Aerodynamics question...

[Larry Gude]

OK, it's 1700 mikes from BWI to ABQ. It took 4 hours 20 minutes from take off to touchdown to get there, an average ground speed of about 400 mph on a plane designed to fly, what 600? The pilot told us we had a 120mph head wind, so, does that make air speed 520 or so?

And on the way back it took 3 hours flat, or 570 mph ground speed.

Now, does all that sound right, am I using correct terms and facts, ground speed, air speed, commercial aircraft speed limits, and is it a simple aerodynamic fact that the plane can't cover 570 mph ground speed with a head wind of 120 or do they not go faster because it is too inefficient?

Thanks in advance.

 

[Bustem' Down]

OK, it's 1700 mikes from BWI to ABQ. It took 4 hours 20 minutes from take off to touchdown to get there, an average ground speed of about 400 mph on a plane designed to fly, what 600? The pilot told us we had a 120mph head wind, so, does that make air speed 520 or so?

And on the way back it took 3 hours flat, or 570 mph ground speed.

Now, does all that sound right, am I using correct terms and facts, ground speed, air speed, commercial aircraft speed limits, and is it a simple aerodynamic fact that the plane can't cover 570 mph ground speed with a head wind of 120 or do they not go faster because it is too inefficient?

Thanks in advance.

What kind of plane?

 

[Bustem' Down]

Boeing 737, 767and McDonnell Douglas MD-80 all have cruising speeds under 550 mph.

 

[MMDad]

Wirelessly posted (Mozilla/4.0 (compatible; MSIE 6.0; Windows CE; IEMobile 7.7) 320x240; VZW; Motorola-Q9c; Windows Mobile 6.0 Standard)

Due to lower air pressure at altitude, indicated airspeed is far lower than true airspeed. For example, an indicated airspeed of 350 at 40K feet is roughly equivelant to 630 knots. That's why you cruise at high altitude - the lower pressure means you can travel at speeds that would be supersonic at lower altitudes.

 

[huntr1]

headwind = slows them down
tailwind = speeds them up

best I can do for ya.

 

[Larry Gude]

...

What kind of plane?

...SR71 or a 737. One or the other. I can't quite remember...

 

[Larry Gude]

...

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Due to lower air pressure at altitude, indicated airspeed is far lower than true airspeed. For example, an indicated airspeed of 350 at 40K feet is roughly equivelant to 630 knots. That's why you cruise at high altitude - the lower pressure means you can travel at speeds that would be supersonic at lower altitudes.

...Ok, obviously, I didn't have a clue of the indicated airspeed, so, I mean, I'm just working off of miles from A to B and time. What you're saying here is 350 indicated is covering 630 in terms of actual distance from A to B?

 

[Larry Gude]

...

headwind = slows them down
tailwind = speeds them up

best I can do for ya.

...I knew that part. I'm just finding myself curious if they Can't go any faster in terms of how much ground they are covering into a head wind due to the fact that 400 mph forward into a 120 mph head wind is the equivalent of the plane flying, going through the air at 520 and that they just can't go any faster which seems obvious the more I think about it...

 

[Gwydion]

Better question is what happens when you put a plane on a treadmill.

 

[Pete]

OK, it's 1700 mikes from BWI to ABQ. It took 4 hours 20 minutes from take off to touchdown to get there, an average ground speed of about 400 mph on a plane designed to fly, what 600? The pilot told us we had a 120mph head wind, so, does that make air speed 520 or so?

And on the way back it took 3 hours flat, or 570 mph ground speed.

Now, does all that sound right, am I using correct terms and facts, ground speed, air speed, commercial aircraft speed limits, and is it a simple aerodynamic fact that the plane can't cover 570 mph ground speed with a head wind of 120 or do they not go faster because it is too inefficient?

Thanks in advance.

The airframe of the plane is load limited to a particular airspeed. It is not so much the engine that limits speed in the air it is the max speed the airframe is rated to before the wings peel off. To an airplane groundspeed means nothing, airspeed (IAS)means everything. I don't think a 737 can hit 600 IAS more than likely it is just over 500 IAS and that is full throttle. Cruising at max speed all the time would be inefficient. The engines and airframe are engineered for a modest airspeed capability with reasonable fuel efficiency.

So on your trip you were probably flying at 520 IAS, factoring in the headwind going west of 120 your groundspeed was 400.

Coming back you had a groundspeed of 570 MPH but because of a tailwind coming east the IAS was under 500 mph. The tailwind got you more giddyup and saved them a bunch of fuel.

 

[Larry Gude]

...

The airframe of the plane is load limited to a particular airspeed. It is not so much the engine that limits speed in the air it is the max speed the airframe is rated to before the wings peel off. To an airplane groundspeed means nothing, airspeed (IAS)means everything. I don't think a 737 can hit 600 IAS more than likely it is just over 500 IAS and that is full throttle. Cruising at max speed all the time would be inefficient. The engines and airframe are engineered for a modest airspeed capability with reasonable fuel efficiency.

So on your trip you were probably flying at 520 IAS, factoring in the headwind going west of 120 your groundspeed was 400.

Coming back you had a groundspeed of 570 MPH but because of a tailwind coming east the IAS was under 500 mph. The tailwind got you more giddyup and saved them a bunch of fuel.

...OK, so, given that the load was probably the same, and it was people wise, and it did take the equivalent of 400 mph ground speed to get to ABQ in 4:20 and he told us the head winds were 120 and the 737 runs at about 500 IAS, then he was presumably running about full on the way out?

