption per mile and per hour with slower cruise speeds.
If you mostly use longer runways, (say 3,000 ft or more at S.L.), or fly at lighter weights, then a cruise prop will give you slightly less hp for takeoff/climb, but better cruise speeds and better fuel consumption numbers. (This prop combination would also likely result in a lower service ceiling also, due to poorer climb capability.)
Russ's analogy is an excellent one. I wish we all had a lightweight, ground-adjustable aluminum prop that'd be a bolt-on for our airplanes and still fit inside the standard spinner.
Let me add something, Koop. You had to be at or very near gross weight on that takeoff, (approx. 1350 Empty wt? + 340 skinny adults + 240 kids, + 40 bags + 222 full fuel = 2192 lbs) and it matters very little with these 170's which prop you have with a summertime takeoff at gross. The 170 is very sensitive to that last 200 pounds or so. These things do not jump off the ground and climb like homesick angels when they're loaded to gross. They get airborne more due to curvature of the earth. Plan accordingly.

Thanks Gahorn:
I have a lot to learn. I guess that is what life is all about. Thanks again for you insight. koop

Koop
To give you some insight to some different performance figures, I will list the spec's that I get. I have an A model (like Joe's these are the fast ones ) with a DM 76x55 prop. I have not done a true static run-up since I purchased it but have noted that it will turn up over 2200 rpm before take-off. It will turn 2650 straight and level with a moderate load, and will indicate 125 mph at 2400 rpm (loran gives ground speed at 103 kts). I live in Texas and the elevation at home field is 420' with temps generally in the 90's. My R.O.C varies between 400 to 700 fpm depending on weight. As a side note I have not checked my tach either! The last empty weight of my plane listed in the logs at 1295 lbs. I was told by the previous owner that the prop was a factory 53 pitch and that he pitched it to 55 for better cruise. Another member told me that a DM 76/55 is over pitched for a 170. I hope this helps. Randal N1745D

I also think a 51 pitch prop is probably the best
compromise if you're looking for a little more
take-off performance. I have a 48 pitch that
I can red-line in a shallow climb, but the airplane
only goes 90mph with this prop on it....

My 51 pitch cruises at 112mph at 2475 rpm (I have
850 tires & double-puck brakes). Prior to the tires/
brakes upgrade, (had 600-6s and single-pucks) the
thing went 116mph at the same power setting. Static
rpm is 2350 and creeps up to 2400rpm 50' into the
take-off roll. Full power climb is 2500rpm at about
70-75 indicated. I can just go over red-line rpm in
level flight.

Unfortunately, you have to pick a prop that works best
how and where you operate the airplane. If you're a flat-lander
and don't load the thing up, the cruise props will work OK and
give good cruise speeds. Fly in the mountains and/or with a load,
and the cruise props will make you have to clean your upholstery
now and then as a result of the reduced take-off performance.

I also wish there was a 2 position (electric?) prop we could use
on our 170s....

Bela P. Havasreti
'54 C-170B N170BP