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GazzaMataz
08-04-2004, 02:04 AM
My dad said this to me the other day and I don't know if it is true or not. When you are flying the air low down is more denser than air at higher altitudes. Now does this mean, as he said, that when flying at higher altitudes you can fly faster?

I am not sure since you would probably need more power at higher altitudes to get the lift in the lighter air and so would need to travel faster anyway.

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GazzaMataz
08-04-2004, 02:04 AM
My dad said this to me the other day and I don't know if it is true or not. When you are flying the air low down is more denser than air at higher altitudes. Now does this mean, as he said, that when flying at higher altitudes you can fly faster?

I am not sure since you would probably need more power at higher altitudes to get the lift in the lighter air and so would need to travel faster anyway.

Answers on a postcard‚...

Tickety boo...
Gazzamataz
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ploughman
08-04-2004, 02:13 AM
Generally, yes. Most planes enjoy their best performance at altitude (for this read fast and efficient), it's why modern jet-liners like to fly really high.

But on the other hand less air means less for the prop or blade to push against, less air to create lift or effect control surfaces etc., Helicopters have a relatively low ceiling (say 16-18,000 ft because they can't generate the required lift in the thin air above that altitude to remain airborne. In addition to all that, humans need oxygen above about 11,000ft or they start acting weird. I've been up to 17,600ft in the mountains without oxygen and even sitting down is tiring, run around or do anything strenuous and you'll be seeing stars and passing out. I thouroughly reccommend it.

x6BL_Brando
08-04-2004, 02:16 AM
Your Dad is essentially correct. There are other considerations for flying at high alt - prop pitch needs to be coarsened to enable the prop to 'grip' in the thinner air; also fuel mixture needs to be weaker because there is less oxygen present in the upper air.

Rab03
08-04-2004, 02:24 AM
Simply put, yes and no. You need more power when you're flying low, as the drag is much higher (resistance of air). Aircraft speed rise almost linearly with increase of altitude, till it reaches certain maximum speed. If the plane continues rising above height related to maximum speed, the max speed itself starts dropping and in one moment reaches minimum flying speed for the given altitude-the height at which this occurs represents ceiling.

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KarayaEine
08-04-2004, 08:54 AM
Why would you doubt what your dad told you? http://ubbxforums.ubi.com/images/smiley/16x16_smiley-tongue.gif

But yes, he is correct. Air density decreases with altitude. Air at sea level is quite dense so it takes much more effort (read: power) to move through the air at a given speed.

Remember 500mph is 500mph whether you're at sea level or 50,000ft. However, your I.A.S. (indicated air speed) may be different depending on your power setting. You may be at 100% power at sea level and only show 450kts IAS but when you go upstairs to 50,000ft your IAS would read 600kts (I'm just pulling numbers out of the air for illustration purposes only)

The air density also affects engine performance as well as human performance. Normaly aspirated engines (those with a carbuerator) don't run well or even at all above 14,000ft. The air is not dense enough to support combustion. You'd have such a lean mixture that there just would not be enough to move the piston. hence the need for superchargers or turbochargers. These devices 'cram' more air into the intake/combustion chamber so the engine will function. They make the air more dense, so to speak. Humans don't do well either above 12,000 ft altitude(unless they acclimate themselves over a period of time like Sherpas or mountain climbers) so you need a pressurized aircraft to keep the environment you're in at a level where there's enough oxygen at a pressure where we can still function.

High flying planes also need different wings than lower flying ones do. Look at the Ta-152. Nice long, thin wings are superb for the altitudes it's designed to fly at. they provide more area for lift to compensate for the decreased air density and allow manueverabilty. try taking your Fw-190A to 50,000ft and you'll see you're flying right at the edge of a stall even though your IAS is 550kph. The wings just don't produce enough lift in that thinner air.

Hope this simplistic explanation helps.

Johann

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