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Constant Speed Prop and failure modes.

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budyerly(at)msn.com
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Posted: Thu Mar 19, 2009 2:29 pm Post subject: Constant Speed Prop and failure modes.

<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]--> It has been great reading your experiences with the Woodcomp.

  

 It is possible for most electric props to put you in a position  where a failure in the electrical circuit can cause what they used to call  a "runaway prop" situation.  The point where the fine pitch stop is reached  by the controller or manual settings or a failure of one or both.  

  

 My experience with the Airmaster prop is being cited here...

  

 The fine mechanical stop on the Airmaster is typically 10 degrees of  pitch.  For a 914 equipped aircraft, this means you can achieve 5800 RPM  with very little throttle movement, because the prop is absorbing almost no  torque.  This can put the aircraft in a position where you are producing  insufficient thrust to continue flying in anything but slow flight.  This  failure is well spelled out in the Airmaster propeller operating  handbook.

  

 One owner had this malfunction because of a self inflicted switch  error by going manual operation thinking he was in automatic  operation.  He took off and flew.  When coming in to land in cruise  setting, he  reduced the power to idle and the prop controller  commanded fine pitch, (The prop will adjust the pitch in auto to try to get the  RPM up to the 5000 rpm, the cruise setting commands, which means the prop went  to the fine stop).  As he approached the field he set the prop controller  to Takeoff and inadvertently hit the manual switch.  When he added power,  the rpm shot to 5800 in a heartbeat.  The prop did as commanded, nothing,  as it is in manual.  Even at 5800 rpm the plane would only barely stay  airborne.  He luckily set down and called me.  We went over the  switches and behold he found the problem. 

  

 If an electric prop controller fails, especially at low altitude, a  fine pitch/runaway prop condition puts you in a position, where only your skill  and cunning, cool airmanship, and knowledge of your propeller and aircraft can  save you and or your plane. (Good Job Frans!)

  

 The Airmaster has a dedicated manual control which bypasses everything to  give you electric pitch control and a mechanical stop.  Propellers,  such as the Woodcomp, with Beta (reversing) do not have mechanical stops, only  electric.  A good manufacturer will insure the prop has a double  backup and clear instructions on operations because the prop limit switch  or controller failure can cause the prop to go into reverse or bypass an  electrical stop, causing an over rev or worst case, reverse thrust in  flight.  

  

 A number of aircraft were lost in the 30's because of runaway props.   Consequently reputable manufactures make sure they have manual backups and  mechanical stops to prevent aircraft loss.

  

 Thanks for your running comments because we all could use a review of our  prop manuals.

  

 Bud Yerly 

   [quote][b]

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josok-e(at)ukolo.fi
Guest

Posted: Thu Mar 19, 2009 5:13 pm Post subject: Constant Speed Prop and failure modes.

Thanks for the clear story Bud,



[quote:022a4d2f87]The Airmaster has a dedicated manual control which bypasses everything to give you electric pitch control and a mechanical stop. Propellers, such as the Woodcomp, with Beta (reversing) do not have mechanical stops, only electric. [/quote:022a4d2f87]

Just one correction: The SR 3000 does not have mechanical stops, not on any non-reversable or feathering either. 



[quote:022a4d2f87]A number of aircraft were lost in the 30's because of runaway props. Consequently reputable manufactures make sure they have manual backups and mechanical stops to prevent aircraft loss.[/quote:022a4d2f87]



And i might add that manual backup does not help if the pitch motor belongs in a toy car.



Regards,



Jos Okhuijsen

Visit -  www.EuropaOwners.org

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budyerly(at)msn.com
Guest

Posted: Thu Mar 19, 2009 8:00 pm Post subject: Constant Speed Prop and failure modes.

<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]-->  Jos,

 I typed Frans instead of you Jos for the congrats....No excuse...In a  rush to another Eagle Scout Board of Review and didn't proof my work.  Too  many irons in the fire today.

  

 Sorry, I was only generally commenting on electric reverse pitch props, and  did not intend to segregate the SR3000.   Beta control  props normally do not have a mechanical fine pitch stop (without a lot  of mechanical do dads and overrides) because the prop must go from fine to flat  to negative to operate.   In my Air Force career I never flew  C-130's but did see that they have quite a system of hydro-mechanical and  operational restrictions on reverse thrust for safety.  A great deal of  operational and emergency training covered all the failure modes.  I wish  our light plane industry did the same.

