Early Cessna 150 Vx Speeds
 

Good Morning,
please excuse my ignorance, but I seem to be seeing discrepancies within the Cessna 150 climb speeds. The 1968 POH says that 57MPH is the best angle of climb speed, whereas the 1969 POH says that 64MPH is the best angle of climb speed, is there really any difference or is it just the newer POHs were more accurate as I don't believe there are any significant differences between the 1968 and 1969 Cessna 150s that would cause this.

Are the later figures used as a safety margin against the stall speed?
Or am I just being extremely pedantic? :roll:

Interesting observation. Indeed, referring to the POH for the 1966 150, Vx at sea level is presented as 52MPH, as you note, 1968 150 is 57 MPH, and the 1974 150 is 70 MPH. The only difference which may be found between these airplanes which could affect performance would be the pitch of the prop, which is not stated. That said, the pitch of the prop shouldn't have much affect on climb speed. The engines, gross weights and wings are the same for all of these years, and the differences in landing gear and cabin width shouldn't affect climb speed. And, the certification basis for each of the planes is the same, so same climb criteria.

If I had to guess, I'd say that Cessna began to be liability aware. Super slow climbs will give a steeper gradient, though have a much lesser margin of safety. Remember that if you're climbing at an airpseed slower than glides speed, an engine failure will likely result in a crash, as entering a glide and flare to land may not be possible. I have a STOL kit on my 1975 150, and I would never climb at a speed slower than the fast '60's, unless it were life or death!

I note that in Caravan flight testing I did years back, I found that Cessna had prescribed a deliberately fast after takeoff and VX. I got a better climb angle at a slower yet speed, but it was terrifying in the case of an engine failure (which was what I was to test).

As for the 150, if you need to articulate this to an examiner for a flight test, state the speeds in the POH, and then suggest that as the later models are presented with faster speeds, that's what you'd like to do, just to be safe. A climb at 52 MPH in a 150 is not worth the risk!

Yes i agree, I fly in the U.K from a relatively short strip and I use 60 knots/70 Mph as my usual climb figure, or 65 at the absolute minimum.

Back in the day when I was full time instructor I did a bit of experimenting with a Cessna 150 with a curious student. One day on a flat calm morning we climbed from 1500 ft to 2500 ft ASL at Vx 56kts and Vy at 68. By measuring the time to climb we could calculate how far the aircraft had traveled at each speed and see what the difference was. I can't remember the absolute numbers but there was a significantly steeper climb gradient at Vx. The interesting thing was we tried a climb at 60 kts and found virtually the same gradient as 56 kts. Since 60 is a nice round number marked on the gauge I told my students that unless you were in a bad spot and needed absolutely every bit of performance, just use 60 for Vx and 70 for Vy.

Easy to remember and easy to fly

Part 23 (I only checked current regs though) only requires Cessna to determine the climb performance and confirm that it meets a climb gradient of 8.3 percent. If the C150 meets that gradient at a slightly higher (and thus safer) speed than the absolute Vx that they determined, they can certainly publish a different speed and having done so, you are then obliged to use that (at least when quoting it to an examiner). So use the figure from the POH that is valid for that particular C150.

If you ever have to get yourself out of a sticky situation (read: emergency) and using the lower (not from the POH) figure will prevent the accident, you'll be congratulated. If you ever smash up an aeroplane and they find out that it was (partly) because you flew a lower (not from the POH) Vx figure, you'll be blamed.

Yes, my problem is that the cessna 150g Vx is 52 Mph, which seems very low to me.

I've looked at a three different years available to me; 1967, 1969 and the 1977 Commuter. The only C150 POH that I can see, that gives 52 mph, is the 1967 Commuter. However, this is a figure for a maximum performance T/O at 50ft, thereafter the POH gives 72 mph for a maximum performance climb. These are all generic manuals and I did not have access to the NAA amendments.

All three models have the same engine and a 69 inch prop, I presume all are McCaulay. Each POH give the same wing loading, power loading and stall speed. There is a tiny difference given for the 1977 Commuter wing loading of 10.0 pounds/sq ft, all others being 10.2. The main anomalies seem to be recommended speed at 50 ft for a max. perf. T/O, although the other max. perf. climb speeds vary by the odd 1 mph.

For low powered single engine aeroplanes the figures seem to be the observations and prejudices of the test pilot. To be fair there is probably no other way to do it. No one appears to have looked back or forward for consistency. Aviation is fond of giving the illusion of precision but it usually isn't. There's an awful lot of shoulder shrugging.

Before EASA we used have the air test on every third annual (Star annual). The anomalies we found in manuals were common together with errors, which in my experience manufacturers were never keen to correct. Sometimes an NAA becomes aware of the mistakes and anomalies and requires amendments to be entered in the POH. I don't know if these discoveries were ever fed back to the manufacturers.