The Cessna 120-140 Association Forum

*WARNING* These values are for reference use only. Always use your Operation Manual instead.


I was studying my POH & I'm a little bit confuse with the ''Best Rate of Climb'' speed & I didn't found the Best Angle of Climb.

Questions:
1. There is the Vy that we should use at initial climb (73 mph) & the Vy at sea level (81 mph). Why there is a big difference like this.
2. What value are you using for Vx.


Here is the values that I'll be using on my C140:

Hi,

This is only an educated guess from doing some back of envelope sums but I think the difference might be Indicated airspeed Vs. True Airspeed as described on the table found on page 11 of the manual.

Si

Under STD ISA temp. at ASL the TAS = IAS. There is a diff. of ~ 3 to 4 mph due to our super pitot tube (CAS) that is influencing the TAS.

1. They are mentioning 73 mph for Best Rate of Climb on page 5 & 23 under takeoff section
2. There is 81 mph (Sea Level) for Normal Rate Climb at the bottom table of page 11
3. There is 81 mph (Sea Level) for Rate of Climb & Speed for Best Climb at the top table of page 12


It's the table at the top of page 12 that make me confuse:

The RATE OF CLIMB & SPEED for BEST CLIMB at various altitudes for gross weight of 1450 lbs & equipped with wooden Sensenich 74FK-49 propeller is given in the table below:
Attitude (ft.):...................Sea Level
Best Climb Speed (T.I.A.S): .. 81


1. It's a strange that the value are the same as the ''Normal Rate Climb'' table at the bottom of page 11 so maybe it's another normal climb table using word like ''Rate of Climb & Speed for Best Climb" just to confuse me. They are just using different altitude...
2. The 73 mph (page 5 & 23 under Take-off section) for Best Rate of Climb make more sense to me for the Vy

The thing is, back when the plane was certified, there were no Vx, Vy, etc. required to be established or published. The figures that were published are more like "guidelines" (apologies to Capt. Jack Sparrow). The original flight manual was a two page affair, ACA-309, and only included "limitations". The multi-page handbook we all know came later.

6643 wrote: ↑Mon Mar 21, 2022 11:51 am The original flight manual was a two page affair, ACA-309, and only included "limitations".

I'm learning something new everyday. Thanks for the info!
FYI, the ''true indicated airspeed'' aka TIAS (T.I.A.S.) is another acronym for IAS... I was confuse with TAS.

I have been updated my personal spreadsheet for reference.

That's my ACA-309!

Like you I worried about this initially. Tried to get good data but finding a really smooth day, reading the VSI (located on RHS in my plane) was problematic at best. GPS data wasn't any better. Timing off the Alt also didn't really work. At least not well enough to say at better than 5 mph. I might go back and do a better job, but its really an academic exercise. I have numbers based on experience that seem to work.

Add to that the poor to non-existant calibration of the pitot tube and pitot static. There is a Cessna document on how to bend the pitot to proper angle, but that only lasts till you bump into it by accident!!! When you bend it the reading changes. So I would have very little confidence that my Vx,Vy, etc, yours, and the original Cessna test pilots would agree on the ASI.

Bottom line is you have to learn to fly your plane using the numbers your plane gives as guidelines.

Best, Jack Sparrow

cdoughty wrote: ↑Tue Mar 22, 2022 8:54 am Like you I worried about this initially. Tried to get good data but finding a really smooth day, reading the VSI (located on RHS in my plane) was problematic at best. GPS data wasn't any better. Timing off the Alt also didn't really work. At least not well enough to say at better than 5 mph. I might go back and do a better job, but its really an academic exercise. I have numbers based on experience that seem to work.

Add to that the poor to non-existant calibration of the pitot tube and pitot static. There is a Cessna document on how to bend the pitot to proper angle, but that only lasts till you bump into it by accident!!! When you bend it the reading changes. So I would have very little confidence that my Vx,Vy, etc, yours, and the original Cessna test pilots would agree on the ASI.

I did some work 20 years back test flying for a propeller company getting data for certification. We used no special equipment or techniques and flew in all conditions much like I suspect Cessna did in 1946 for their data. it was only through repetition that we obtained confidence in the numbers but what was interesting is that they were in the end rather consistent from the start.

Si

cdoughty wrote: ↑Tue Mar 22, 2022 8:54 am So I would have very little confidence that my Vx,Vy, etc, yours, and the original Cessna test pilots would agree on the ASI.

Totally agree! Even on my AC it can change from day to day based on the Configuration & the Environment. I was just looking for other references to compare with my calculations.

Here is my airspeed (in knots)... same values as my last post for those who are commonly using mph.
*I also did a checklist & it's available in another thread : viewtopic.php?f=6&t=642&p=11776#p11776

I'll test these values this summer when I'll be done with my mini-restoration.

simonlowther wrote: ↑Tue Mar 22, 2022 2:48 pm

cdoughty wrote: ↑Tue Mar 22, 2022 8:54 am Like you I worried about this initially. Tried to get good data but finding a really smooth day, reading the VSI (located on RHS in my plane) was problematic at best. GPS data wasn't any better. Timing off the Alt also didn't really work. At least not well enough to say at better than 5 mph. I might go back and do a better job, but its really an academic exercise. I have numbers based on experience that seem to work.

Add to that the poor to non-existant calibration of the pitot tube and pitot static. There is a Cessna document on how to bend the pitot to proper angle, but that only lasts till you bump into it by accident!!! When you bend it the reading changes. So I would have very little confidence that my Vx,Vy, etc, yours, and the original Cessna test pilots would agree on the ASI.

I did some work 20 years back test flying for a propeller company getting data for certification. We used no special equipment or techniques and flew in all conditions much like I suspect Cessna did in 1946 for their data. it was only through repetition that we obtained confidence in the numbers but what was interesting is that they were in the end rather consistent from the start.
Si

I’ve done quite a lot of flight testing on several different models of Ag planes for Certification, we used a flight test boom with an articulating “bird” as a pitot tube but that was the only test gear, it was all done with regular instrumentation and a stop watch. FAA required instruments calibrated within the last 90 days
It’s been done that way forever, and it’s surprisingly accurate with enough data points to average and throw out the outliers.
GPS has made air speed cals a lot easier and more accurate, but you know honestly for basic stick and rudder aircraft like a 140 or an Ag plane, close is good enough. You fly hand them by seat of the pants and with some experience you go by what feels right.
I’ve got some book somewhere that says best rate climb speed is 73, but in my airplane it is a little over 80 mph, maybe my metal wings are the difference?