Jet speeds

[lambsie]

Why is there a difference between engine out drift down speed and best hold speed? Which speed in the envelope represents best L/D ratio? Doesn't this then give me best glide?
T.I.A.

[Crossunder]

Not a full answer this but holding speed will not give you maximum L/D, because at Vl/d you'll have very poor speed stability. You'll probably end up using more fuel by increasing/decreasing TLA all the time. Not to mention the uncomfort of your PAX. In addition, jet engines do not operate very efficiently at low RPM (as at speed for best L/D). Dunno 'bout driftdown speed. Would make sense to set this at/close to Vl/d as this will give you best gliding range in zero wind.. Never seen Vl/d (or Vdmin) mentioned in any AOM for JAR 25 aircraft though...

[Oxford1G]

Hi Lambsie,

The holding speed put simply is an endurance speed, it gives you close to the minimum kg/hr, minimum thrust and drag are required. The best endurance speed is where the drag is least.

The maximum range speed would give us the maximum nm per kg. This speed is higher than the endurance speed. We are looking for the highest IAS for the least drag, or thrust, or fuel consumption. This would then give us the maximum range. Close to Vmd, but not quite. :ok:

[Eckhard]

I think that best holding speed is usually quoted as 1.1Vmd (for reasons of speed stability as explained above) whereas best driftdown speed would in theory be exactly Vmd. If we assume a Vmd of 200kts, the best hold speed would be 220kts which sounds reasonable for a medium-weight airliner. The best drift-down speed at high altitude would also be around 220KIAS which, due to compressibility, would give an EAS of about 200kts (Vmd).

Also, remember that the drag characteristics will vary between all-engines, wings level (zero sideslip) and engine-out, wings level (small angle of sideslip). The drag of the windmillling engine will also have an effect. Therefore the two configurations will probably have a different Vmd, even at low altitude.

[lambsie]

In practise however:

Say a 767 GE powered, GW 140,000 kg, D/D speed 239 kts IAS.
Yet, holding at FL250 would be 223 kts IAS (from perf. manual).

What I'm trying to figure out: wouldn't best L/D speed give us greates range (good for D/D) and also min fuel flow (good for holding). What speed IS best L/D? And why would the two speed figures quoted above be different?

Thanks gurus!:D

[quid]

Lambsie-

All good info so far. Let's try to wrap it up.

With any given fuel load onboard the a/c, you can only optimize 2 things - greatest number of hours and minutes aloft, or greatest number of miles flown. The speeds (IAS) for both are very different.

As pilots we have to make the distinction between theory and real life. We also like things to be very simple to understand and use. In theory, one IAS would be optimum. In real life, altitude changes during drift down, so there would be changes to the DD speed. In holding, buffers are built in to account for G loads in turns and turbulence. For my a/c, I've seen significant differences in the holding speed charts from different airlines.

For this reason, your holding speeds would be somewhat higher than min drag. (Hours and minutes.)

If you have access to the Specific Range charts for your airplane (your tech section probably has them), it becomes quite evident that the max range (NM per # of fuel - miles flown) is much faster than the min drag speed. For example, on my a/c (DC-8) at FL 350 ISA, the max range speed is M.74, long range cruise (99% max range) M.78, holding M.67, max endurance M.58.
(These speeds would vary, of course, with altitude.)

The changes in speeds are not linear with altitude changes. From SL to around FL 200, there is little change in IAS for stall speeds, but as you near aerodynamic ceiling, the increase in IAS is more rapid with increases in altitude.

In summation, L/D is much slower than max range, holding speed is faster than theoretical L/D in real life.

[LOMCEVAK]

Recommended performance speeds are often a compromise compared to the true optimum speeds. A few points to consider:

Mach number is the speed parameter relevant to range and endurance theory. Any quoted IAS is calculated from the required Mach number.

Going back to the original thread, in a drift down situation with one or more engines inoperative, the drag will be different (usually greater) than in an AEO hold.

When holding, you do not want a small speed reduction to put you on the backside of the drag curve and have to make a large power increase to recover the situation. Also, you need to maintain an adequate margin above the stall. Note that there is no relationship between stall speed and min drag speed.


Much of this has been said before - just a different way of putting it with a few new thoughts.

[Flamgat]

Actually range flying is based on best L/D ratio, and that is an Alpha, not a speed. So it is independant of weight, altitude, etc.

Since most aircraft dont have Alpha indicators the only indication we have of the best L/D ratio Alpha is the IAS.

Secondly the lift and drag is a function of EAS, so in real life we have to make compromises. That is only the airframe.

When we add engine requirements, even more compromises.

Who said life is fair?