C150F owners - is the ASI in IAS or CAS?

[Matthew Rogers]

There seems to be some confusion (at least in my mind) over whether the ASI in a 1966 C150F reads in CAS or IAS. There is a chart for converting IAS to CAS in the POH and no mention that the ASI reads in CAS in the POH. But many people keep saying that the older ASI read in CAS so no conversion is needed.

This is in regards to a close call I recently had where I desperately needed to hit the correct Vx speed and in my plane POH that is listed at 52 MPH CAS.

Please chime in if you have actually owned and flown a 1964-1967 C150. The Vx speeds changed in 1968 to 57 MPH and then again in 1969 to 64 MPH for no apparent reason.

 

[Clip4]

Your problem is that before 1978, when AFMs were mandated, aircraft handbooks were not very good. But the values in the climb, takeoff and landing data are IAS. It amazing how getting the crap scared out of you will get the performance charts out.

It appears that Vx for your aircraft is 52 IAS with flaps up. See max performance take off on page 1-1.

 

[Matthew Rogers]

Yes, that is my problem. So your thoughts are that V speeds are listed as IAS but stall speeds are CAS?

 

[kgruber]

ALL Cessna 150 airspeed indicators report "Indicated."

 

[RyanB]

It’s called an Airspeed Indicator— it’s indicated. That’s why you have a chart to convert it into CAS.

 

[Matthew Rogers]

Ok, I understand that. The second question is whether the V speeds are listed as IAS or CAS? Stall speeds are explicitly listed as CAS, but he V speeds are not noted whether they are IAS or CAS. Are the Best Rate and Best Angle speeds in the POH the Indicated speeds to fly by the ASI or are they CAS?

In my plane, there is a 5-10 mph difference between the CAS and IAS at slow speeds close to Vx.

 

[chemgeek]

Well, the 1969 C150F POH specifies stall speeds as CAS "because IAS is unreliable near the stall speed." Operational speeds are in IAS since that is what you can monitor in the aircraft. Usually IAS and CAS are only a few mph apart throughout most of the operating range anyway, and does not pose a practical problem. These op speeds should provide adequate safety margin. I don't know about the 1966 model POH but suspect it is similar.

 

[dmspilot]

I already discussed this in my other post.

Your POH states: "If an obstruction dictates the use of a steep climb angle, the best angle-of-climb speed should be used with flaps up and full throttle. These speeds vary from 52 MPH and sea level to 60 MPH at 10,000 ft."

The specified speed of 52 is the speed you "use". You should not be doing conversions on it before you use it. It would be silly for the POH to tell you a climb speed that you can't use because you have to convert it to another speed first.

And you mentioned in your other post you weren't at sea level but at 2,300' so it would actually be higher than that, more like 54.

 

[Pilawt]

Well, the 1969 C150F POH specifies stall speeds as CAS "because IAS is unreliable near the stall speed." Operational speeds are in IAS since that is what you can monitor in the aircraft. Usually IAS and CAS are only a few mph apart throughout most of the operating range anyway, and does not pose a practical problem. These op speeds should provide adequate safety margin. I don't know about the 1966 model POH but suspect it is similar.

From the 1966 150F owners manual, page 2-9:

 

[Shepherd]

C-150?
There are only two speeds available for a 150:
1. Flying
2. Not flying.

Everything else is just advertising blather.
Figure out what they are for your particular aircraft and mark them on the airspeed indicator.

 

[Matthew Rogers]

Glad I kept it at the "Flying" speed. I'll use that speed all the time for now on.

 

[dmspilot]

From a '67 model.

No conversion is necessary for Vy, the same should be true for Vx.



Now, why CAS and IAS seem to be intermixed sometimes, I don't know. The 60s were a weird time with manufacturers doing a lot of nonstandard stuff.

To add to the mystery, the takeoff performance chart expects you to be at 64 mph. Don't know exactly what speed your should be using in your scenario, but the 42 you insist on is probably so far on the back side of the power curve you're probably lucky to be alive right now.

 

[MauleSkinner]

Where the confusion comes in a lot of times is that ASI MARKINGS pre-1978 were usually calibrated speeds, not indicated...so for instance to be technically correct, you can't extend flaps at the top of the white arc...you have to convert that to IAS to know what the max speed is.

 

[MauleSkinner]

Now, why CAS and IAS seem to be intermixed sometimes, I don't know. The 60s were a weird time with manufacturers doing a lot of nonstandard stuff.

the bold portion is actually a stand-alone statement. You're being redundant.

 

[Matthew Rogers]

I can look back at a recording of the flight on FlyQ to get some basic information. Not instantaneous data, but at least an average speed over the time in question.

Not counting any head or tailwind (0-5 shifting, could have been either at that moment), my FlyQ recorded ground speed over the trees was around 57 mph so that converts to 52 mph IAS with flaps down which is right at Vx. Perhaps my brain just thought I was going slower than I really was traveling. Or maybe I pulled up just a little right at the tree line and traded a few mph for a few feet.

Hard to know without some video. Which of course I didn’t bother to setup for this short flight.

 

[kath]

GPS has to amalgamate position readings over some time period, to give you a speed. So there's time lag there, and if you were doing a lot of maneuvering like in the pattern, I wouldn't trust it at all for speeds. The airspeed indicator is reacting much more closely to instantaneously.

 

[RyanB]

Well not counting any head or tailwind (0-5 shifting, could have been either at that moment), my GPS recorded ground speed over the trees was around 57 mph so that converts to 52 mph IAS with flaps down which is right at Vx. Perhaps my brain just thought I was going slower than I really was traveling. Or maybe I pulled up just a little right at the tree line and traded a few mph for a few feet.

Hard to know without some video. Which of course I didn’t bother to setup for this short flight.

You’re making this all too complicated.

 

[Matthew Rogers]

Yes agreed. Close the thread. Question answered.

 

[Art Rose]

Air Speed Indicator in a 1966 C150F reads Indicated Airspeed miles per hour. The ASI takes ram air pressure directly from the pitot tube. The higher the pressure on the instrument diaphram, the higher it pushes the airspeed indicator needle. POH numbers are IAS.

Do I detect something about trying to fly by using, or referencing GPS ground speeds? Don't even try that ignorance. It will eventually get you dead. Put the GPS away for now. Flying airplanes don't care about ground speed.

I don't understand why there are so many new pilots these days who think they're supposed to simply fly by numbers. Maybe it's something they're missing in their training?
This is not a numbers game. Those POH numbers in those old books are for reference only, and it's adviseable for your safety to add a few mph to those numbers. Especially down low and with little or no unusual attitude recovery skills. Never explore those edges until you have experience and altitude.

Changes in the aircraft, the load, or the density altitude can produce dramatic changes in performance. Be extremely careful about pushing the limits on airspeed numbers from POH's, especially for 50 year old airplanes. Too many reasons to start listing here.

When was the last time the aircraft was weighed, and the weight and balance calcs were updated? Do you still have one of the older O-200's that were detuned to 90hp? Did your instructor explain to you that a C-150, in the heat, with full tanks, two up, and with full flaps, is likely not to leave ground effect?

Always fly out of the sock!