best rate of climb speed

[stuckwithmassey]

Tell me guys is there a change in airspeed for a change in weight with regards to an aircraft best rate of climb speed?
I understand where this speed occurs on the Pa vs Pr but would a heavier aircraft require a faster best ROC speed??

Your thoughts.

Also a reference from a website or if ur a driver then u dont need to.

I thought that AC Kermode said a faster speed is required.

[TopBunk]

SWM

On the B747-400 the best rate of climb airspeed is the best angle climb speed + 25 kts.

The max angle climb speed (below 320 tonnes) is Vf30+100.

Practically, max angle speed is Vref30+100 / M0.84 - source BA Training Study Guide from 1998.

Obviously (?) the speeds are a function of aircraft mass, as the mass increases, Vfe30 increases.

At 380tonnes, the Vf30= 179kts, so V(best angle)= 279, V(best rate) = 304kts.

At 280 tonnes, Vf30 = 152, hence 252 and 277 kts resp.

[Keith.Williams.]

Assuming that we are talking about jet aircraft then best angle of climb speed (Vx) is Vmd and best rate of climb speed (Vy) is about 1.32 Vmd. If we increase weight we will increase vmd in proportion to the square root of weight. We will therefore increase Vx and Vy. For small changes in weight we can (for exam purposes) use an approximation that for any % change in weight we will get about 1/2 that % change in Vs, Vmd, Vx and Vy.

If we are talking about props the same holds true, but Vx is slightly less than Vmp and Vy is slightly more than Vmp. But Vmp varies in proportion to weight so the overall relationship between weight Vx and Vy is about the same.

[Capt Scribble]

767 best ROC is Vref80 + 50 kts. Heavier= faster

[stuckwithmassey]

Yes that makes sense guys thanks for the answers there, it seems that operational knowledge can usually make sense of awkward theory

[Old Smokey]

And all of the above holds good until Mcrit is reached, whereupon climb speed (expressed as a Mach No.) remains constant. If you really want to nit pick, Mcrit is slightly lower at higher weights, thus climb speed (expressed as a Mach No.) reduces slightly with increasing weight.

[mutt]

Old Smokey,

Can you please tell me what aircraft that happens on??

Thanks.

Mutt.

PS, welcome to Pprune, i have really enjoyed reading your contributions....

[Old Smokey]

Mutt,

Not type specific, answered in general principals consistent with the status of the original poster (assumed as an ATPL theory student from the call-sign, and other posts made).

The point to be made was that whilst best Rate of Climb speed does increase with increasing weight, a specific Mach No. defines the upper limit of this increasing speed schedule, and that that Mach No. is governed by Mcrit for the aircraft. This has been indicated by examples from TopBunk in stating BA’s recommendation for the B747-400 as Vref30+125 / M 0.84 (putting his additives together). TB’s statement validates the variable CAS for weight situation, but further illustrates a ‘non-variable’ Mach No. over the weight range. It is accepted that these are practical ‘all purpose’ speeds, and usually, practical speeds are a little off optimum.

It is also accepted that, for the jet aircraft, the practical best ROC speed may significantly differ from the absolute optimum with only minimal loss of ROC due to the low rate of Divergence / Convergence of Power Available Vs Power Required in the vicinity of the optimum speed.

My post alluded to ‘nit picking’, and, if an aircraft is considered at best ROC speed a moment before Mcrit is encountered, Pa and Pr are essentially parallel. Even a small speed change would reduce the Delta PA / Pr. Passing Mcrit would lead to small drag rises initially, reducing ROC even if by negligible amounts. As Mach No. climbs further above Mcrit and drag divergence increases against a disproportionately lower Power available increase, ROC loss becomes more marked. It is somewhere between this point and (back to) Mcrit that the ‘practical’ climb Mach No is extracted, a higher speed for negligible loss of climb rate. My contention is (and caveated in the next paragraph) that the upper limit of the increasing CAS schedule occurs at Mcrit.

The flaw in this argument is for an aircraft with low transonic drag divergence and a disproportionately higher rate of ram thrust recovery, where Thrust X TAS Vs Drag X TAS is still divergent above Mcrit – that WOULD be type specific.

Now, if we’re considering Long Range / Best Econ climb etc., let’s start chewing up some of the wave drag, tolerate a small ROC loss, but achieve an improved Time and/or Fuel burn for the flight.

Tried to keep the answers short, simple, and generic…..