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BMA Viscount Spool-up Behaviour

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BMA Viscount Spool-up Behaviour

Old 5th Feb 2008, 20:29
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BMA Viscount Spool-up Behaviour

As a lad standing at East Midlands perimeter in the 70s, I used to be fascinated at how the engine note of a Viscount (813/4/5) on the takeoff roll would rise and then fall slightly before rising again for takeoff. This would appear to be the onset of prop speed governing (somewhere around 1100rpm?) However: as a (slightly older) student doing turnround cleaning at the same airport in the late 80s watching the same aeroplanes (not so many Viscounts by then but remnants of the same 70s fleet) the engine note appeared to rise at takeoff no dip as I recall .

Is this right? if so why .

Did I also notice a change from 4-engined taxying to 2-engined taxying over the same period. What would have driven this? fuel conservation? noise reduction? engine life stretching ? air-quality around airports? ...

Thanks
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Old 6th Feb 2008, 11:49
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Viscount engine note..............

As an ex 815-er, and to put it simplistically, on any Dart if the power is introduced slowly and gently, when the prop starts to govern, it will drag down the engine rpm, as there is not a whole lot of power applied to overcome the effects of the first coarsening. A good fist of power at this point will result in a steady rpm as the prop absorbs the applied power. (If the power stays low, more hunting of rpm will result). A steady determined application of power in the first place will result in one slight decrease as the prop "catches", and a steady note thereafter.

Think of your car. As you let in the clutch, the rpm dies away as it is engaged, then accelerates with the car. If you increase the power as the clutch is engaged, you can prevent the rpm from dying down.

Hope that helps.

Last edited by ABUKABOY; 7th Feb 2008 at 16:13. Reason: Amend Viscount Mk.
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Old 6th Feb 2008, 12:13
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hmmm now yer asking

well, i worked for BMA at LHR 1977-85 so often we would have 5 or 6 in at once, they always taxied in on 2 engines, fuel issues were quite a big thing i recall.

re the t/o engine noise hmmmm i really am sorry i cant remember,
at LHR my office was on A9 on the ramp close to the runway i/s where often our VC8's would sneak in for a short take off BUT the noise was usually drowned by a trident taxying past!
i used to go to SEN alot in the 60's and 70's to watch channel airways vc8's too..

i obviously flew in BMA ones dozens of times both in the cabin and on the jump seat but i still dont recall an engine noise dip on t/o after initial power going on....

love to go on a viscount now sigh
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Old 6th Feb 2008, 17:28
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ABUKABOY - Thanks, I wondered whether rate of change of power demand had anything to do with it, though I would have expected it to be the other way around: i.e. if throttles were slammed forward, I'd expect the prop speed governor to struggle (and hence produce my 70's dip) especially if the demanded speed had changed ; for a slower throttle advance, I'd expect the governing to be much better.

Either way, do you think there might have been a change in BMA's operating procedures recommending a different throttle advance rate between the 70's and 80's ...?

Just curious. Cheers.
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Old 6th Feb 2008, 20:23
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Pink Floyd - Dark Side of The moon - On The Run, about 3 minutes in, that has just got to be a Viscount running up for takeoff
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Old 6th Feb 2008, 21:07
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Javelin - I think you're right - and what's more I reckon it's doing a little dip too !
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Old 6th Feb 2008, 22:22
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Same thing happens with a recip if throttle is opened briskly. The prop starts out on the low-pitch stop, and takes a second or two to "grab" the overshooting engine as it goes into governing.

If the throttle is opened in a more moderate fashion, the prop can respond quickly enough that there is no overshoot.
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Old 7th Feb 2008, 03:31
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If one listens very carefully, one can hear the water methanol start to flow during the takeoff, a very slight pitch (noise) change results.
Likewise with a large recip that uses ADI.

RollsRoyce Dart, a mighty fine engine.
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Old 7th Feb 2008, 07:49
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I think The Beatles "Back in the USSR" had a Viscount on the soundtrack too.
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Old 7th Feb 2008, 13:39
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A G1 went that way. Co-pilot hit the water meth after an engine failure on t/o.
Aircraft crashed inverted.
Also, as far as taxying a Viscount on two is concerned, I seem to recall that the 700 and the 800 had the hydraulic pumps reversed, outboard and inboard.
Sometimes caused confusion!
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Old 7th Feb 2008, 13:56
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Nearly caught out a few times taxiing out at LHR on two engines when cleared for an immediate intersection departure (we could hear the controllers sniggering in the background) The last engine started would go all the way from stationary up to takeoff power as we rolled with no apparent ill effects!
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Old 8th Feb 2008, 09:59
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Doppler Effect

I think you will find the simple answer is that the doppler effect makes it sound as if there are changes to engine / prop noise as the aircraft passes an observer.
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Old 8th Feb 2008, 10:16
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Sorry Paradism, as a former Dart (and Twin-Wasp) stoker I can't let your suggestion pass uncorrected. There's no Doppler effect when you are strapped in the cockpit ! The above guys know what they are talking about.

