View Full Version : Turbojet RPM Gauge Readout

19th Oct 2005, 18:06
In older jet aircraft such as the DH Comet with its non-bypass Avon turbojets, would the engine RPM gauge read out actual engine RPM or a Corrected RPM figure?

I'm afraid I don't understand the subject enough to work it out for myself!

Can anyone set me straight?



L Peacock
19th Oct 2005, 18:46
Can't answer with authority but just wondered why the lack of bypass would change the way RPM was presented.
Single or twin spool, aren't N1/N2/Nh/NL whatever still the same, regardless of bypass ratio?
EPR would be another subject.

Only A Few More Seasons
20th Oct 2005, 01:25
As far as I can recall the Avon as fitted to the Comet were expresed in RPM I seem to remember 12k being about max - Now we come to the Spey as fitted to the 1-11 100% HP rpm (high pressure turbine speed) this should have been full power - but oh no they started blasting demin water into the thing then (as far as I can remember ) full power then became 107.9% - you must remember there was no A/T in those days

The Comet also had a thing called a thrust lost indicator for each eng which under most curcumstances was the governing factor a to whether or not to continue the T/O if the needle was not at 12oclock when the throttles were opened.

All this made the British A/C industry what it is today!!

Swedish Steve
20th Oct 2005, 11:15
N speed indicators present actual N speed. The max speed is not always 100%, can go from 90% to 110% depends on engine and spool.
On the older British engines the gauge was corrected by a setting knob by the crew so that it always showed 100% on take off.

20th Oct 2005, 15:09
Thank you all for your comments.

As I understand it the RPM is in effect an N2 reading. On the Comet 4 the max RPM was 8050 and climb setting 7300-7400 RPM.

If one maintained a constant throttle setting, would the RPM gauge show a change (decline?) with altitude - thereby requiring adjustment of the throttle to maintain desired climb RPM?


3 Point
20th Oct 2005, 17:44

A jet engine burns a lot less fuel at altitude than it does at sea level (this is because, as the altitude increases and the density reduces and so the mass flow through the engine reduces). In order to maintain steady state the fuel flow must reduce in proportion as the aircraft climbs.

In a simple (ie old) fuel control system the throttle controls fuel flow directly and so, as the aircraft climbs there is progressively too much fuel and the throttle must be reduced to prevent an increase in RPM. In a more modern engine there is provision in the fuel control system to compensate automatically for the reducing air density by making a consequent reduction in fuel flow and thus the RPM will be relatively stable.

A modern FADEC (Full Authority Digital Engine Control, ie a computer is in control) will of course adjust fuel flow to maintain RPM, EGT, EPR and a whole host of other parameters in the desired range.

Simplistic I know and I'm sure there will be many who will give a more detailed explanation but good enought for government work!

Early engines (de Haviland Ghost, RR Avon) had RPM gauges showing actual RPM of the rotor (there was only one); later, (RR Viper, Pegasus), %RPM gauges came to the fore; some display the N1, some the N2, in some there are possibilities to display both. A modern system (eg Airbus 320 with IAE V2500 engines) displays N1, N2, EPR, N1 vibration, N2 Vibration, EGT, Oil pressure and temerature, fuel flow and plenty more besides.

Rolls Royce publish a very good book on jet engines, it's called "the Jet engine", ISBN 0 902121 04 9. Don't know if it's still in print.

Hope this helps.

3 Point

20th Oct 2005, 21:03
The cockpit gage always shows the mechanical speed, not corrected (i.e. performance-related) speed - because the certification authorities want you to know how close you are to the max certified rotor speed. (Today that's not much of a concern to pilots; most engines have a triple-redundant overspeed protection system.)

BUT - in the very early days the gage expressed actual rpm like a recip engine. Sometime in the early 50's it became standard to express this as % rather than real rpm, and that's how it is today.

21st Oct 2005, 11:08
3 Point

Thanks for that. I hope I'm getting the hang of this.
As the turbojet engine uses less fuel as it climbs, for a given set RPM (adjusted by throttle as you say) does the engine also produce less thrust as it climbs?

Barit 1

Very clearly put! Thanks.


21st Oct 2005, 14:02
As the air density decreases, the fuel/air ratio to maintain rpm remains about constant, and so the fuel control (be it hydromechanical or electronic) gradually trims back fuel to hold rpm.

Thus with lower mass flow (lower density air) the thrust of course decreases. Usually the most critical point in the transport mission (thrust required compared to thrust available) is at top of climb.

3 Point
21st Oct 2005, 22:50

It's as barit says, as the air density reduces the fuel must reduce to maintain the correct ratio and, as the thrust is in proportion to mass flow through the engine that reduces too. Modern engines handle the requirement to reduce fuel flow automatically but in early jet engines the pilot has to monitor RPM and reduce the throttle setting to maintain it.

Happy landings

3 point

22nd Oct 2005, 11:24
Well thank you both. Very clear!

I can now get on with my "project" - but I may return when I uncover the next factor beyond my grasp....


16th Nov 2005, 10:48
Hi, could one of you help me with another question on this topic, please?

As an example the DH Comet 4 used a climb setting of 7350 RPM for its Avon turbojets.(8050 max RPM)

Would the crew adjust the throttles to maintain this read-out on the (uncorrected) mechanical RPM gauge, or would they use a calculated Corrected RPM (CN2) as the benchmark to be maintained during the climb?

Does anyone know or remember?

Many thanks

18th Nov 2005, 01:56
most engines run off some kind of governor, ie you set an rpm, and get the desired effect. in the climb the governor should automatically adjust to maintain engine speed.

3 Point
21st Nov 2005, 20:07

I'm not a Comet pilot so I can't really say but rigpiggy is right as far as modern jet engines are concerned.

I do know about the DH Venom and Hawker Sea Hawk; in both of those you have to manually adjust the throttle to maintian climb RPM (incidentally this is indicated in actual RPM, not %).

Happy landings

3 Point

23rd Nov 2005, 05:42
does the engine also produce less thrust as it climbs?Less fuel will always produce less power so, yes, the thrust does decrease during the climb. most engines run off some kind of governor, ie you set an rpm, and get the desired effect. in the climb the governor should automatically adjust to maintain engine speed.On old turbines (i.e. before %age RPM gauges) the 'governor' was the pilot! There was no FCU to look after speed topping or surge protection and you didn't just have to watch the RPM, it was just as likely to be the Jet Pipe Temperature (JPT) that would limit the power setting. Advancing the throttle was often a journey into the unknown - especially on one of those clapped out old "Sno-blows" that the RAF conjured up out of time-ex engines in the sixties...

Later on, engines became more automated and RPM began to be measured as a percentage of maximum continuous RPM. It isn't just the RPM that creeps up during the climb. That excess fuel also increases the EGT - or JPT as it was more usually called back then. An electrical "Top Temp & Speed Control" system (TTSC) came into use on old Rolls engines like the Avon, providing a second line of defence after the the pilot and/or the FCU's centrifugal governor. This TTSC system measured both RPM and JPT and backed off the fuel if either approached the preset limit.