Best angle vs best rate

[Hedncld]

Can someone clarify where best angle and best rate appear as it relates to the drag curve? I have read a few things which seem to contradict one another. I understand Vy occurs at maximum excess thrust, however does this speed reside around best L/D? Any thoughts??
Thanks for the help

[Genghis the Engineer]

Re: Best angle vs best rate

If you are talking about climb speeds, they aren't directly related to the drag curve.

You may be mixing best angle/rate of climb and best angle/rate of glide.

G

[Old Smokey]

Re: Best angle vs best rate

This entire discussion will depend very heavily whether you're talking about a propeller or a jet aircraft (or perhaps even a glider or a rocket?)

Regards,

Old Smokey

[Hedncld]

Re: Best angle vs best rate

I was referring to jet aircraft. We always use cruise climb from our performance management system. It does have best angle on one of the pages but how it arrived at that figure is tucked away in the box behind the screen - with no other references in the manuals we are issued. Your take old smokey? As I think you're on target a lot.
Thanks

[Gary Lager]

Re: Best angle vs best rate

For jet aircraft, best angle of climb occurs at maximum specific excess thrust - i.e. at the speed where the difference between drag and thrust is greatest.

Since the thrust output of a jet engine is relatively constant with speed, max excess thrust occurs close to Vmd (where drag is minimum).

Example: B737, best angle speed usually around 210-220 kias.

Best RATE of climb is related to specific excess POWER (I remember it as POWER being a RATE of doing work) - in other words, we are looking for the maximum difference between aircraft power available and power required (drag * velocity).

Since power is Force x velocity, and thrust(force) for a jet doesn't vary (much) with speed, the relationship of Available power versus speed for a jet is now a (straight-ish)diagonal line. So maximum specific excess POWER occurs at a higher speed.

Example: B737 best rate of climb speed occurs nearer to 270-280 kias.

The FMC works these things out accurately because it knows, based on actual ambient conditions, how 'straight-ish' or not the various thrust & power curves actually are.

[Old Smokey]

Re: Best angle vs best rate

Gary Lager has answered the question very very well indeed. Gary, stand by for incoming flak from those who would have you believe that jet engine thrust declines with speed. Indeed it does, until about M0.5, whereafter ram recovery steadily restores the thrust to approximate static speed values in the vicinity of M0.75. After that, thrust in SOME engines will exceed static thrust. The Net result is that as the Power required curve bends upwards, so too, does the Power available, as the now increasing (recovering) thrust is increasing commensurate with the speed increase, yielding a Power Available curve also curving upwards, approximately paralleling the Power Available curve over a fairly significant speed range (30 to 40 knots). It's not unusual to see a best rate of climb speed in excess of the final Maximum Range Cruise speed.

It's absolutely true that ram recovery is not as great for modern high bypass engines as opposed to their earlier ancestors, but those internal compressor/turbine cores driving the ruddy great fans just loooove that ram recovery.

I've had one (alleged) expert try to convince me that ram recovery in high bypass engines was insignificant, with supporting data which proved that the modern jet couldn't hope to achieve much more than 300 knots in level flight

Regards,

Old Smokey

[hawk37]

Re: Best angle vs best rate

Surely, Smokey, there can't be any engines flying on airliners these days (maybe 727's, or older 737's?) where thrust output, at any altitude, increases with increasing mach, below .80?
Hawk

[Hedncld]

Re: Best angle vs best rate

Thanks Guys for your clarification. I appreciate your time responding

[Old Smokey]

Re: Best angle vs best rate

hawk37,

Read you loud and clear. It's difficult to be specic when replying to a generic statement. That's why I used such terms as "approximate", "vicinity of M0.75", and "SOME engines".

Frankly, I don't have specific information for the aircraft which I currently fly (B777 with very high bypass engines), but day to day experience shows me that if I reduce my climb speed from the 310-320 KIAS range to 270 KIAS in turbulence, the whole guts drops out of the climb.

The slight sarcasm that I poked at some other posters (on other threads) was in light of their statements that ram recovery was negligible, or none at all for the high bypass engine - ABSOLUTELY NOT SO!

One of the sad facts that I found bemusing from your post, where you referred to older aircraft "(maybe 727's, or older 737's?) ", was that the ram recovery on those JT8 engines was excellent at high speed, but the pre- super-crical wings on those aircraft negated the advantages of higher speed flight.

