This question if so basic [to you all] that it probably belongs here. From what I understand as an aircraft climbs into thinner air, drag is reduced and higher airspeed results for the same thrust settings, the trade off is having to fly near 'q corner' I guess.

My question is: If you read the 763ER [did I say that right?-767-300ER?] specs and compare them to the A330 specs in wiki there is a small discrepancy in the 'cruising speed'. The A330 claims a 20km/h higher cruise speed at FL36 vs the 767 @ FL35. Would calculating cruise speed at the same FL for both aircraft close this 20km/h discrepancy?

Any links to useful reading or documentaries on this subject would be greatly appreciated.

Many thanks in advance, Jason

 

Lies, damn lies, statistics and aircraft manufacturers brochures. All in the same category, you have focused on a very small speed differential(which may or may not exist-subject to weight, temperature, etc.) but when a company buys an aircraft they are looking at many other factors-cost of acquisition, mx, payload/range, comfort, safety record, fleet commonality, fuel economy,financing,etc. You have to look at the whole complex mess under a wide variety of conditions and understand that stated performance may or may not equal actual performance.

A good indicator is who sells the most aircraft in a particular category, who has the most orders and who is offering the best overall value. Airbus wins in some and loses in others.

 

....................

 

Power required curve shifts up and to the right with an increase in altitude (with TAS as the horizontal axis). Takes more power required to fly the higher TAS at cruise altitude.

Fuel flow however drops with a decrease in OAT. So although the aircraft takes more power to fly the high TAS at greater height, the overall decrease in FF makes it more efficient to do so.

These were the lies I was trained to teach MIL students. Not an engineer by trade.

P. S. I like turtles.

 

Actually mate I'm fighting the good fight. I have been told 4 times now IRL [3 times by travel agents, once by the MSM] that travelling on the A330 is preferable over the 767ER. The Airbus is "more comfortable"... very subjective. I imagine this might be a 'fuzzy' reference to less cabin noise. And it "is faster". After reading this recently I started to wonder where this disinfo was emanating from, of course its Wikipedia[tm]. So I have clipped 20km/h and 305 meters off the Airbus specs: Mach 0.80 470knots at FL35 / 11000m. Looks like the French 'metricate' with this formula a bit. Also the brand tragic media monkey that created this forgot to express airspeed in knots, have a look at the A330 entry.. Maybe this will go a small way towards the media getting it right....

Overall, as one that has airliner landing as a wallpaper, and habitually looks skywards I cant get the brand thing. It seems to me that Airbus and Boeing seem to have complimentary products... for the most part.. I guess people just like to barrack for their teams..

@propfails2FX Thanks mate,I was way off the mark, I had forgotten about volumetric efficiency.

Turtles, yes. Nothing says 'Politically Correct' like the slaughter our national and UN protected fauna by those whom should know better....

 

Zombie Kid Likes Turtles - YouTube

Lost in translation, sorry. Would have been just as much of an obscure comment as if I said, "Beached as" or "Trent from Punchy" to a Yank.

P. S. Volumetric efficiency? Is that the same as airplane flies higher and needs more throttle, but it's cold up there and the engine likes cold so it burns less gas? HAHAHA...just having you on mate.

Chur chur bro

 

Ah, i thought it was a reference to the ABC story about turtles.

Sweet broo, its like a big, free-range intercooler up there. , fully sick maaaaate

 

I got curious to do a review of the theory behind thrust required (TR), thrust available (TA), and velocity with altitude. I got out my trusty "Aircraft Performance and Design" by John D. Anderson as a guide, and put together few excel plots with the 767-300 in mind.

• This is true, although normally we plot things as a function of only one variable at a time so you would want hold one variable constant. In my first plot I held speed constant. In the second, I let speed be variable. I could make a chart with multiple curves on it to show such a shift, but it is easier to make one curve and show what one variable does at a time.

• Power required = TR x velocity. Assuming a given velocity, we can study TR curve and also get the power required curve. And since TR = drag in steady flight, that curve is the same as the drag curve as well. Hence I did not make a power required plot.

• Even though drag is given a break due to the high thin air, the actual total drag value the airplane "sees" goes up strongly with altitude, because CD is tied to CL. (Much) higher AoA is required to fly in thin air. Higher induced drag is reflected in a higher CD, hence higher total drag, although the form drag due to air density goes down.

• TA drops off more gradually with altitude than total drag does, for turbofans at least.

• This is where we get our speed bonus with altitude, not from the thinner air specifically, as many believe. In the "Velocity with altitude chart" below we see strong speed increase and an even stronger mach (M) increase with altitude. Note that the chart values are inflated, due to not considering mach drag divergence. In reality of course subsonic designs are limited to speed to about M=.95 if not less.

 

very cool, thanks for posting

 

@ everyone, many thanks.

@ cubdriver, cheers for the explanation and charts you created, muchly appreciated.

"Aircraft Performance and Design" by John D. Anderson - ORDERED