The Cessna 120-140 Association Forum

I have this manual sourced by a previous owner that says 73 mph is the best climb speed. I think maybe Aircraft Spruce and others sell them.
Actually it’s gone missing so I’m working off of memory here, I don’t remember if it claimed 73 to be best rate, or best angle, But I think it simply said best climb speed without clarifying best rate or best angle.
Anyway I’ve always felt that it’s certainly not best rate, and I have always climbed at a higher speed, 80’ish, but she’s still climbing well at 90.
I’ve always done this based on a few factors, first engine cooling, my little airplane isn’t a rocket ship so climbs can be long, and faster speeds give much better engine cooling, plus of course if your trying to go to a destination, higher speeds get you there faster with less fuel burned, and this is s personal belief but if the engine quits I feel better with a little speed.

Anyway David sent me a copy of his original tentative flight manual, one that apparently didn’t make it to full acceptance, I assume as it had more info than required and anyone who has ever worked with the FAA learns very quickly to never give them excess information.
I’m attaching a copy, but it seems to indicate that speeds well above 73 are best rate, but interestingly clips at 80 mph with 80 showing higher standard rate, so it would seem that we should be climbing at at least 80 MPH, if we aren’t tying to climb over an obstacle anyway.
Maybe this is something that is known, if so sorry to waste your time, but maybe some others like me don’t have any actual performance data and are going off of that manual that AS and I’m sure others sell.

If you look at that chart,it indicates a rather large increase in climb rate at speeds well above 73 mph

a64pilot wrote: ↑Mon Jan 04, 2021 9:56 am If you look at that chart,it indicates a rather large increase in climb rate at speeds well above 73 mph

Hi,

I am pretty sure this is true.

The check list I use is from Checkmate, and states: Vr = 55, Vx = 76, Vy =89
Note I don't have the list here, I am on holidays now, but I think these speeds are correct.

But... an old print of the same brand, ten years ago stated Vr = 40.

In flight I get with full load and standard conditions (15 Celsius, 1013 mBar, sea level) about 650/700 FTMin Up. That value is the same your chart shows for 60 degrees F. Chart shows 640.
But... Chart suggests using 81 mph and in my opinion Vy is more likely in the 85-90 range. And engine cooling will answer "thank you".

I have a C85, standard pitch McCauley 1A90 propeller. MarvelSchebler carburettor and UL91 AvGas. It runs 2280 rpm static.

Kind regards,

I think you may be misinterpreting the chart. What it's saying is that, at sea level, climb at 81 indicated and expect 740 fpm on a zero degree day. On the same day, at 6000 feet, climb at 74 indicated and expect 455 fpm. The difference in both speed and climb rate is due to the altitude.

I read the chart the same way that John does,

In my example I used 1st row and 4th column, so at 81 mph, sea level, and 60 Fahrenheit expect 640 fpm.

In John's example 1st row and 1st column, so at 81 mph, sea level, and 0 Fahrenheit expect 740 fpm,

I have noticed in my 140, in the conditions I wrote before, that if I increase the speed from 81 up, flying hand off on a calm wind day, I get best variometer readings. That makes me think that 81 mph is NOT the best rate of climb speed. For that reason I am convinced (with my 140) that Vy is higher than 81. That is coincident with Checkmate list, that gives Vy = 89.

For that reason I use 85 to 90 mph after clearing obstacles at 75 mph aprox.

Observation about the wording of the chart:

The chart (1947) says NORMAL RATE OF CLIMB. What means "NORMAL" ?
There are of course infinite possible climbs, but the two interesting and most used are "BEST ANGLE" and "BEST RATE".
For that reason, I think that we must interpret "BEST" where the chart says "NORMAL"
Am I right ?

Kind regards,

By reading the disclaimer at the top of that chart I would assume that what they are showing in those charted numbers is what you can expect at those conditions. In other words if you aren't seeing at least a 560 ft ROC at 2000 Pa at 60F at gross then you might want to investigate why. Same with the landing and take-off numbers. No where does it say that those numbers are either best rate or best angle. Remember, that chart and these tests were all done over 70 years ago

8474 wrote: ↑Tue Jan 05, 2021 11:37 am By reading the disclaimer at the top of that chart I would assume that what they are showing in those charted numbers is what you can expect at those conditions. In other words if you aren't seeing at least a 560 ft ROC at 2000 Pa at 60F at gross then you might want to investigate why. Same with the landing and take-off numbers. No where does it say that those numbers are either best rate or best angle. Remember, that chart and these tests were all done over 70 years ago

Agree !

That page in the tentative AFM is largely identical to what ended up as Page 11 of the Cessna 120/140 Operations Manual. I might theorize that the number of potential variables and the method in which Cessna performed the tests might have led to its removal from the official AFM and ended up in the Operations Manual. Just a guess...

6643 wrote: ↑Tue Jan 05, 2021 8:36 am I think you may be misinterpreting the chart. What it's saying is that, at sea level, climb at 81 indicated and expect 740 fpm on a zero degree day. On the same day, at 6000 feet, climb at 74 indicated and expect 455 fpm. The difference in both speed and climb rate is due to the altitude.

I don’t understand what your saying, the chart indicates to me that al ALL altitudes tested, the rate of climb is higher at 81 mph than it is at 73 mph.
unfortunately they didn’t continue the chart until ROC began to decrease, so we can a only assume that logically they clipped the chart at max ROC, but maybe didn’t.

Yes Jody, you are not understanding what both John and I are saying.

Think of those numbers as a minimum acceptable number, like a power check on a turbine engine. You do a power check, check those numbers against the charted numbers and if you are running cooler and with less N1 than the chart then your engine is acceptable.

Thats my take on those charted numbers.

a64pilot wrote: ↑Tue Jan 05, 2021 2:48 pm

6643 wrote: ↑Tue Jan 05, 2021 8:36 am I think you may be misinterpreting the chart. What it's saying is that, at sea level, climb at 81 indicated and expect 740 fpm on a zero degree day. On the same day, at 6000 feet, climb at 74 indicated and expect 455 fpm. The difference in both speed and climb rate is due to the altitude.

I don’t understand what your saying, the chart indicates to me that al ALL altitudes tested, the rate of climb is higher at 81 mph than it is at 73 mph.
unfortunately they didn’t continue the chart until ROC began to decrease, so we can a only assume that logically they clipped the chart at max ROC, but maybe didn’t.

The way I read it which is pretty typical even in some of the newer Cessna POHs is you are getting a Vy IAS for each altitude and OAT combination. So the best performance you can expect is 740 fpm at sea level and 0F. If it gets warmer or pressure altitude gets higher, then your climb rates decrease. Vy indicated does go down as you go higher. That's why you get best climb at 6000 ft at 74 IAS however, you have thinner air so climb will be less FPM because you are producing less HP at 6000 ft.