RE: gasser vs elevation
 

About the landing speeds. Very true. Indicated airspeed remains the same but ground speed increases due to the change in density altitude. This generates problems for a lot of different things that might have runway length limitations. If you were landing a plane from inside of it the numbers on the gauges would reflect the same vlaues. Unfortunately we land from outside the plane and must recognize that more apparent speed on final is required to avoid the stall/spin on short final trap.

RE: gasser vs elevation
 

My students seem to understand it best when I put it like this:

Let's say an airplane stalls at 25 mph.
That means it "needs" more than 25 miles per hour worth of air molecules hitting it to be able to fly.
There are less air molecules in less dense air, so the airplane has to fly faster to "feel" 25 miles per hour worth of air molecules at high density altitudes.
So the air speed indicator indicates 25 miles per hour, but the airplane is traveling across the ground, (ground speed) faster.

Oversimplified, but it seems to help them grasp the concept.

Note: At "ISA" or sea level, 29.92 barometric pressure, and 59 degrees, with NO wind, 25 mph on the airspeed indicator would actually be 25 mph ground speed, (if the air speed indicator was accurate).

AV8TOR

RE: gasser vs elevation
 

That is very true.

An airplane's air speed indicator senses stagnation pressure in the Pitot tube, which is later translated into mph, km/h and/or knots.

According to Bernoulli's principle:

Stagnation pressure = 1/2 x air density x (air velocity)^2

In order to read the same air speed for reduced air density, the plane must fly a little faster (note that the influence of air velocity is quadratic).

Exactly the same thing happens to lift, since the formula relates air density and air speed in similar way:

Lift = CL x Wing area x 1/2 x air density x (air velocity)^2

In this link, we can see charts showing the variations of air density with altitude:

http://www.2-stroke-porting.com/altiden.htm

The reciprocating engine has a fixed displacement; hence, it can only suck so much volume of atmospheric air.

The OP is moving his engines from 1,400 ft elevation to about 7,000 ft.
The air density will change from 0.073435 Lb/cuft to 0.0620816 Lb/cuft.
Hence, when operating at 7,000 ft, his engines will be breathing the same volume of air, but only 85% of the mass of air respect to 1,400 ft.

Since combustion is based on mass of air (number of oxygen molecules per number of molecules of fuel), the fuel-air mix must be leaned in a similar percent.
Since 85% less mix is burned, the engine will generate less power.

Accordinly, the air speed to sustain flight for similar trim (and to read the same airspeed in an imaginary air speed indicator) would be:

Air speed @ 7,000 ft = (1/square root of 0.85) x air speed @ 1,400 ft = 8.5% faster

The airplane will have one thing on favor: lower temperature (see attachment).
That means that air will be less viscous at 7,000 ft (20 degrees cooler), and it will imposse less drag to the air frame for the same velocity.

RE: gasser vs elevation
 

Not always true about the lower temperature.... Ask some of us whom have flown from the airport at Tusayan near Grand Canyon in the summer! ;)

The airport is around 6600 feet, but the temperature is often 80 degrees plus! Makes for a density altitude of around 9500 feet!

And then the normally aspirated engines lose about 3% power per 1000 feet of density altitude. Yeah, it has caught a lot of pilots "off guard"...

Well, I guess we about covered this one.... ;)

Hey Ralph, you know what it is like to fly from true altitudes of 7000 feet, aren't you going to get into this? :D

AV8TOR

RE: gasser vs elevation
 

Just reread the post previous to my last one. You lost me on the "less viscous due to lower temperatures". Lower temperature air means More Viscous.....

AV8TOR

RE: gasser vs elevation
 

Not for the air, av8tor1977.
See attachment of post #28.

Copied from http://en.wikipedia.org/wiki/Viscosity

"Viscosity in gases arises principally from the molecular diffusion that transports momentum between layers of flow. The kinetic theory of gases allows accurate prediction of the behavior of gaseous viscosity.

Within the regime where the theory is applicable:

Viscosity is independent of pressure and
Viscosity increases as temperature increases."

RE: gasser vs elevation
 

We flew the Kelly F1GT 80cc and the Tsunami Unlimited 289cc Saturday at Williams, 6835 feet...No problems....Engine speed according to a computer program that uses the sound, about 9500 rpm with our 31 pitch prop....air speed ? 250 ++..
OS 160 glow engines work very well up here wiith 20-6 props....
Altitude is a non factor, you just allow for a little little less power..NO long scientific dissertations...;)

RE: gasser vs elevation
 

We do all get carried away sometimes, don't we? ;):D

AV8TOR

RE: gasser vs elevation
 

Guilty :eek:

RE: gasser vs elevation
 

Let's talk about who makes the best oil:)

RE: gasser vs elevation
 

How about the best drug store ;)

RE: gasser vs elevation
 

Blondes, brunettes, or redheads?

RE: gasser vs elevation
 

Which ever has wings..

RE: gasser vs elevation
 

That one's too easy:)

RE: gasser vs elevation
 

:D[8D]:);)

RE: gasser vs elevation
 

Ralph, it's great to see you in such a good mood .... no Grumpy here. Sure hope no one goes and causes it to change anytime soon :)

Karol

RE: gasser vs elevation
 

Someone will, count on it..[>:]