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Speeds of a different kind

January 18, 2018

 

Yes, it is another non flying day in the life of a flying instructor. Too much rain, too much thinking about things and not enough time to do everything. My previous blog about flying around Mont Blanc,  drew some questions about the relationship of Indicated Airspeed and True Airspeed and how this would affect the Vne of the aircraft.

 

As we all know airspeed is indicated by the "Air Speed Indicator", this is the difference between the Total Pressure and Static Pressure as the aircraft moves through the air. The Pilot's Operating Handbook tells us the speed range at I.S.A. (International Standard Atmosphere), which means the stall speeds and never exceed speeds are shown to be at sea level.

 

 In the bad old days small planes with 50 hp engines could hardly reach, now modern light sports aircraft regularly fly at altitudes reaching 15,000 feet, the thinner, less dense air at such altitudes affects the performance of the aircraft in quite marked manner. 

 

The positive aspect of flying at altitudes higher than 6000 feet is that the True Airspeed increases due to the less drag experienced in less dense air. When Indicated Airspeed is 100 knots, the True Airspeed can be more than 120 knots at such high altitudes. This helps us travel faster thus Ground Speed reads higher as well. 

 

However, the aircraft will Stall only when in exceeds it critical angle of attack, regardless of indicated airspeed.

 

But when in thin air, due to density altitude, the faster the aircraft goes, the more closer it gets to its Vne and potentially susceptible to flutter. The lighter the airframe and less draggy its shape, it is easy to exceed the Vne without much effort from the pilot. This can even happen inadvertently. The flutter is caused by flexing of the airframe by uneven airflow over control surfaces such as elevator and ailerons, at high speeds, even though the indicated airspeed may be lower than Vne.

 

The POH for the aircraft, if written in enough detail, will show a graph of how the Vne (TAS) is decreased as altitude is increased. Please remember that most simple POH show speeds for sea level. 

Even Gliders with no motors have to be careful not to exceed their Vne when they are at high altitudes. No sane glider pilot will dive from high altitude to lose height, they will put their airbrakes out at a slower speed to reduce chances of flutter.

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