The International Cessna^® 170 Association

bagarre wrote:

jrenwick wrote:...But Va on your airspeed indicator is the same, regardless of air density. I believe the same is true for all the V-speeds, isn't it?

...Va isnt marked on your airspeed indicator. It's based on your weight and density altitude....

Sorry, I didn't mean to imply that Va is marked on the airspeed indicator. It never is. I was referring to the fact that it's an indicated airspeed, not true airspeed (which is not indicated on a basic airspeed indicator). I know it's a function of aircraft weight, but nowhere have I read that density altitude is a factor.

I agree with George's discussion about the negligible effects denisty altitude has on the indicated or caibrated airspeeds proscribed for the 170. My understanding of things is this; Indicated airspeed is the uncorrected reading obtained from the airspeed indicator. Calibrated airspeed is indicated airspeed corrected for pitot-static installation and instrument errors. True airspeed is Calibrated airspeed corrected for pressure and temperature. The speed experienced by the airframe is True airspeed.

A 170 at sea level, on a standard day,full flaps and with zero angle of bank, will stall at an indicated airspeed of 52 MPH. The true airspeed experienced by the aircraft will be the same or nearly so. The same 170 at 10,000 feet on a non-standard day, with an OAT of 10 C, in the same configuration will also stall at 52 MPH IAS. However, the true airspeed experienced by the aircraft at the point of the stall will be approximately 62 MPH.

A 170 at 13,000 feet, on a non-standard day with an OAT of 10 C, having crossed the Sierras or the Rockies, begins a long descent. The air is still, so the aircraft is allowed to accelerate to 130 MPH IAS. The True airspeed experienced by the aircraft will be approximately 164 MPH.

The Same aircraft experiencing vertical gusts during the descent would have to slow to 90 MPH IAS, or less, in order to remain below a 115 MPH maneuvering speed.

Aerodynmaics for Naval Aviators - "Airspeed Measurement", pg 14: "..the airspeed indicator system measures dynamic pressure and will relate true flight velocity when instrument, position, compressibility, and density corrections are applied."

Robert Eilers wrote:I.... The speed experienced by the airframe is True airspeed...."

With regard to flight-control deflections, I think perhaps you meant to say Indicated airspeed?

TAS is the result of correcting EAS for air density. EAS and CAS (in a 170) are effectively the same. Although an airframe may travel thru the air experiencing a high TAS... the effects of the atmosphere impinging upon the airframe are more closely that of CAS (essentially IAS for a 170.)

Gust loads, such as your example, are an entirely different discussion than Va, as you noted.

Geez you guys. I fly for fun now days and like Jimmy Buffet said..."Math Sucks" I don't think most of us go to the extremes of our flight envelopes but maybe we do. I like the middle ground myself. A superior pilot relies on his superior knowledge to avoid having to use his superior skills.

It's good brain food but really doesn't change the cost of getting a cheeseburger at the next airport over
But, I have to disagree. Math is fun.