Flight below optimum altitude

[Dplus$]

I noticed in "preparing for your cathay pacific interview" book by capt. xyz that one of the questions on the written quiz says,"a flight carried out below optimum altitude would result in... and the answer is less time,more fuel.

I understand why it would result in more fuel, but why less time?

[hetfield]

I noticed in "preparing for your cathay pacific interview" book by capt. xyz that one of the questions on the written quiz says,"a flight carried out below optimum altitude would result in... and the answer is less time,more fuel.

I understand why it would result in more fuel, but why less time?

Greater TAS.

Regards

[karimi]

No change in TAS if above tropopause (tropopause definition: layer of more than 2000m when the temp gradient is less than 2°/1000m) so flight time might be the same at same mach if below optimum

[Dplus$]

yea but wouldn't you have a greater tas at a higher altitude...

[Old Smokey]

yea but wouldn't you have a greater tas at a higher altitude...

No, at typical cruising levels, cruise is scheduled at a Mach Number, and with decreasing temperatures with increasing Pressure Height, TAS will reduce. Above the tropopause where the temperature is constant, TAS will also be constant.

All of what I've said applies to cruise at a constant Mach number, in these times of cruising at Cost Index speed (now leaning heavily on the Fuel cost side and approaching Maximum Range Cruise), the sum of the Low speed and High speed drag polars around MRC usually indicates an increasing Mach Number with increasing Pressure Height. Below the tropopause this increasing Mach is usually insufficient to completely reverse the TAS reduction trend, but above the tropopause will lead to small TAS increases with increasing Pressure Height.

Regards,

Old Smokey

[hetfield]

http://upload.wikimedia.org/math/0/6/2/062ea0945a840608d2697bdf6b072743.png

So with constant Mach, TAS increases with increasing SAT (lower altitude).

[santiago15]

Dplus$

Check your private messages

[karimi]

Once again for Hetfield ,:}
If flying above tropopause NO CHANGE (or little change) IN SAT!
T is constant so if constant MACH Nr ==> constant TAS
( TAS = M x 39 SQRT T with T in Kelvin wich gives TAS in Knots or
TAS = M x 20.1 SQRT T wich gives TAS in m/sec)

[Alpine Pilot]

I would explain it like that.

optimum cruising altitude: is where we can set optimum thrust (90-95% rpm).
there we will have the optimum SFC. AND at optimum cruising altitude the thrust will be equal to maintain the optimum angle of attack.

OK, so now imagine you are cruising at optimum cruising altitude. Hours later you will have burned quite a lot of fuel, and thus the aircraft is lighter. So what will happen?
The angle of attack requiered to maintain altitude will be less due to the less weight. So if you maintain the thrust setting the aircarft will speed up.

in normal operation you would now climb to a higher cruising altitude where the thrust produced again gives you the optimum angle of attack.

Now one can clearly see if I fly below optimum cruising altitude, I will fly faster but not economic.

[Regain]

Given that principles of flight are being questioned in some quarters, including why lift is created (I read it somewhere, can't remember where,) may I throw a dumb it down question in?

Might you get there faster because the journey is shorter at lower altitude, therefore quicker?

[santiago15]

Regain

My understanding, no doubt somebody wiser will correct me, is that by flying below optimum altitude you would get there quicker; however, at the expense of burning more fuel.

Let's say - plucking out random figures here - two identical aircraft set out on a flight, ac A flies at 37,000' whilst ac B flies at 27,000'. Assuming they both fly at the same mach no, ac B will arrive at destination before ac A. However, ac A will have burnt less fuel for the same journey. All assuming still air, of course.

[CV Donator]

It was my understanding that the angle attack during cruise remains constant for MRC and LRC (99% of MRC) no matter what the weight, with a subsequent reduction in weight the aircraft would climb due to excess thrust hence the requirement for a reduction in thrust.

[Old Smokey]

santiago15,

Correct within limits. The Flight Level for maximum speed is at the CAS/Mach changeover height. Any flight higher OR lower will result in lower speed. Thus, speed is often-times better below Optimum Altitude, but not always. Fuel burn, however, will always be worse above or below optimum altitude, with the greatest penalty (for the same vertical displacement) above Optimum Altitude.

CV Donator,

The minimum fuel burn will be achieved if the aircraft is kept at it's MRC speed (or other speed schedule chosen) and the engines set for optimum TSFC. Thus, as fuel burns off, a 'creep' climb will develop, with both the airframe and the engines at their optimum.

Regards,

Old Smokey