I am curious what speed you guys use for x-country planning. When I bought my M5 a couple of years ago the fellow told me 105 kts (120 mph). I thought that was a little low based on the specs I had read on the 180 with a C/S prop. But over time I found that number to be pretty accurate on average. I see that some of you with 235's talking about 135 to 140 kts (155 - 160 mph). That is considerably faster than what I am seeing. (jealousy!)
Now I know it depends on altitude, temps, load, etc. but a 35 to 40 mph difference is considerable between a 180 and a 235. Is there really that much basic difference? I had understood there to be a 5 - 10 mph difference. Maybe I have a slow plane or I am a slow flier!
With my M5 I have calculated 162 mph cruise at 6000'. For crosscountry calculations I use 150 mph...it's a good round number. There is a price for that faster speed. I have not tried to see how slow I can go somewhere. I would like to try that sometime to see what speed would produce optimum range.
In my 1968 M4-210C, I get 135 mph (117 kts.) indicated, using 23 square at 3000'. I would be curious to hear from other M4-210C owners, I've never compared!!
The best I can get is around 134 kts. at 2300 RPM, firewalled, 50 deg ROP between 9 to 11 thousand feet. M-6/235., But that is with about 500 lbs. of Pax and 150 lbs of stuff and maybe 300 lbs of fuel. I'm sure it would be faster if it were lighter.
True airspeed. In order for comparisons to make sense, it has to be apples to apples. I doubt for example that I'm even indicating 120 Kts. How rich or lean you are effects power quite a lot too.
Karl Hartlen wrote:In my 1968 M4-210C, I get 135 mph (117 kts.) indicated, using 23 square at 3000'. I would be curious to hear from other M4-210C owners, I've never compared!!
Karl
Karl, thats pretty close to what I get with a 235 under those conditions. My longer wing probably cancels out a lot of the extra power.
On a standard 59deg F at sea level and standard lapse rate, the M5 180 c/s prop will meet the book figure of 145mph TAS and so will the MX7-180 c/s while the MX7-180A f/p prop will accomplish 140mph TAS.
These figures are with VFR antennas and stock 7.00x6 tyres. Drag factors can change this , so can rigging.
I've played with the range thing. Keeping as many variables as I can the same, with my airplane, the lower the power setting, as far as I checked, the better the range. I would bet that the max range airspeed is close to the max rate of climb airspeed.
If you plot the lift and drag vs airspeed on a graph it will be bucket shaped. A lot of things happen at the bottom of the "bucket". Max endurance, max rate of climb, min rate of descent etc.
What is the max glide speed for a Maule? I'll bet that is real close to your max range airspeed, especially if you slow the prop way down and lean it as much as possible. I'm just not willing to travel that slow.
Best mpg per time enroute is 55%+ for headwind, 55%- tailwind
Best L/D is 45%
Power needed to increase from 55% to 75% is cube of airspeed gain
Volumetric efficiency is better at lower rpm
Decrease 55% to 45% increases mpg 10% but time eroute 20%
Remember 300rpm decrease raises MP 1"
Add or subtract .18"MP per 10degF variation
High MP low RPM = higher CHT
High RPM low MP allows more agressive leaning
I think the speed of the planes in general maybe the plane its self and the way it is set up. I've ownd two defferent M-7's. My first was a 2001 M-7 235C and at about 22 to 23 inches MP and 2425 RPM I would see about 125Kts to 127Kts indicated at about 2,500 ft MSL. (it had a 81" McCauley). I now own a 2004 M-7 235C and it has a 78" three blade hartzell Scimitar which is the only real difference in the two planes and at the same power setting I will see 132Kts to 135Kts. The 04 seems to be set up better and about 7Kts faster. I was always told that a three blade prop would be slower than a two blade. So I'm not sure if the plane is set up defferent or if the engine just runs stronger. The GPS on a no wind day at 2,500Ft will read the same as indicated airspeed.
If everything else is equal, the more the blades, the higher the drag and therefore the slower the plane. However, I think the scimitar blade design may be a slightly more efficient design and may help offset the additional drag of the third blade. That's just an opinion though. Just like I think the length of the leading edge of a scimitar is longer than 78" due to it's shape. There are some differences between scimitar props though.
I fly new airplanes for a living and can tell you that some are just faster than others, when they are supposedly identical. It's the same for helicopters also.
I have a 1975 M5 210C. I generally fly between 2000'-5000' range. Usually run 50 degrees rop and 24 square. For a general year round average I plan on 115 KNOTS.
