Fuel Flow Reduction With Altitude (Constant Power)
 

A question arose during a flight the other day and I was unable to come up with a satisfactory explanation at the time. After some thought and several hours searching the internet (to include this forum) I think I now have a basic understanding of the phenomenon. However, I'm wondering if it's really as simple as it seems on the surface.

The question:

What causes fuel flow to decrease with altitude for a given torque setting in a turboshaft powered helicopter?

My understanding of the explanation:

As altitude increases, air density decreases, so the gas generator must spin faster in order to supply sufficient compressed air to satisfy the power demand and maintain equilibrium.

Typically, gas turbine engines operate most efficiently toward the high-power end of their operating spectrum. So, as the gas generator spools up the compressor and gas generator turbine approach their design points (optimum angle of attack) and become more efficient at producing that compressed air. That is, less power is required by the compressor to generate the required discharge air pressure, and the gas generator turbine becomes more efficient as well, thus reducing the fuel flow.

This increase in efficiency continues until the design point is passed or an operating limit is reached.

Additionally, with the increase in altitude comes a decrease in ambient temperature (within the altitude range where helicopters operate, at least). This increases the allowable temperature difference within the engine, increasing its thermal efficiency. However, it seems to me the contribution of this aspect would only be realized when the engine is operating near its temperature limit.

Have I got this right and what other factors are involved in this phenomenon?

-Stan-

Mostly due to the reduced air density, which reduces the drag it takes to turn the compressor.

Fuel flow is proportional to the MASS of air pumping through the engine because the FCU meters the fuel accordingly. As altitude increases, air pressure decreases, mass of air flowing through engine therefore decreases, so fuel flow decreases. That's why jet airliners fly as high as they do, to reduce the mass of air flowing though the engine, hence reduce fuel burn. It's to do with the stoichiometry of combustion.

Perhaps (and I may be very wide of the mark here) the temperature of the fuel in the lines themselves (and therefore through the fuel flow meter) is an additional factor in all this?

Would cooler/denser fuel passing a gauging device show as a slightly reduced rate on the gauge? It is still the same 'amount' of fuel for combustion so the stoichiometric ratio would be maintained :confused:

I guess it depends if the gauging device is located on the fuel line in a warm spot (near engine) or at the tank.

I guess it also depends on how the particular gauging device measures the actual fuel flow too? The mass of fuel passing through the line doesnt change but the volume would?

My ignorance is showing :O It certainly wouldnt be the major contributor but could be an additional factor.

To get back to the question - What causes fuel flow to decrease with altitude for a given torque setting in a turboshaft powered helicopter?

At a given torque setting on a free shaft engine, the Ng (or whichever particular term is used) will increase with altitude and the engine will be operating at closer to its optimum design with respect to SFC. Most FCU's these days account for fuel SG and the FCU deals with Wf (Weight of Fuel)

Check out the info here - RR Allison C30 for an example.

As you can see the SFC (lbs/shp/hr)varies from 60% Cruise 0.719 to 0.588 at OEI 2.5 minute. This pretty much average for most turbines that are out there. TM's, RRA's, PWC's etc. A GE CT7/T700 has an SFC down in the 0.450's or so. 30-40% better SFC does make a difference.

Max range and fuel flow
 

New member to the forum, hope this makes it to the originator. Great refresher on the engine aspects, think I found the right place to read up on helo issues. One other aspect about constant power settings and fuel flow is the performance of the rotor system itself and the variable weights of the helo. If the desire is to make the most out of your fuel, you must use a combo of the two aspects, engine performance and rotor efficiency. Max range in a helo can be deceiving in concept since it may only be a gain of 10 miles by flying the more conservative power setting. Either way, when the helo is heavy, it is better to fly lower where the rotor blades can be more efficient with denser air, but as you burn off that fuel, the engines won't have to work as hard to keep Nr 100% and you can climb to take advantage of the very principals you brought up. If you have to perform a mission at the max range of your helo, go low on the way there, return high on the way back...works everytime (unless there are unfavorable winds aloft)

Sorry Phrogman, but I disagree with at least part of your statement.
There is an optimum altitude for each weight, in a no-wind situation. Unfortunately, the civil flight manuals won't show you that in the approved performance section, and you'll have to do a bit of digging the 'manufacturer's data' to find range charts. And then you'll have to add the wind into the equation.
You're generally right that you need to climb as weight decreases to get maximum range, but the general statement of go low on the way out and high on the way back is overly simplistic - what if you weigh more on the return trip???

Concur with you Shawn, it was very simplistic, intent was to bring in another element I did not see being discussed. If I burn 600 lbs/hr on the way out and then pick up 3 people (conservative 200 each), nothing has changed an hour later. Point well taken though, thanks.

Shawn, isn't varying altitude with weight another justification for designing in variable Nr (over a limited range)? FADEC already has an altitude (static pressure) input, and weight could be seen as a lowpass filtered input on torque demanded. As long as Nr selected allows enough pitch margin for manouvres.

My take on fuel flow is that the engine is designed with a maximum temperature in mind for the turbine entry stage. At higher altitudes the cooler inlet air allows a higher temperature differential between combustion chamber and ambient, hence a higher pressure ratio, hence a higher thermodynamic efficiency. I imagine that turbine blades are operating at similar AOA - does N1 change with altitude for a given torque setting?


A brief bit of googling on the subject, although too detailed really:

http://en.wikipedia.org/wiki/Brayton_cycle
http://www.grc.nasa.gov/WWW/K-12/airplane/brayton.html
http://ocw.mit.edu/ans7870/16/16.uni.../chapter_5.htm

Some fun stuff if you are a sad engineer, who doesn't fly as often as you would like:
http://www.faculty.virginia.edu/riba...hermodynamics/
http://thermo.sdsu.edu/testcenter/te...r08/index.html

Ooooooohhhh Yes a very good question ...

