The International Cessna^® 170 Association

Hello

I am curious if anyone knows the IAS to CAS conversions for the 170B. There is no mention in my manual. Is it just because it is negligible?

Thanks!

Kevin

Related question: the V-speeds in the AFM are given as "True Indicated Airspeed." What's that?

If the speeds were given as "Calibrated Airspeed" or "KCAS," as in more modern manuals, then you'd expect to find an IAS-to-CAS conversion table. But our AFM doesn't mention Calibrated Airspeed, so maybe that's why there's no conversion offered. I'm stumped.

John

jrenwick wrote:Related question: the V-speeds in the AFM are given as "True Indicated Airspeed." What's that?

I never noticed that. Perhaps "true indicated" can be interpreted as calibrated? It seems a very odd choice of terminology. Hopefully someone can shed some light.

Frankly, I've never given determining calibrated airspeed much effort. Isn't it a correction for installation? So should it be different with every serial number, or just a standard modifier for the type? And how important is it at the speeds and altitudes we're flying in our little planes?

1SeventyZ wrote:...Frankly, I've never given determining calibrated airspeed much effort. Isn't it a correction for installation? So should it be different with every serial number, or just a standard modifier for the type? And how important is it at the speeds and altitudes we're flying in our little planes?

It's a correction for the fact that the airspeed indicator is most accurate when the pitot tube is facing directly into the relative wind, which it does not do at low airspeeds and high angles of attack. The correction should be the same for all aircraft of the same type, which is why the conversion table is published in the POH of newer aircraft.

This is also the reason you may notice that many aircraft can maintain level flight at an indicated airspeed below the stall speed given in the book. That's because V-speeds, at least for newer aircraft, are given as CAS, not IAS.

I've noticed this is true for older aircraft as well, including my J3 and 170, so maybe you're right -- True Indicated Airspeed may just be an older term meaning the same as CAS. This would beg the original question: why is there no conversion table?

When I was a flight engineer and flying big airplanes over great distances, it was important to know exactly what was going on and keeping the airplane operating at its most efficient speed and configuration. If you went on a bombing mission to target "X" and got home with 500 pounds of fuel over the required reserve, you can bet your bippy that the next trip to target "X" you would leave with 500 pounds more of bombs and 500 pounds less fuel. Air speed calibrations and calculations were damn important. The airplanes were much heavier and faster and there were significant differences in the various speeds.
I don't think you could really gain anything at all on a C-170 going through all the calculations because of the relatively low speeds, the crudeness of the instrumentation, the limited weight change and length of flight endurance.

blueldr wrote:When I was a flight engineer and flying big airplanes over great distances, ...

So you're moving UP in the world, BluElder! You're now flying little airplanes over short distances and you're the PILOT!

Yea - but he is still telling people where to go.

Actually I was a pilot first (1943), next I was a flight maintenance mechanic (1947), then a flight engineer (1951), then back to being a pilot (1953), then they retired me out in 1964. During my 22 years I had two periods of enlisted service and two periods of commissioned service. Old Sam has made the Eagle S--t the last day of every month every year since '42. And, I still really like SOS for breakfast just like I had this morning to celebrate Sams EFT to my bank. I think the thing I miss the most is chewing ass like I did when I was a Master Sergeant Line Chief. Those were sure the good old days!

BluElder, You really should have made the GLS convention. I'd have made you a key-note speaker!

Any chance of Branson?