And making the trip in 3 flat on the way back with a ground covering speed of about 520, then they were running maybe 70-75% or so?

That sound right? Am I thinking right? Then they simply couldn't go any faster Westbound and would be over engined if they could, would be exceeding the design? And structurally wouldn't want to go any faster East bound in this example?

 

[dgates80]

OK, it's 1700 mikes from BWI to ABQ. It took 4 hours 20 minutes from take off to touchdown to get there, an average ground speed of about 400 mph on a plane designed to fly, what 600? The pilot told us we had a 120mph head wind, so, does that make air speed 520 or so?

And on the way back it took 3 hours flat, or 570 mph ground speed.

Now, does all that sound right, am I using correct terms and facts, ground speed, air speed, commercial aircraft speed limits, and is it a simple aerodynamic fact that the plane can't cover 570 mph ground speed with a head wind of 120 or do they not go faster because it is too inefficient?

Thanks in advance.

Flight time BWI --> ABQ = 260 minutes

Distance is actually 1447 Nautical Miles

1447 / 260 = 5.565 NM / Minute average

5.565 * 60 = 334 knots groundspeed, average.

Flight Time, ABQ --> BWI = 180 minutes

1447 / 180 = 8.04 NM / Minute

8.04 * 60 = 482 knots groundspeed, average.

482 - 334 = 148 knots difference in groundspeed

Split the difference, no wind average 408 knots True Airspeed, average.

True Airspeed is Calibrated (indicated corrected for instrument error) Airspeed, corrected for air density and temperature. (I'll ignore compressibility)

Ground speed is TAS corrected for the prevailing wind vector.

Commercial airliners typically run about 550 knots TAS at FL390, indicating roughly 250 to 300 knots IAS at altitude, typically about .75 Mach

 

[limblips]

OK, it's 1700 mikes from BWI to ABQ. It took 4 hours 20 minutes from take off to touchdown to get there, an average ground speed of about 400 mph on a plane designed to fly, what 600? The pilot told us we had a 120mph head wind, so, does that make air speed 520 or so?

And on the way back it took 3 hours flat, or 570 mph ground speed.

Now, does all that sound right, am I using correct terms and facts, ground speed, air speed, commercial aircraft speed limits, and is it a simple aerodynamic fact that the plane can't cover 570 mph ground speed with a head wind of 120 or do they not go faster because it is too inefficient?

Thanks in advance.

Nobody has mentioned the FAA controllers and airways separation requirement. Very few flights are straight line nor are they as fast as the aircraft can cruise. In areas of heavy traffic (DC, BWI, etc) aircraft takeoff and landings are closely timed. Aircraft coming in or going out will be slowed or sped up to allow for proper separation.

 

[ylexot]

The airframe of the plane is load limited to a particular airspeed. It is not so much the engine that limits speed in the air it is the max speed the airframe is rated to before the wings peel off.

The standard practice is to make the airframe load limits 1.5x the loads that are predicted to be experienced. Airframe loads are not the limiting factor. Airliners are designed to fly close to the critical Mach number where the drag coefficient begins to rapidly climb. Flying above the critical Mach number is inefficient. Airlines don't like inefficient. The airframe can easily go faster (structurally) and the engines can probably push the plane a little faster than critical Mach, but not by a whole lot. The engines are sized to the airframe based on the critical Mach number.

BTW, fun trivia...does anyone know how the Boeing 707, 717,...787 series of airliners got their name?

 

[PulseStart]

The standard practice is to make the airframe load limits 1.5x the loads that are predicted to be experienced. Airframe loads are not the limiting factor. Airliners are designed to fly close to the critical Mach number where the drag coefficient begins to rapidly climb. Flying above the critical Mach number is inefficient. Airlines don't like inefficient. The airframe can easily go faster (structurally) and the engines can probably push the plane a little faster than critical Mach, but not by a whole lot. The engines are sized to the airframe based on the critical Mach number.

BTW, fun trivia...does anyone know how the Boeing 707, 717,...787 series of airliners got their name?

717 is a DC (Douglas Commercial DC-9, MD-80) jet but no, how did they?

 

[ylexot]

The wing sweep of a 707 is 45 degrees. Sine of 45 is 0.707...

 

[PulseStart]

The wing sweep of a 707 is 45 degrees. Sine of 45 is 0.707...

45 is too much!

 

[ylexot]

Yeah, you're right. I had heard that in college and never questioned it. I just looked it up and they were wrong.

 

[PulseStart]

Yeah, you're right. I had heard that in college and never questioned it. I just looked it up and they were wrong.

A Tomcat wings forward is 20 degrees and full aft is 68. 45 for an Airline would make for long take offs.

 

[blacklabman]

The standard practice is to make the airframe load limits 1.5x the loads that are predicted to be experienced. Airframe loads are not the limiting factor. Airliners are designed to fly close to the critical Mach number where the drag coefficient begins to rapidly climb. Flying above the critical Mach number is inefficient. Airlines don't like inefficient. The airframe can easily go faster (structurally) and the engines can probably push the plane a little faster than critical Mach, but not by a whole lot. The engines are sized to the airframe based on the critical Mach number.

BTW, fun trivia...does anyone know how the Boeing 707, 717,...787 series of airliners got their name?

The Boeing 707 got designated that because the wing's aspect ratio is 7.07. The aspect ratio is based upon wing area, span, and chord. The higher the aspect ratio the more efficient the wing. This was a test question during aero-performance class.

The later series of Boeing liners were a logical progression of the 707.

I don't know what happened to the 717.