Transient and static droop
 

Hi there, i am just trying to find a good explanation of transient and static droop.

droops...
 

Transient droop is the reduction in rotor speed following a sudden change in power demand (increase), for instance recovery from an autorotation will produce for a short (transient) time an underswing in rotor speed below the nominal governed value before the engine governing system reacts and recovers the Nr to nominal.

Static droop is a function of a non isochronously governed rotor system where the nominal rotor speed reduces as collective pitch increases. The Lynx for example, when fitted with the Gem exhibits a 'droop line' characteristic with a lower Nr at higher collective, this seemed to be a fucntion of the control system technology available for that generation of engine. FADECS have eliminated static droop in most cases.

The other type of static droop is where the power demand exceeds the power available (such as when an engine limit is reached) in a steady state case and the rotor doops for that fixed collective setting. This may be caused by either engine or transmission limiters working. In some cases this limit may be 'soft' and when a certain % of droop is experienced (for instance when you REALLY need the power) the limiters disappear and rotor speed will recover

hope that helps

DM

Ronnie, Static droop is the RRPM that you get when you apply power to a rotor system;ie, on the ground with ,say 100% set and collective fully down,Tq, say 40%; now as you increase tq in 10% steps the RRPM may `droop` to 99.9,99.8 ,99.7%,etc, up to 100%tq. this can be plotted on a graph and will give you a line; most RRPM gauges are somewhat inaccurate so the gauge should have been calibrated ,or you use a data recorder.That `line` will give you the response of the governor on the engine,be it hydromechanical ,or electronic(FADEC),and should follow the engine manufacturers set-up; if not the governor needs to be adjusted.; that is the Static Droop line.
Transient droop is the RRPM that you get when you apply a rapid collective input/up/down,ie, if you apply say 70% tq in one second the RRPM may droop to say,98%,apply 90% and it droops to 96% and in each case should recover to the static figure. This gives the dynamic response of the governor system. These tests are normally carried out with the helo on a tie-down to get the system sorted and adjusted; and if you mave multiple engines they should all follow the same pattern- not always true as in service ,engines can be changed,components too.
Having sorted it on he ground ,we then go and fly it ,and then do the same tests again,only now ,we also do it from /into autorotation,and one has to be careful because we may overspeed the turbine as we get to zero Tq,or damage the freewheel units if we don`t `join the needles` before applying power.
The `transient` droop checks are important because if you have a sluggish governor,you can well droop the engine to the point that it `surges`,ie it is getting too much fuel ,but not enough air and it will `cough,splutter` may flame-out,overtemp,.
This also depends on the type of compressor in the engine.
Further,there is also `droop cancelling` which means that if we apply power, the RRPM will rise,instead of fall.
Also , on earlier helos,an `anticipator` is fitted in the system which basically measures the rate at which the collective lever is being moved up/down,and by how much; this sends a `short-cut to the governor to either increase /decrease fuel flow,before the RRPM has time to respond,thus `anticipating `the response.
Best helos for engine response,before the Fadec systems ,were the Gazelle,and Scout/Wasp,where you could pull full power in less than half a second,and would only surge if the governor was a bit worn,or the compressor was in need of a wash.. Syc

Hi Dangermouse. This has helped alot. do you know alot about the Lynx and the Gem engine? as i am trying to find out more info reference the lynx.

many thanks RB

Droop governing is characteristic of most turboshafts pre-1970. It's relatively easy to implement with hydromechanical controls of that era, and has the advantage of giving a fairly crude means of load sharing in multi-engine choppers.

A few engines had "droop compensation" - wherein the droop line was mechanically "nudged" through a link to the collective stick. (Pull more collective, and the droop line is repositioned to hold constant rpm.) GE's T64 in the H-53 series is an example.

For true isochronous governing, electronic controls (either analogue or FADEC) have been the only practical solution.

Transient droop can also be largely overcome with electronics - the black box senses the rate of collective movement, and applies power to attempt to "get ahead of" the load demand.

Turbine Engine Droop Governors
 

Here is a pretty thorough paper by someone at Woodward :

http://www.woodward.com/pdf/ic/26260.pdf

Most/all of the helicopter engines prior to recent FADEC* equipped engines operated with droop type governors, and the later ones had a droop line reset device, known by various names, such as collective bias etc.

* GE did implement an analog electronic control ( named PMS, for Power Management System ) on the T-58-5 for the SH-3D, which provided isochronous governing, and it worked well......when it worked. Unfortunately there were enough instances of failure at the wrong times so that a PMS-OFF switch was added to the SH-3D cyclic grip.

Thanks,
John Dixson

Minor correction - my notes show the PMS as an analog electronic overlay on the T58-GE-10 Navy engine, not the -5 USAF model.

T-58-10
 

You are correct. -5 was USAF HH-3E, -10 was the SH-3D.

Thanks,
John Dixson

Ronnie,

Try to get your hands on a copy of the book 'Helicopter Test & Evaluation' (Cooke & Fitzpatrick ISBN 0-632-05247-3). Expensive to buy but free from the library.

Pg 247- 257 gives a good explanation of governor operation and it's relationship to static and transient droop. The explanations are also closely modelled on the Lynx/ Gem.....as is alot of other stuff in the book.

Pm me for the chapter on turbine engines out of my book Cyclic and Collective that discusses the same thing.

Rotor Droop BH-206 BIII
 

Any suggestions are appreciated. Pilot and Pax, no problem. Add 2 more pax and still under gross weight, pull collective and get same to hover increase torque to less than 100% there is severe rotor droop. My sentiments were either governor or fuel control unit. Mechanic replaced governor and same condition exists. He will try new fuel control unit. If anyone has experienced this would appreciate a reply.

Have you had the Nr gauge calibrated,or are you `hearing` a lower RRPM than normal.? Don`t change/calibrate before the FCU change,only one at a time..

Is there a recent W&B check on the copter? You might be heavier than you think.

Also - what is core speed (Ng or Nh or N1) doing the static droop? Likewise turbine temp?

Rotor droop while less than full rated torque tells me the engine aero performance is down. Changing control bits is unlikely to help.

Borescope inspection is in order, methinks.

You have confirmed my conviction: fling-wing people talk scribble.

Hi barit1, I'm still struggling with this concept. When you say it's "relatively easy to implement"; How is this done? Looking at the internals of the B206 governor, I simply don't understand why it doesn't return to the previous RPM. (ignore the compensation for a minute). In my mind, after a load change, the PTG will let the FCU accelerate N1 until the PTG thrust bearing is back in the prior position. How can this happen at a lower RPM if the speeder spring hasn't change. Tearing my hair out!