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Effect of weight and temperature on operating ceiling and speed

I know airliners climb during cruise as the fuel is burnt off.

However they also fly a lot closer to Vbg (best glide speed) than piston GA which normally flies way above Vbg and thus weight plays a smaller part on the speed per fuel flow.

Does anyone have any suggestions on how one could work out some sort of table which relates the ceiling to the weight and ISA+0 +10 etc conditions?

The ceiling would have to be defined consistently e.g. +100fpm climb rate.

Administrator
Shoreham EGKA, United Kingdom

By experiment

EGPD / OMDW / YPJT, United Kingdom

Adjusting for temperature should be simple. Each 8°C air temperature rise is equivalent of close to 1.000 ft higher density altitude. As piston engines (non FADEC, non turbo) senses only density, and the airframe does the same, ISA + 8°C should give an approximately 1.000 lower ceiling.

Weight is much more complicated. During climb, reduced weight will let the excess power translate into a higher rate of climb. Very roughly said, at half the weight, rate of climb will double. But this is only the first order effect, not taking into account that the induced drag wil decrease, further improving climb. But to fly best rate-of-climb at a lower weight, speed should be reduced, further reducing drag, once again increasing climb even more, if only a bit.

At lower weights the ceiling will obviously increase. At this increased ceiling altitude the engine power will be reduced compared to the basic ceiling. But due to thinner air, true airspeeds will be slightly higher, requiring more power for a given IAS (e.g. the IAS for lowest drag).

Piston engine power reduces fairly linearly with altitude, and at almost the same rate for all engines. (Valid for constant speed propeller installations - fixed propellers complicate matters so I skip them for now). But the problem is that the change in induced drag from a change in weight is not the same for different wings, and for that reason I don't think it is possible to produce a generic table that relates weight to ceiling.

It has been my intention for a long time to put some numbers into the above - I hope to get the time for that soon. I have a pile of climb data for different airplanes waiting to be used for calibration of some formulaes.

huv
EKRK, Denmark

However they also fly a lot closer to Vbg (best glide speed) than piston GA which normally flies way above Vbg and thus weight plays a smaller part on the speed per fuel flow.

Well that's true at the sort of levels that GA normally flies at, but if we're talking ceiling then it's the same deal. You have to fly close to the bottom of the drag/power curve because it's the only speed at which you can stay in level flight. And that drag (or power) is then proportional to weight.

I have a canister with water in my hanger which I put in the baggage compartment if I fly alone or with two. It gives me 2-3 knots at the same fuel burn. I have not noticed any speed difference just based on the weight.

I have a canister with water in my hanger which I put in the baggage compartment if I fly alone or with two. It gives me 2-3 knots at the same fuel burn. I have not noticed any speed difference just based on the weight

Different effect. Aft CG reduces the elevator down force to keep level (and hence reduces elevator drag and increases speed)

At normal cruise speeds the overwhelming power consumer is parasitic drag (for my aircraft parasitic drag will be c. 4x higher at cruise than at best glide and induced will be reduced by a third. So in a symplistic way, the effect of weight on drag will be over 6x greater at best glide than normal cruise.

To Peter's original question. I was surprised at how signigicant the propeller map is to Vy and Vx performance, so I would have thought a general table would be very difficult to develop.

EGTF

I know one thing for a fact; turbofan engines develop a lot less power at ISA+15 than at ISA.

It has been very warm, windy , and turbulent this last weekend, we noticed the difference on Saturday

Darley Moor, Gamston (UK)

My experience with the DA40 TDI so far is that OAT makes little difference on the operating ceiling. The diesel engine is relatively underpowered. A lot of the nice grass strips are out of reach. You can land there, but not get out anymore.

At altitute the engine keeps on going. I was supprised to be able to keep a steady 500fpm climb rate all the way up to FL180 on a warm summer day. Temp was -9 at FL180 (which is ISA +12).

Does the ISA deviation affect cruise speed, for a given fuel flow?

On a recent trip to Brac, I had ISA+17 on the way there and around ISA on the way back. ISA+17 makes life really hard. It knocks maybe 2000ft off the ceiling, so you burn a lot more fuel to get to any level at which peak EGT is not sufficient.

OTOH I once saw ISA-14 and I think the ceiling would have been somewhere over 22000ft then.

Does anyone know the formulae for working out any cruise speed effect?

Huv’s formula above of 8C = 1000ft seems about right in reality for the ceiling.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

Does the ISA deviation affect cruise speed, for a given fuel flow?
Yes. Dynamic pressure depends on air density, so an ISA deviation of 8°C will have the same effect on cruise speed as a pressure altitude change of ≈1000 ft, provided you have the same engine power setting. (I am disregarding changes in fuel flow due to variations in how much you can lean the engine at different altitudes.)

Does anyone know the formulae for working out any cruise speed effect?

You don’t really need a formula, just translate the ISA change to a density altitude change and look in your POH performance tables/graphs.

Last Edited by Airborne_Again at 25 Sep 19:06
ESKC (Uppsala/Sundbro), Sweden
53 Posts
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