I guess all depends on the aircraft and the design compromise ....

I have always wanted to experiment with this properly ...however I havn't been allowed to play with a suitable aircraft.

I want a machine with a good GPS with air data computer AND equiped with a fuel flow meter .... having said that ....

I would imagine the ideal would be to fly the machine at the highest altitude commensurate with the highest N1, best TAS, lowest fuel flow within the confines of its maximum continuous power (ie highest Tq allowable).

:hmm::hmm:

In my experience TOT is the limiting factor as it increases with increasing altitude due to less cooling resulting from lower overall mass flow.

Bob

All good points. I've hardly ever seen an engine that is TOT limited in cruise, except in very hot climates, and even then there is a slight decrease in OAT with altitude, or at least there should be....

On long overwater SAR flights in the S-76 (E. Asia) we experimented with cruise altitudes, speeds etc as we were quite severely WAT limited for fuel load for most of the year. I found that a low level departure (which suited ATC) followed by a gentle cruise climb to about 6,000 ft worked very well. The R-Nav had a specific fuel consumption page (showing nm/lb of fuel), which was extremely useful to watch. A cruise descent was made towards the "scene" of the job. The same on the way home.

Prouty later put an article in Rotor & Wing, advocating exactly that - his calculations and our trial and error method tied up nicely.

We once managed 219 nm offshore and back to an injured sailor but had to find a "secret" chinese oil rig to give us some fuel for that one.

You'd be limited from obtaining the full potential from a 76 by the Vne limitation, wouldn't you? I recall it became quite restrictive above 4000 ft. I ferried a 76 A++ from Redhill to Lagos once, and wanted to use warranty cruise power during the high level Africa legs, but couldn't because of this.

With the EC-155 the fuel flow is fairly constant at MCP until you get to about 5000 ft, when the transition from Tq limiting to N1 limiting occurs, then it falls away dramatically. 330 kg/hr at SL drops to about 280 kg/hr at 8000 ft. I can recall the endurance reading about 2:55 on take off, with full fuel (1000 kg,) then showing 3:30 at top of climb! Combined with a significant increase in TAS with altitude, you could really stretch the range.

I've seen the very same Tq to N1 transition in BH206s and BH407s at roughly the same altitude. Almost always starting at MGTOW on 1-2 hour legs, what works for me is the gentle climb ShyTorque mentions until reaching a temperature or Vne limit then reducing power as necessary. Roughly 6000' works well as a practical enroute altitude finishing with the same cruise descent.

Bob

I take on board the comments about greater effeciencies with altitude but in the real world, the wind has usually by far the greatest influence upon performance. A 15kt wind on the surface will often be 30kts+ aloft. That's a 10% performance advantage/disadvantage for a 150kt helicopter; and there aren't too many that can achieve that. Of course, wind direction and it's associated changes with altitude is also a major factor to be considered.

Shawn

The A109A and C series often have TOT limited cruise over here in the UK winter. Generally as a result of the need to turn on the air-bleeds (anti-icing and cabin heat). A little bit off-topic but another factor to consider in performance planning in the real world.


JJ

Jellycopter - thanks for the info. You're right about performance planning being more complicated that it first appears.
The folks who set the cross-country record from NY to LA recently in an A109S flew the helicopter from Italy to the USA. They regularly flew at altitudes up to 11,000 feet, and got phenomenal fuel consumption.
I recall one trip in a Bell 212 at 10,000' where the fuel flow was down to under 500 pph, where it was normally +600pph when we were lower.

I think the S76 was what the OP was referencing. The first limit at higher altitudes is often Vne, not any engine or transmission limit, so you are forced to operate at reduced power settings. This can account for at least some of the lower fuel requirements. At 6,000 ft, you have to bring the power way down to stay below Vne.

The original question was about fuel flow reduction with constant "power" but a change in altitude. That's really just a theoretical question, good for understanding but not as applicable as what is being discussed now.

All the performance chart information, wind calculations, practical experience, and rumours just can't replace a fuel flow gauge and groundspeed. Fuel flow divided by groundspeed tells you how much fuel you use to travel 1nm. Find the speed that minimizes fuel/nm and you've got the best range speed, for the conditions you're flying in (i.e. wind, altitude, bleeds, etc.).

Of course, climb schedules won't jump out of that calculation, but I'd recommend the AFM for your climb schedule. In fact AFM will probably be validated if you use the trial and error method of minimizing fuel/nm.

If you don't have the effect of wind in your max range performance charts, then speeding up in a headwind and slowing down with a tailwind will improve your range. The question is how much? Rule of thumb is 1/2 the head/tail wind component, but you'd need power required curves to properly calculate it on the ground.

I'll probably generate some discussion with this point, but for most helicopters, the max range speed occurs around the speed where tip effects start to influence your power required curve. (Tip effects are where the speed of the advancing tip is fast enough that compressibility of air becomes a concern). Of course airframe limitations may not allow you to reach that speed, in which case Vne would be your max range speed, or whichever speed max continuous power gives you.

Matthew.

Jellycopter, The R-Nav fuel page takes into account the wind, as it works off groundspeed. These days, even with more regulated airspace above, I often change altitude, as much as is reasonable, to gain the odd knot or two of groundspeed.

Gomer,
Yes, but on the other hand you have the IAS/TAS relationship to your advantage with increasing altitude so it's not as bad as it might first appear from "raw" numbers on the Vne placard. :)