  

 Re reading my post, I did not intend to say any particular prop  manufacturer is more reputable than any other.  I believe they all try to  give the best product they can for a competitive price.  Buyer be ware is  generally not a factor in aviation, as nearly all manufacturers are sensitive to  feedback on their products and their clients safety.

  

 As far as the motors in an electric prop, it does amaze me how little power  it takes to move the prop blade given the mechanical advantage.  During a  troubleshooting test, I have run a 7 amp test battery completely flat running  the Airmaster from fine to feather and back to test how long it takes for a  feathered gliding position to fine pitch power transition and to try and see how  quick the prop can respond from the fine stop at 10 degrees min to the  normal full power pitch of about 19 degrees (depending on blade length)   and the motor never got hot or squawked, and neither did the  controller.   If you're smooth on the throttle and  don't jamb  the throttle from idle to full, it keeps pace nicely.  

  

 For the guys looking for props, especially when they have never flown  constant speed props, we need to share our experiences.  But for those  searching for info, they need to understand the operational and emergency  response necessary for the eventual equipment malfunction,  then through personal training and emergency procedure training  keep their skills up in the event of failure.  Of course I'd rather be  lucky than good any day, but you can't always trust to luck.

  

 Hydraulic props are not without their problems either...  But that is  another subject for another day.  

  

 Good night,

  

 Bud

  

  

 [quote]   ---

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nwcmc(at)tiscali.co.uk
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Posted: Fri Mar 20, 2009 3:49 am Post subject: Constant Speed Prop and failure modes.

It should also be remembered that a prop stuck in a coarse setting can 

be an issue as well. To this end it is worth checking that the aircraft 

can climb away safely from a go-around at short finals at low speed in 

this condition.



I flew C-130’s for 9 years and without doubt the propellers with their 

control systems were studied more closely than any other technical 

aspect of the aircraft. On this aircraft there are 6 different safety 

features that are designed to limit the rpm of the engine. This was 

particularly important for an engine/prop combination that was designed 

to run at constant speed. On the RAF C-130’s the engines produced about 

5000shp each and the propeller angle could reach 55 degrees in the 

cruise and keeping the rpm within the 98-102%rpm limit needed some high 

quality engineering. The incident below whilst not relevant to the 

Europa might make interesting reading.



An Orion aircraft (which uses the same engine/prop combination as the 

C-130) had a very testing incident which makes one aware that, even 

after many years of trouble free operation, it is possible to get into 

a situation where a prop can bite. In this case the crew was flying in 

a naval military exercise many miles offshore. This entailed a long 

transit at high level, a significant amount of time of slow flight at 

low level, followed by another high level transit back to their base. 

The problem came when a fault in one of the propeller control systems 

caused an outboard prop to pitch lock during the low level phase of the 

flight. This is one of the safety features to prevent the prop going 

too fine. However because of the power/altitude/speed combination the 

prop was stuck in low pitch. This isn’t normally a problem as the 

engine speed can be contained by continuing to fly at a low TAS. It 

still takes the total attention of one crew member using the throttle 

to keep the rpm within limits. However in this case, to be able to 

reach the nearest airfield, the aircraft needed to climb as it had 

insufficient range at low altitude. The crew was faced with the choice 

of a premeditated ditching or to intentionally increase the TAS by 

climbing and accept the risk catastrophic failure as the prop was 

forced into an overspeed situation. They decided to choose the latter. 

The highest rpm that the safety features can normally cope with is 

108%. As they climbed the prop rpm increased until finally the engine 

and propeller broke away from their mountings. Afterwards this was 

calculated to have happened at about 170%rpm. Fortunately the 

engine/prop combination fell away without damaging the aircraft. The 

crew managed to continue their flight to a safe landing. 



When William Mills first flew into my strip with the first Woodcomp 

prop with beta range capability I had a chat with him about the dangers 

of props that can operate in the beta range. I thought it a risky 

feature which was totally unnecessary on the Europa. As Jos found, even 

in forward fine pitch the situation can very quickly become 

uncontrollable should something go wrong. 



I would suggest that owners of Europas with VP props may like to check 

that the settings for the mechanical stops for both low and high pitch 

on their propellers enable safe low speed flight.



Nigel Charles



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