They do?

[TransformerRobot]

Do planes really fly faster when they fly at lower altitudes?

[EdEarl]

I believe it depends on the airplane. In general jets fly faster at higher altitudes, and propeller airplanes are limited by altitude. But, I am no expert.

[michel123456]

Do planes really fly faster when they fly at lower altitudes?

Are you talking about the ground effect ?

[TransformerRobot]

No, I'm talking about something I heard related to air racing.

[Cap'n Refsmmat]

As an off-topic aside: could we all please pick topic titles that are descriptive? "They do?" gives me absolutely no idea what question you're asking or whether I know how to answer it. You could attract more attention to your threads with good titles.

[Klaynos]

Ground speed, true air speed or indicated air speed? Could you be more specific in what you read and the context of it?

[TransformerRobot]

I'm talking about air speed actually.

[Enthalpy]

Aeroplanes use to fly higher to be fast. Racing is at low altitude for the show and because pylones mark the base distance.

 

I could imagine that, as piston engines give more power with denser air, the penalty of higher resistance is less crucial for them.

[TransformerRobot]

Ah, thanks.

 

But why then why do jets fly faster at higher altitude?

[arc]

I am not an aeronautical engineer, but I do know that jet engines are less efficient at lower altitudes. It is not only easier for the engines to compress cold dense air rather than warm air, but more cold air can be compressed thus producing more power. This combined with less aerodynamic friction at high altitude and you can see the advantages. The atmospheric pressure is not important because the compressor portion of the engine negates those values.

The most critical aspect of air temperature is during takeoff and landings at or below sea level in higher temperatures where an engine failure could require the remaining engine to save the day. ( Death Valley is 86 meters below sea level and holds the record for the highest reliably reported air temperature in the world, 56.7 °C (134 F) on July 10, 1913.)

The increase in temperature proportionally decreases not only the jet engines efficiency but its rated maximum power at a most critical time. This reduction in power can in some circumstances like at airports in equatorial regions require the reduction of the planes MTOW (Maximum takeoff Weight) to increase the safety margin. If the pilot tried to compensate for the reduction in rated power output with more throttle the engine could over heat and possibly melt the turbine blades or even completely fail. arc

Edited June 30, 2013 by arc

[D H]

Planes don't fly at high altitudes because they can go faster at altitude. They do so because this lets them operate much more efficiently.

 

Efficiency doesn't count for beans in air racing. Speed and maneuverability are all that count. Drag adversely affects speed, but only if the airplane doesn't have sufficient power to overcome that drag. Lack of power isn't a problem with the unlimited class of race planes.

 

One of the key factors that limits the top speed of a prop-driven plane is the speed of sound. People have tried making supersonic propellers, but they never did work. The tips of the propellers need to be moving at less than the speed of sound lest all kinds of problems occur. In addition to prop issues, lift also becomes problematic as speeds approach mach 1.

 

So why is this relevant? Mach 1, the speed of sound, is not an absolute. It varies with air makeup, but most importantly it varies with temperature. Since temperature normally drops with altitude, so does the speed of sound. This in turn means a prop-driven airplane will fly fastest at low altitude if atmosphere conditions are normal (positive lapse rate) and if the plane has sufficient power to deal with the increased drag.

[TransformerRobot]

Okay, then how would a plane perform if it had a wingspan of 10 feet, body length 8.5 feet, had a small jet engine, and it's overall body-weight was 800 pounds?

 

For that matter, how high of an altitude would it need to fly for better performance.

[arc]

Okay, then how would a plane perform if it had a wingspan of 10 feet, body length 8.5 feet, had a small jet engine, and it's overall body-weight was 800 pounds?

 

For that matter, how high of an altitude would it need to fly for better performance.

 

Good news, they make one like you need the specifications for.

 

http://en.wikipedia.org/wiki/Bede_BD-5

 

Specifications (BD-5B)

General characteristics

• Crew: one, pilot
• Length: 3.88 m to 4.11 m (12 ft to 13.5 ft) w/stretch kits
• Wingsspan: 4.26 m to 6.55 m (14 ft to 21 ft 6 in)
• Height: 1.6 m (5 ft 2 in)
• Wing area: Depends on wing used (-5A, -5B or -5J)
• Empty weight: 167 kg and up ()
• Loaded weight: 184.6 - 366 kg (407 lb to 809 lb)
• Max takeoff weight : 530 kg (1,100 lb)
• Powerplant: 1 × Various reciprocating engines, from Rotax to Turbo Honda; turboprop with modified SolarT2; jet with Microturbo Couguar or TRS-18

Performance

• Maximum speed: 320+ km/h (200+ mph) recip, 500 km/h (300 mph) jet
• Range: 1,152+ km (720+ miles) recip, 500 km (300+ miles) jet
• Service ceiling: 3,700 m (12,000 ft) recip, 7,000 m (23,000 ft) jet
• Rate of climb: 579 m/min (1,900 ft/min) recip, 1,219 m/min (4,000 ft/min) jet
• Wing loading: Varies depending on wing selected and aircraft weight

[TransformerRobot]

Now I wanna see these things racing.