Min drag vs Min fuel flow speeds

[esreverlluf]

Would any of you learned gentlemen be able to furnish me with a simple discussion of the above speeds and why they are often not the same!

[Genghis the Engineer]

The fuel flow .v. airspeed curve is the drag.v.airspeed curve ADDED TO the thrust.v. airspeed curve, and the fuel-flow.v.thrust curve.

Because the last two relationships are not uniform (linear) you end up with the bottom of the fuel flow curve not being at the same airspeed at the bottom point on the drag curve. This is basically down to engine / propeller efficiency varying with airspeed and a non-linear power curve.

G

[carpe.d.m]

Please clarify if you are indeed wanting feedback on-
...best lift to drag ratio speed
vs...min fuel flow speed

Also are you operating Boeing and hence Boeing FMC.This really helps as we can throw in some practicals on the FMC inputs

[babble]

First, if you are flying straight and level then min drag = min thrust (always).

The next thing to consider is the relationship between fuel flow and thrust.

If the thermal efficiency of the engine were constant (which it probably isnt) then fuel flow would be proportional to power developed. (with a jet, of course it is difficult to measure power developed anyway)

The relationship between power developed and thrust depends on the propulsive efficiency of the engine/ fan/ prop etc..

In the case of a "theoretical" jet engine which had no conversion losses due to fans, turbines etc, (ie a pure actuator disk) the propulsive efficiency increases linearly with speed and is given by

efficiency = 2*U/(V+U)
where U=TAS and V=speed of the efflux relative to the aircraft. This of course is the reason why big fan engines are more efficient than turbojets.

In this theoretical case, thrust is proportional to fuel flow and the two speeds are the same.

In real engines, of course things are not so neat. With a prop in particular, propulsive efficiency does not obey the above equation, and min ff speed is likely to be lower than min drag.

[esreverlluf]

carpe.d.m
You are indeed right. I'm after best L/D speed vs min fuel flow speed - and any practical info relating to Boeing FMCs would be appreciated.

[carpe.d.m]

O.K Here we go.
Best lift/drag is the speed you operate at to get from A to B with the least fuel burnt.Nil time considerations here.This is the "best range speed". With a Boeing FMC,one would derive this speed by entering a "cost index" of zero. " long range cruise " is or has a fuel burn of 1% above best l/d.

Now min. fuel flow is what it states.The speed that will result in the min f/f per hour.This is the speed one would hold at.One can derive this speed in Boeing FMC by selecting "present hold" (don't execute).

I hope this info makes some sense.If not re-query.

[quid]

The speed from A to B with the least fuel burn would be best "specific range". It is significantly faster than best L/D. It is the speed on the peak of the specific range curve. LRC is 1% faster than that. "Specific range" is the speed to fly (no wind) when you want to convert fuel to MILES.

L/D max is the speed to fly if you want to convert the fuel to HOURS and MINUTES. It's also known as Max Endurance and most operaters use this (+ or - a few knots) as HOLDING speed.

Least fuel flow (due to varying efficiencies of jet engines) is even slightly slower than L/D max. It is a very unstable speed (due to the angle of attack) and really has no practical application.



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[esreverlluf]

Thanks guys, now absolutely clear as mud.

Anyone care to have a stab at what the E/O speed as generated by the B757FMC represents?

Thanks in anticipation.

[carpe.d.m]

OK I apologise for the bum-steer.QUID is correct.Best L/D is the holding speed.I do beleive however it is the speed ,the only speed,to give one the lowest burn on the fuel flow guage.That is the name of the game here.And one does attain it by bringing up hold information on the FMC.

"specific range" and "optimum range" I think we are talking about the same thing.Although I believe the latter to be a clearer term.The thing to know here though is how to get it out of that box and I believe one gets it by entering 0 in the cost index field.

As for your next question re E/O speed in the -57 I believe that is best climb gradient speed.After all what is the objective in this scenario....well there is prob. several poss. objectives....but one of the more likely ones is that you are terrain critical,you will therefore want to descend with as shallow angle or path as is poss.

QUID also offers another important consideration in relation to any of the lower type speeds and that is they to tend to be a little unstable

[manuel ortiz]

How is best lift over drag speed affected on a twin turbo fan aircraft after becoming one eng . inop. ?

[sprucegoose]

I wouldn't think best L/D speed would vary with engine failure. It is afterall an aerodynamic consideration not a thrust consideration.

[quid]

You're right, sprucegoose. Has nothing to do with how many engines are operating.

L/D max is a very important number for pilots to know aobut their a/c. It gives best angle of climb, best driftdown speed and max endurance; all the same indicated airspeed. (Of course, the airspeed varies with weight.)

manuel-

If you mean becoming inop right after gear up and still in the takeoff configuration, the L/D max will be approx V2 +20.


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[This message has been edited by quid (edited 26 October 2000).]