Regards Hawk et al,

Old Smokey

[bookworm]

Re: Best angle vs best rate

FWIW, I've got some charts for the JT4A (707, DC8) and the PW 4056 (747-400 etc.).
The JT4A at 30,000 ft shows a marked increase in maximum continuous thrust from about 5100 lb at 250 KTAS through to more than 5700 lb at 550 KTAS. Wow those TSFC numbers look ugly. Did they give Jet A1 away?
By contrast, the PW 4056 is almost perfectly flat at 30,000, while at 35,000 ft there's about 12,000 lb at M 0.5, rising to about 12,500 lb at M = 0.85. It's not much, but it's something.

[CV Donator]

Re: Best angle vs best rate

Slightly off topic would I be right in thinking that maximum range glide occurs at vy (VMD X 1.32) and max endurance glide at Vx or Vmd.

[172_driver]

Re: Best angle vs best rate

For a jet that is correct, speed for best glide at Vmd x 1,32 and minimum ROD at Vmd, from an aerodynamic perspective. Due to engine efficency both speeds may vary slightly in practise.

[Old Smokey]

Thanks bookworm, for the information, yes, ram recovery certainly exists, with INCREASING thrust at higher speeds, even for the high bypass engines. Maybe not as much as for the old-timers, but recovery nonetheless as opposed to various assertions that thrust continues to decline as speed increases.

Multiply that increased thrust by the increased speed, and we have GREATER THAN LINEAR increase in POWER, which directly impacts upon Jet climb performance.

I rests me case your honour.

Regards,

Old Smokey

[Macgyver]

Originally Posted by 172_driver

For a jet that is correct, speed for best glide at Vmd x 1,32 and minimum ROD at Vmd, from an aerodynamic perspective. Due to engine efficency both speeds may vary slightly in practise.

I think that this statement may not be correct. Max glide range occurs at L/D max for both jets and props (I know that max range is not L/D max for a jet, but we are talking about glide performance here). When considering jet performance, this means that max glide range occurs at minimum drag, which is the bottom of the thrust curve (i.e. Vmd).

From the book "Flight Theory for Pilots," :

For jet aircraft, the following performance items occur at L/D Max:
- Minimum drag.
- Max engine-out glide range.
- Max climb angle.
- Maximum endurance.

[hawk37]

Macgyver, you are correct. 172 Driver should have said that the speed for a jet powered aircraft to achieve best ** RANGE ** is at approximately Vmd x 1.32, and no reference to Vy at all. This would be for the powered situation, not for gliding. I can’t seem to find an explanation for this 1.32 number. Max range occurs when the ratio V / Drag is at a maximum, which further requires the ratio Cl^.5 / Cd to be a maximum. Perhaps this further simplifies to approximately 1.32 Vmd. Various assumptions apply, such a thrust output and fuel flow is linear with speed, and the drag polar is parabolic (no mach/compressibility effects).

Smokey and Bookworm, thanks for your contributions. I always learn from your contributions.

Hawk

[Dick Whittingham]

I think that 1.32 derives from the assumption that the drag curve is the sum of the profile drag curve, a pure x squared curve and the induced drag curve, a pure 1 over x squared curve. In that case the tangent from the origin touches at 1.316 times the x value at min drag. (4th root of 3, mathematically) Of course, in real life these curves are not pure curves, so 1.32 is a rough approximation, and goes seriously wrong when, eg. wave drag upsets the drag curve at higher M.

Dick W

[Old Smokey]

Bookworm,
Further remarks from your post -

"By contrast, the PW 4056 is almost perfectly flat at 30,000, while at 35,000 ft there's about 12,000 lb at M 0.5, rising to about 12,500 lb at M = 0.85. It's not much, but it's something."

Actually, it's VERY significant, as opposed to "It's not much, but it's something". In terms of ram RISE, that is, where the static thrust increases beyond it's original value, true, it's not a great deal. In terms of ram RECOVERY it's VERY significant, even if the thrust remained 'flat lined' at 12,000 lb. With increasing speed, the speed differential between the jet efflux and the aircraft forward speed results in an almost linear decrease in thrust, to the point where a jet aircraft could not hope to achieve much more than about M0.6. Even for a 'flat line' situation, the very significant thrust RECOVERY back to normal is due to increasing ram as Mach number increases.

Regards,

Old Smokey

[172_driver]

Macgyver, hawk37, thank you for the correction. I feel a bit embarrased to admit that I had not thought about it that way. It is quite logical to assume that gliding performance does not depend on whether there are jets or props fitted on the wing.