Originally Posted by
172510
I'm not working on ATPL theory, I've passed that several years ago.
As I'm a CRI now I think it's a good idea to check my knowledge.
I'm just considering subsonic flight here.
This is below what I understood, would anyone tell me if I got it right?
1a TAS is the true airspeed. I don't think TAS is of any use for the pilot. When we use TAS, it's because we want to know our ground speed. But TAS in itself will not help to fly the aircraft.
Agreed. The progression, not as well understood as it should be, is:-
IAS --> (system error corrections) --> CAS --> (compressibility corrections) --> EAS --> (density corrections) --> TAS --> (wind corrections) --> groundspeed.
1b Nevertheless at high speed the TAS will cause the aircraft to flutter.
Not quite. Flutter *may* occur, and the speed at which it occurs is defined in TAS, and most commonly it's at high speed. I've seen flutter under 100kn, and in theory at least increasing speed through that - if stuff hasn't fallen off on the way through, may damp the flutter back out.
For small aircrafts the VNE/VNO are in IAS, meaning that at VNO, I'm closer to the risk of flutter at high altitude because my TAS is higher.
For all aircraft Vne/Vno are declared in IAS. Both are set by a safety margin below Vdf - the flight test diving speed, and a further margin below Vd - the design speed limit. There are numerous reasons why Vdf may be declared. Ones I've dealt with have included canopy buckling, flutter, aileron reversal, longitudinal stability flattening or reversal. Flutter's the only one of those likely to be defined in TAS, most will be declared in EAS from an analytical viewpoint.
Mne/Mno are a separate issue, but defined in similar ways. Clearly you fly to the lower of the two, so at low altitude Vne is the critical factor, and at high altitude, Mne takes over.
2 To fly the aircraft, the only useful speed is the EAS, as it's directly linked to the dynamic pressure, which is directly linked with lift and drag.
Nope, the only useful speed is IAS, because that's the one the dials read in.
3a I don't see any use of the CAS. I think it has been invented only because it's easier to mesure than EAS, and that CAS is very close to EAS in the area of the flight enveloppe (low altitude low speed) where manoeuvres are required.
It wasn't invented, it's an intermediate step that you need in calculating these things at any time.
But yes, for all reasonable purposes below about 0.6M and FL100, EAS=CAS.
But an ASI which would provide a best approximate of EAS instead of CAS would be better fit for purpose.
CAS is only useful directly when it approximates to EAS, but we do need it when doing performance calculations at the high level of flight testing, certification and design.
3b Is it a certification requirement that the IAS be the best approximate of the CAS, or is it permitted to design the IAS so that it indicates the best approximate of the EAS?
Most civil airworthiness standards require that IAS is within 3% or 5 knots: whichever is larger of CAS from 1.3Vs to Vne.
There's no standard preventing an EAS calculator in the cockpit - indeed some ASIs are designed to do exactly that, and some FMS work out TAS from IAS, via EAS - although I suspect that many of those fail to allow for PECs (IAS to CAS corrections) in the transition, so there will be errors from that source in the TAS value determined, as well as any other errors that creep in en-route.
G