Chris
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Old 8th Feb 2008, 11:24
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Doppler

Thank you Chris, but if you read the first post you will note that the observer was not in the cockpit. I bow to your obviously greater knowledge in this matter. In the instance quoted, an outside observer would note a decrease in note as the aircraft passed him.her, that is straightforward physics. However, I agree it does not explain the increase in note as the aircraft lifts off.
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Old 8th Feb 2008, 16:10
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Paradism... ! I didn't want to be pedantic, but perhaps I needed to be.

Let me try to explain (this is the Tech Log forum, after all):

1) The rise and fall of the engine note that they are talking about is at the beginning of the take-off run, as the throttles are being advanced, not when the aeroplane is getting airborne.
2) On the Dart, the propeller is in ground-fine pitch initially (for taxying without overheating the engines, and to provide engine braking if the throttles were closed).This effectively stops the propeller coarsening its pitch enough to provide the thrust necessary for flight.
3) As the power comes on, the propeller spool and propeller therefore accelerate rapidly. At some point, the system recognises that take-off power is required, and removes the ground-fine-pitch stop. The pitch now coarsens suddenly, momentarily slowing the propeller spool (shaft) of the engine down until the rapidly-increasing power from the core spool wins the contest. [The Dart is a 2-spool engine.] From then on, the propeller continues to accelerate, even though its pitch may continue to coarsen.
4) The transition from ground-fine pitch to flight pitch - and vice-versa - is one of the key aspects that turbo-prop crews have to monitor, certainly on the Dart engine.
5) It is even more important on landing. On the Dart Herald, for example, a lever had to be pulled back when the aircraft was safely on the ground. This engaged ground-fine pitch, resulting in a momentary increase in propeller rpm and useful drag to slow you down. The Viscount may be slightly different, but the end product would be the same.
6) NB: Woe betide any Dart crew which fails to ensure that the props are in ground-fine pitch for taxying after landing. If not, opening the throttles on the taxiway may result in the TGT (turbine gas temp) going off the clock, and melted turbine blades.

Phew!

The sudden coarsening of propeller pitch in (3), above, is the cause of the rise and fall of the engine note, referred to by jh5speed. Lads who inhabit the perimeter fences at airports all over the world are more than familiar with the Doppler effect. I should know - having been one of them myself. So I've heard the Dart from the perimeter fence and the cockpit.

Hope you are now convinced!

PS: The Dart sounded great, but what about the whispering Proteus...
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Old 8th Feb 2008, 16:21
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Paradism - I assure you that this is not about the Doppler effect. What I (i.e. the lad at perimeter fence all those years ago) am talking about is the spool-up behaviour at the end of the runway before brakes-off. Looking at my first post - I guess I could have made this clearer. This is all about stationary aircraft.

The response so far have confirmed my suspicion that it is to do with rate of change power demand. I think the subtle change in note as the water/meth comes in is not the answer we are looking for - although I'm sure it is audible at close quarters.

The only remaining question is why things (apparently) changed in the 80's.

One thing occurs to me: were the 70's Viscount pilots those who had progressed from pistons (Argonauts in BMA's case) and tended to treat the power-levers in a different way to the 80's pilots who might have had different previous experience ? ...
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Old 8th Feb 2008, 16:36
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Chris - Thanks - you just got in before me:

Just to note the Dart is a single spool animal - prop is connected to the one (and only) shaft. 2 stage of compressor (centrifugal) and 2 turbine stages though - perhaps that's what you were thinking of?

My only recollection of a Proteus (thread drift; pull up!) is the rumble I used to hear from miles away when a Redcoat Britannia used to make an occasional visit to EMA. I always wondered whether 'Whispering' referred to the experience seated within, or watching it go by.

Cheers
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Old 8th Feb 2008, 17:27
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jh5speed.

True all Darts are single spool but I think the ones on the Viscount 810's were -525f's with three turbine stages. I worked on some back in the 70,s. Earlier Viscount 800,s had -515,s with two turbine stages.

When the throttle is advanced the Prop starts to govern at 11000 engine rpm and if the fuel is not being increased at a sufficient rate the speed will decay a bit.

As to Water Methanol, this only comes in at about 14500 rpm if the system is armed and operates at take off rpm (15000) until first power reduction to 14200 rpm. (or the W/M runs out).
Also the -520 series Dart, on Viscounts and F27s, W/M restores power at hight ambient temps so would not be used in the UK very often except on hot summer days, where as the -532 on the Dart Herald boosted power at all temps. so was used more often.

Hope this helps.
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Old 8th Feb 2008, 18:30
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Chris Scott

Many thanks, a most comprehensive and articulate explanation.
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Old 8th Feb 2008, 20:56
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Pink Floyd - Dark Side of The moon - On The Run, about 3 minutes in, that has just got to be a Viscount running up for takeoff

Thank you for adding second and third opinions to what I have thought ever since the album was released! Sad really!
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