Calibrated airspeed questions

C

clestudentpilot

Well-Known Member
Hey guys, I just post questions in another thread regarding a Cherokee B 180, which led me to ask a few questions on calibrated airspeed. The POH for this airplane does not say whether the speeds listed are IAS, or CAS. What would you think this means? The POH also does not have any conversion for CAS to IAS, so I don't even know what I would do to convert anyway.

My second question is in regards to the white arc, compared to IAS and CAS. In the Archer that I fly, the POH say that Vs0 is 61 MPH, and it specifies that this is a CAS. The white arc also bottoms out at 61 MPH. Using the conversion chart with 40 degrees of flaps, 61 MPH CAS=54 MPH IAS. Shouldn't the white arc bottom out at 54 MPH on the airspeed indicator, since that is IAS. I am confused my this, and don't know what to think.

Any help is appreciated!
 
How are you converting 61 MPH CAS to 54 MPH IAS if the POH does not have a schedule to convert between CAS and IAS?
 
clestudentpilot said:
Hey guys, I just post questions in another thread regarding a Cherokee B 180, which led me to ask a few questions on calibrated airspeed. The POH for this airplane does not say whether the speeds listed are IAS, or CAS. What would you think this means? The POH also does not have any conversion for CAS to IAS, so I don't even know what I would do to convert anyway.

My second question is in regards to the white arc, compared to IAS and CAS. In the Archer that I fly, the POH say that Vs0 is 61 MPH, and it specifies that this is a CAS. The white arc also bottoms out at 61 MPH. Using the conversion chart with 40 degrees of flaps, 61 MPH CAS=54 MPH IAS. Shouldn't the white arc bottom out at 54 MPH on the airspeed indicator, since that is IAS. I am confused my this, and don't know what to think.

Any help is appreciated!
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Older CAR 3 airplanes painted the arcs in calibrated airspeed. As for the POH airspeeds, I don't think the CAR 3 regulations provided any guidance as contents of the manual, unlike present Part 23 regulations. My guess would be CAS, but you might can tell for sure by comparing the POH speeds to what's in the TCDS:

Never exceed 171 mph (148 knots) CAS
Maximum structural cruising 140 mph (121 knots) CAS
Maneuvering 129 mph (112 knots) CAS
Flaps Extended 115 mph (100 knots) CAS
 
tgrayson said:
Older CAR 3 airplanes painted the arcs in calibrated airspeed. As for the POH airspeeds, I don't think the CAR 3 regulations provided any guidance as contents of the manual, unlike present Part 23 regulations. My guess would be CAS, but you might can tell for sure by comparing the POH speeds to what's in the TCDS:

Never exceed 171 mph (148 knots) CAS
Maximum structural cruising 140 mph (121 knots) CAS
Maneuvering 129 mph (112 knots) CAS
Flaps Extended 115 mph (100 knots) CAS
Click to expand...


Thanks for the reply. Excuse my ignorance here, but what does CAR and TCDS stand for. Also, thanks, that clears it up, I just couldn't figure out how it is 61 MPH CAS=54 MPH IAS, while the white arc end at 61 MPH. It seems odd that they would do that, but I don't really care, just as long as I know the facts on it. This also makes sense even moreso, because the POH specified in the Archer, that all speeds are CAS, unless otherwise noted, and Vfe and Vno did not make sense initially, but now it does (even though I think it only changes it by like 1 or 2 MPH

The speeds you listed are the speeds that are in the Archer POH. The only speed in the Cherokee 180 of those 4 speeds is Vfe, and it onyl says that in the Operating Instructions in the landing instructions. Unfortunately, it says nothing about whether the speeds are CAS or IAS. I would assume most speeds are the same between the 2, since the Cherokee 180 is just an older Archer, but the stall speeds are not the same, so I guess nothing can be assumed
 
clestudentpilot said:
Thanks for the reply. Excuse my ignorance here, but what does CAR and TCDS stand for. Also, thanks, that clears it up, I just couldn't figure out how it is 61 MPH CAS=54 MPH IAS, while the white arc end at 61 MPH. It seems odd that they would do that, but I don't really care, just as long as I know the facts on it
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CAR = Civil Aviation Regulations, which is what preceded the FARS. CAR 3 contains airworthiness regulations which later became Part 23. The TCDS is the Type Certificate Data Sheet which contains a summary of aircraft info from the type certificate. You can find them online for almost all airplanes.

I would assume most speeds are the same between the 2, since the Cherokee 180 is just an older Archer, but the stall speeds are not the same, so I guess nothing can be assumed
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Probably the main difference is the maximum gross weight, which affects the stalling speed.

This may actually be a personal challenge for you. You can probably derive all the appropriate speeds for yourself with some careful flight testing. It could be fun.
 
Haha, yeah you're right. I can't believe they failed to put some speeds in the POH. Vr, Vs, Vs0, Vfe, Vx, and Vy are in there, but not Va, Vne, or Vno. I saw a picture of the cockpit, so I saw on the airspeed indicated the green and white arcs, so I was able to derive Vne, and Vno. I would now have to assume based on your reponses that the speeds listed are calibrated. Unfortunately, without a conversion table, that does tell me much. I will now have to do some homework and see if it is in the TCDS for the Cherokee. Thanks again
 
For best glide, pitch for level and full aft trim should get you very close, assuming the mechanics have set the trip to the right limits.

There isn't any speeds referenced in the emergency section of the POH?
 
Codyjp said:
For best glide, pitch for level and full aft trim should get you very close, assuming the mechanics have set the trip to the right limits.

There isn't any speeds referenced in the emergency section of the POH?
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Haha, that would require there to be an emergency section in the POH. The POH for this airplane is just terrible. This is not a cheap poor mans replication either, I believe it is a copy of the original.
 
I saw a manual once for a Citabria (I think that is what it was...). I was both the pilot's manual and the repair manual.

6 pages. Total.
 
Codyjp said:
I saw a manual once for a Citabria (I think that is what it was...). I was both the pilot's manual and the repair manual.

6 pages. Total.
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:yup::insane::crazy:

I guess I shouldn't complain after seeing that. This one is maybe 20 pages long. I'm surprised that manufacturers would take so much time building an aircraft, but then the POH becomes an after thought.
 
Codyjp said:
I saw a manual once for a Citabria (I think that is what it was...). I was both the pilot's manual and the repair manual.

6 pages. Total.
Click to expand...

Take a look at the POH for a Pawnee sometime.

"Approach and Landing.... roll steer the airplane with the rudder pedals. Mixture to idle cutoff after landing" No published airspeeds. One performance chart, and that only covers sprayer and duster configurations.
 
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