Cruise Speeds

[BlakeinAlabama]

I am curious what rpm settings people cruise at and what speeds (generally) you cruise at during those settings. Thanks!

[bagarre]

Same here

2550RPM EM7651 Prop @ 3,000FT (40 degrees OAT) shows me around 120ish with wheel pants and no strut fairings.

On a side note, next time you're flying along, look behind you and see how much down elevator you are carrying. Then level the elevator to the horizontal stab and see how much you pitch up. I always wondered how much of a speed gain there would be if we adjusted the horizontal stab's decalage and how much extra force would be needed to three point it in that configuration.

[170C]

Aryana, its refreshing to hear someone with a C-170 report that at 2550/2600 rpm's they are seeing 115-120 mph So many report faster speeds at 2450 or so. My ole Green Machine averages 115 at those same rpm's as you are seeing. I have flown with a gaggle of 170's (of all colors ) and most seem to go faster than I can maintain even at 2600 or sometimes 2650 rpm. I always think I have the slowest plane in the air. Maybe I need to paint my spinner red (NOT):mrgreen:

[ghostflyer]

It is a interesting situation where we all look at the engine and its power output and props etc when discussing speeds. A very quite and much less expensive component of gaining a good cruise speed it is making sure all flight controls are rigged as per Mr Cessna manuals. Plus clean out all the junk that we all carry around in the back of aircraft . A lot of the junk we all carry is of the Just Incase factor and we just might need it. I looked at the last 200hrs of flying and what did I need out of all the junk that I was carrying around . It was NOTHING . However I still carry 3bottles of oil around and some very basic tools .
Having correct rigging is a big saver. Every aircraft that isn't a repeat customer that comes to our business we do a rig check . It's mind blowing how many aircraft that are not correctly rigged. A myth that a lot of aircraft owners have is about the drooping ailerons . We are told it creates more lift , but really in cruise flight you do not need all the lift and you have only created drag.

[blueldr]

If you're really concerned anout a correct air speed for a given power, one of the first things you need to confirm on these old airplanes is the accuracy of the tachometer. Original tachometers are almost all off to some degree. A digital tach checker that reads the turning propeller blades will allow you to make a correction card if you so desire.

[blueldr]

I have a friend that used to have the fastest Grumman Tiger in the air and he advised everyone about it.
I bought an electronic digital tach checker. He was facinated with it and asked to borrow it to try it out.
He came back with a face longer than his arm and was almost in tears. His tach was so slow at his cruise setting he was making about 150 to 175 turns more than the reading. No wonder his Tiger was a racer. The funny part of it was that his fuel consumption was always a little high and he never did connact the two.

[GAHorn]

My airspeed has been calibrated...my tach has been verified, and my EM7655 prop (theoretically equivalent to DM 7653) at 2450 RPM delivers 105 kts TAS. (120 mph). This is at 5500 PA, +10C OAT and 2000 lbs wt. (This was before I stayed at a Holiday Inn.)

All unnecessary antennas are removed, brake lines streamlined with landing gear, bullet spinner and wheel pants.

The elevator/horiz. stab situation has been discussed in other threads and is neither an economical nor simple experiment to make, and it clearly has other issues such as spin-testing and CG envelope variation.

BTW: The Owner's Manual recommends 2450 RPM for cruise, and that happens to equate to about 65% power at 7500'...where wideopen throttle provides that particular power setting and the best cruise numbers/fuel burn occurs.

[lowNslow]

My airspeed has been calibrated...my tach has been verified, and my EM7655 prop (theoretically equivalent to DM 7653) at 2450 RPM delivers 105 kts TAS. (120 mph). This is at 5500 PA, +10C OAT and 2000 lbs wt.

Exact same numbers with my 170B as well. Average burn rate approx. 7.5gal/hr.

[BlakeinAlabama]

Being new to this engine, I was be curious what others get on the RPMs when you go full power at takeoff. also, what is the suggested rpm setting for cruise. I see some posting 2550 and 2650, but I think mine red lines there, but will have to look again.

[blueldr]

On my O-300 engine with a 53 inch prop pitch at 7500 msl, 2450 RPM and leaned to stumble and just richened to smooth, my fuel burn used to be 6.7 to 6.9 GPH with a TAS of about 110 to 115 MPH. This setting was apparently just on the lean side of peak and the CHT was just right.
Strangely enough, the spark plugs needed to be scrupulously clean for this operation. If they were lead clogged at all, the engine had to have a richer mixture.
In a summer up in Idaho, where I had to always run on leaded avgas rather than my usual mogas and could not (or would not) get around to cleaning up my spark plugs, I would have to end up running on the rich side to keep things feeling right.

[GAHorn]

Being new to this engine, I was be curious what others get on the RPMs when you go full power at takeoff. also, what is the suggested rpm setting for cruise. I see some posting 2550 and 2650, but I think mine red lines there, but will have to look again.

Blake, if you examine the TCDS (Type Certificate Data Sheet) you will observe the maximum static RPM for the standard engine/prop combination installed in B-models was 2250-2350 for the MDM prop, and 2230-2330 for the earlier DM prop.

In other words, if your prop is "standard" and your engine is healthy, on a standard day at sea level...that's what you should see.
Your tach must also be accurate, of course.

Those RPMs result in takeoff horsepower in the 120 hp range (according to the TCM Operator's manual graphs), because the engine only develops 145 hp at 2700 RPM, which can usually only be attained in-flight...(i.e., not "static")...or with a different prop.

In other words, our fixed-pitch props are a compromise. They are like driving a car with second-gear only.... not the best for drag-racing...not the best for top-end speeds...but a good compromise. If you want low-gear...then get a climb prop and expect lower cruise speeds/higher trip fuel consumption. If you want higher cruise speeds, get a cruise prop and watch your take-off/climb performance suffer. Most folks are happy with the 7653 props, which are 76" diameter tip-to-tip and theoretically travel 53" forward with each complete revolution.

A-799.pdf