Why not "Zero Thrust" in simulated SE failure?

[justanotherflyer]

A stopped prop produces less drag than a windmilling one. Setting "zero thrust" simulates that situation for practice engine failures in ME aircraft.

Shouldn't the same principle apply in singles? Would it give a more accurate impression of performance when the donkey actually stops?

[eyeinthesky]

Unless you have a fairly catastrophic mechanical failure, such as a seizure, it is unlikely that a fixed-pitch prop will actually stop. Therefore a windmilling prop is a better simulation. Not sure about a VP prop, however. I imagine a loss of oil pressure associated with engine failure will mean it goes to the coarse stop under spring pressure and there's nothing you can do about it (this of course depends upon the way the CSU system is organised).

You can practice in a fixed pitch prop by going up to altitude, pulling the mixture and reducing airspeed until the prop stops. You can compare RoD with that with a windmilling prop. Make sure you have sufficient height left to restart and/or a suitable landing place in mind should it fail to restart.

[Send Clowns]

You can stop the prop if you need slightly better range from a decent height. Open the throttle fully and bring speed back nearly to the stall, the prop should stop, the glide will be slightly better. Hard to practice though!

[IRRenewal]

A stopped prop produces less drag than a windmilling one. Setting "zero thrust" simulates that situation for practice engine failures in ME aircraft.

Because a stopped prop on a MEP aircraft would be feathered and produce les drag than a windmilling prop. Selecting a low power setting simulates this lower drag situation.

Single engine piston training aircraft don't have feathering props (at least, I don't know of any). Therefore the zero thrust thingy doesn't apply.

[QSK?]

eyeinthesky

A VP prop in a single engined aircraft will default to fine pitch in the event of a propeller governor failure, whereas in ME aircraft the props default to coarse pitch.

By defaulting to fine pitch, the SE aircraft with a VP prop then acts like a SE fixed-pitch prop aircraft.

I think that's right, isn't it? Anybody? (otherwise I'd better go and redo my BFR).

[eyeinthesky]

QSK

Thanks for that. I wasn't sure which way it went for single engined, hence the caveat in my post!

Whatever, it seems a bit of a non-starter as a concept due to the inability to feather a single prop (non-turbine, that is!).

[QSK?]

eyeinthesky

Yeah, have to agree with you. I, too, have always wondered why a pilot cannot feather a prop on a SE aircraft.

When I first did by CSU/Retractable endorsement on a SE aircraft, the instructor indicated that, if I was sure I was unable to restart the engine, I should pull the prop lever right back to coarse pitch to minimise the drag from the windmilling prop.

However, this technique is never mentioned in the emergency procedures within the POHs for the VP aircraft that I fly and I have never figured out why.

[ROB-x38]

A variable pitch prop left to its own devices will move into fine pitch due to the centrifugal twisting moment (CTM). Most S/E aircraft use oil pressure in the hub to increase the blade angle.

With a loss of oil pressure in a S/E aircraft the prop will default to fine pitch, and hopefully you'll be able to generate a bit of power to limp to a field.

Multi-engine aircraft use counterwights or springs so that with the with the loss of oil pressure they default to coarse pitch and feather - minimum drag.

[QSK?]

Thanks for the explanation ROB-x38, but is it technically possible for SE aircraft to be designed with VP props that can also go into feather?

I can understand going to fine pitch in the event of governor failure, but why can't a SE pilot also have the option of feathering the prop if the engine stops for other reasons?

Cheers QSK?

[Tinstaafl]

In short, because the extra mechanical complexity isn't worth it.

A brief description of the systems:

When rotating a prop. has a centrifugal twisting moment (CTM) trying to twist the blades towards a flatter angle ie fine pitch. It also has an aerodynamic twisting moment (ATM) trying to twist the blade(s) to coarse pitch. Unless overridden by some mechanism or other the CTM overcomes the ATM leading to an overall tendency to move to fine pitch.

In a s/e VP/CSU setup this is advantageous. Oil pressure fed to the prop hub is used to oppose this movement to force the the blades to a coarser pitch. If anything should happen to the oil control or governing mechanism then at least available power will not be limited by a severely constrained RPM.

If there's an engine failure then the only choice in a single is 'down'. Having a feathering prop only changes the situation by 'how steep'. There isn't an option to try to continue flight**. With or without feathering the only option is to choose a nearby paddock.

