Total Drag Curve: CD VS IAS/EAS/TAS
Thread Starter
Join Date: Mar 2005
Location: over dada way!
Posts: 21
Likes: 0
Received 0 Likes
on
0 Posts
Total Drag Curve: CD VS IAS/EAS/TAS
Should the total drag curve be referenced to IAS, EAS or TAS?
Having browsed through numerous resources, there doesn't seem to be any consistancy. Although TAS is used as the reference for power required vs available charts which is understood.
So, is it IAS, EAS, TAS or doesn't it matter?
Having browsed through numerous resources, there doesn't seem to be any consistancy. Although TAS is used as the reference for power required vs available charts which is understood.
So, is it IAS, EAS, TAS or doesn't it matter?
Join Date: Jun 2004
Location: Australia
Posts: 1,843
Likes: 0
Received 0 Likes
on
0 Posts
Spot on , EAS = Calibrated airspeed corrected for compressibility.
In times of old, and for newer aircraft operating below, say, 10000 feet, there's very little in the difference between EAS and CAS in practical terms. Higher and faster, it begins to matter a great deal.
Let me put it this way, EAS is the only DIRECT indicator of the true "Performance" speed of an aircraft until the Transonic (above Mcrit) speed zone is entered.
As a few examples, for a given weight, Minimum drag speed (Vmd), Total Drag, Stall speed, MANOEUVRE MARGINS, Vmo, Maximum Range Cruise, are all at a constant EAS - it is the "true aerodynamic value" speed. I emphasise MANOEUVRE MARGINS because there's still a helluva lot of pilots out there that think that if, for example, CAS for Minimum Manoeuvre Speed at a particular weight was, say, 250 Kt, then it's still 250 Kt at 35000 feet - WRONG! The reality is that if the aircraft had an EAS indicator in place of CAS, it WOULD remain at 250 EAS at all levels until Mcrit comes into play, for the example, 264 Kt CAS is now required at 35000 feet to provide the same protection as 250 Kt CAS at low level.
CAS is a Dinasour speed that came for the ride with us from a past era where the difference was negligible in the normal areas of operation, it now matters.
I'm sure that I won't see EAS as the standard cockpit indication of speed in my era, common sense dictates that someday it will, and CAS will be sent to where it belongs, in museums with the other Dinasours.
Regards,
Old Smokey
In times of old, and for newer aircraft operating below, say, 10000 feet, there's very little in the difference between EAS and CAS in practical terms. Higher and faster, it begins to matter a great deal.
Let me put it this way, EAS is the only DIRECT indicator of the true "Performance" speed of an aircraft until the Transonic (above Mcrit) speed zone is entered.
As a few examples, for a given weight, Minimum drag speed (Vmd), Total Drag, Stall speed, MANOEUVRE MARGINS, Vmo, Maximum Range Cruise, are all at a constant EAS - it is the "true aerodynamic value" speed. I emphasise MANOEUVRE MARGINS because there's still a helluva lot of pilots out there that think that if, for example, CAS for Minimum Manoeuvre Speed at a particular weight was, say, 250 Kt, then it's still 250 Kt at 35000 feet - WRONG! The reality is that if the aircraft had an EAS indicator in place of CAS, it WOULD remain at 250 EAS at all levels until Mcrit comes into play, for the example, 264 Kt CAS is now required at 35000 feet to provide the same protection as 250 Kt CAS at low level.
CAS is a Dinasour speed that came for the ride with us from a past era where the difference was negligible in the normal areas of operation, it now matters.
I'm sure that I won't see EAS as the standard cockpit indication of speed in my era, common sense dictates that someday it will, and CAS will be sent to where it belongs, in museums with the other Dinasours.
Regards,
Old Smokey