** Unless it's a 'mild' partial engine failure in which case fine pitch is the most desirable anyway.

In a multi setup there is an option to try to continue if an engine fails. The limitation is that any continued flight ability is marginal. In this case it's worth the extra complexity & weight.

To achieve this the overwhelming CTM effect needs to be overidden so that if worst comes to worst the blades would move towards coarse, ultimately ending up feathered.

The system commonly used in light twins uses forces produces by some combination of:

* gas pressure in the prop hub

* angled weights on the blade root

* strong springs in the prop hub

These all act in concert with the previously mentioned ATM to cause the blades to tend towards coarse pitch. Oil pressure is now used to drive the blades to fine pitch.

In the event of a need to feather then removal of oil pressure such as when the engine stops leaves the blades with nothing opposing their being shoved towards feather.

There's catch though: What to do when you want to shut down after flight? The loss of oil pressure as you stop the engine would leave it with a feathered prop. Not good for the next engine start.

So, to counter this there must be latches in the hub that activate to prevent the blades going into feather below a certain RPM - typically 800 or so. Now the whole shebang needs yet another control to prevent the feathering latches getting in the way when you wish to feather. In light multis this is usually integral to the prop RPM lever, although it could also be a separate switch/knob/button/lever etc.

All this adds cost & complexity. In a single it's just not worth it for the sake of maybe choosing a paddock a bit further on.

[QSK?]

Tinstaafl

Thanks for the detailed response. Much appreciated mate.

Cheers QSK?

[nosehair]

...aaahh, the curious mind, it's a wonderful thing.
I too, have wondered about this "windmilling prop" during simulated engine failure training.
I now understand why singles are not made with feathering props - too heavy and expensive - but VP props should be pulled to coarse pitch for minimum drag and most forward lift, or "pull".
The training in this technique is just not there in our systematic approach to emergency training. Simulated engine failure in singles is introduced, usually, in fixed-pitch singles, and current philosophy is to not actually kill the engine with mixture so that the prop can be stopped by slowing to near stall speeds. This works and I personally encourage it - safely within gliding range of a runway of course - but most people don't, and that is my point. We don't teach stopping the prop for best glide distance.
However, as my experience tells me now, the problem with most pilot trainees is not getting the maximum distance, but getting rid of excess altitude and airspeed. Most of the training I do on forced landings deals with the student coming in a "little high-and-fast" as it should be on a forced landing, and then, when making the field is assured, proper use of slips and flaps to get the airplane down into the field in that last 3 or 4 hundred feet.

[Flyin'Dutch']

When training for emergencies the aim is for them to learn a process which will work time and time again.

If you teach them that they can glide further by stopping the prop, which may or may not work, you engrain in them an action which will not work when things are most likely to go wrong. The EFATO scenario.

Yup, no doubt if you have time and altitude you can worry about gliding a bit better if you stop the prop.

The marginal increase in performance which can be achieved does not outweigh the risk of making this issue even more complicated.

Let's face it, how many times have you been impressed by someone doing an unexpected PFL?

FD

[justanotherflyer]

Thanks for a very enlightening discussion, everyone.

Nosehair's observation, that the more usual student problem is that of shedding speed and height close to landing, is certainly true to my own experience.

On a bit of a tangent, I'd appreciate some elaboration of FlyinDutch's reference to the EFATO training scenario being unrealistic.

[Flyin'Dutch']

On a bit of a tangent, I'd appreciate some elaboration of FlyinDutch's reference to the EFATO training scenario being unrealistic.

Can't see I wrote that but if I did not convey myself very clearly apologies.

What I said was that people need to be trained so that they can handle an engine failure off pat. To sex it up by teaching them to do some additional bits which may or may not stop the prop and by doing so gain a bit extra in the glide which follows would to me, seem to be counterproductive.

What you want is that people without any hesitation can execute those skills necessary to set up safely for an engine failure in the minimum of time, cause that is what matters after an engine failure. Especially an EFATO.

Sure enough if at some point you want to talk about what you can do extra if you have height and therefore time to make things even better great. However judging by people's basic flying skills there seems to be plenty of room for improvement in those before embarking on more esoteric forms of FL techniques.

FD

[justanotherflyer]

FD

Thanks for the further elucidation, I misread your previous post.