Another hornets nest

[jigsaw]

Originally posted by hitech
The torque is exactly the same as if they were both on center line.
 

Torque still exists and still has a rolling moment. The difference is how it affects flight.

In a conventional plane if you lose the critical (left) engine, you're going to get a larger rolling effect from the right engine. This is due to torque rolling into the centerline, and the shorter arm to the end of the right wing being easier to lift.

Losing the right engine will create a lesser effect, as torque from the left engine is now in the opposite direction of centerline and has to lift most of the left wing, fuselage, and all of the right wing. However, it does help with Vmc because it wants to help "raise the dead" engine.

With counterrotating props, it's a moot point.

[hitech]

jigsaw: Your asesment of why is incorect. Loosing left or right eng makes no difference in the torque do to the engines. Both effect the plane exactly the same with respect to torque. Belive this first then continue to ask why your example happens, becaue the resone you stated is not why.

Now this is not saying in anyway that loosing left or right engine it is the same. The prop slip streams flow over the wing produces lift hence more lift one wing, hence more roll. Loosing one engine obviously produce a yaw do to off center line thrust. This yaw with any dihedral in the wing also produces roll.

But you realy must belive that torque effects with respect to roll are exatly the same no matter where you place the engine.


HiTech

[hitech]

GScholz:

Turbine spinning in one direction, air in the other. The forces that create these motions cancel each other out.

This would be true on a free flow of air, But what is spining the turbin to accelerate the air? If the torques match with the air spin and the turbin spin, We still need some torque to turn that turbin that would be transmited to the air frame. Im reasonable certain jets do not produce much if any torque. Therfore the air spin can not be equal torque on the turbin.

Therfore turbo props must produce torque.

btw GScholz: Im also trying to convince myslef of this.

[jigsaw]

Originally posted by hitech
jigsaw: Your asesment of why is incorect.  

I'll give you that my explanation is more detailed than what you'll typically hear, but if you think it incorrect, then do these things;

Grab a book: Multiengine Flying: Second Edition. Page 33. Read the first item listed under "What changes the airplane's minimum control speed."

Next time you're at the airport, ask around for a DE or MEI. Ask them to explain the P.A.S.T. acronym.
P: P-Factor
A: Accelerated slipstream
S: Spiraling slipstream
T: Torque

Let me know what they say.

Just for grins, here's a diagram.



edit-fixed link

[GScholz]

Originally posted by hitech
GScholz:

 

This would be true on a free flow of air, But what is spining the turbin to accelerate the air? If the torques match with the air spin and the turbin spin, We still need some torque to turn that turbin that would be transmited to the air frame. Im reasonable certain jets do not produce much if any torque. Therfore the air spin can not be equal torque on the turbin.

Therfore turbo props must produce torque.

btw GScholz: Im also trying to convince myslef of this.

I think I see where you're getting it wrong. The turbines do not accelerate the air. They bleed energy from the air. The air is accelerated by overpressure in the combustion chamber of the jet engine. The only reason there are turbines in a jet engine is that it needs to draw in air at the front; leaching energy from the rear turbines to operate the front turbines that feed the engine with air.

A ram-jet is theoretically identical to a turbo-jet, but does not have any moving parts at all. However it needs to be propelled up to speed before operating independently so the ram effect is feeding it with sufficient amounts of air. In all simplicity a turbo-jet engine is an air-breathing rocket that blows on a windmilling set of fans that helps it breathe.

A turbo-prop leeches energy from the turbines to turn the prop. In modern turbo-jets/props there is usually a large set of counter rotating turbines that incrementally slow the jet exhaust down by deflecting it with the turbine-blades. This generates an enormous amount of torque on the turbines to power either a big fan (turbo-fan) or a prop.




... ... ... at least that how I understand it works.

[Golfer]

there's more to it than that, GS.

The compressors are there to compress air, these are some of the turbine blades.  They work just like the engine in your car...

Suck Squeeze Boom Blow

Air enters engine inlet...suck
Compressors mush the air into combustion chamber...squeeze
Ignition takes place in combustion chamber...boom
Burning fuel/air mixture expands and goes out the back of engine...blow


Turboprops don't leech energy from the turbines to turn the prop, they're connected via a gearbox.  So the turbine speed is controlled by the throttles, the prop speeds are controlled by prop levers and there are also condition levers that regulate fuel flow into the turbine.


The turbine engines on turboprops aren't really big enough to power anything, they're essentially little APU motors.


Here is a short halfassed paper done on the Garrett engines for a B100 turboprop that I had to do to pass my 'Turbine Transition" ground course.  It's a short overview done well enough to only pass but it may help.



AVSC 4200






King Air B100 Powerplants


   The B100 uses Garrett TPE-331-6 Engines (everyone knows the –10’s an a 441 Conquest just ooze testosterone, but since I left my Conquest in my other pants I will stay on subject), which produce 715 Shaft horsepower each.  There are five primary fuel injector nozzles, along with ten secondary nozzles for startup.  100% N1 in the B100 is equal to 41,730 turbine rpm and 2000 prop rpm…that in anyone’s book is a whole lot of spinning.  The gearbox on the engine drives the Starter/Generator, fuel pumps, flow control unit, oil & scavenge pumps, tach generator and the propeller governor.
   A power and engine speed lever located between the two pilots controls the engines.  The power lever basically regulates how much fuel is going to the engine in order to produce the desired amount of power.  The engine speed lever sets the governors.  When the governors are engaged, they limit the power to a 65% minimum to a 97% maximum.  When they are in “propeller governing mode” the prop governor controls the engine speed.  This will keep engine speed within the 95-100% range.
   The fuel cutoff and feather levers do just that.  When selected, they shut off the fuel flow to the engine and feather the prop in case of an emergency.  Friction locks are standard and enable the levers to be secured in any position desired.
   Engine instruments on the panel include the ITT, Torque, Tach, Fuel Flow, and Oil temp & pressure gauges.  These instruments are powered by 28 volts of DC power with the exception of the Tach  (N1 Gauge), which is self-generating.

[jigsaw]

Originally posted by Golfer
AVSC 4200

UVSC?

[GScholz]

Originally posted by Golfer
there's more to it than that, GS.

Unquestionably. I'm just trying to work out the basics.

Originally posted by Golfer
The compressors are there to compress air, these are some of the turbine blades.  They work just like the engine in your car...

Suck Squeeze Boom Blow

Air enters engine inlet...suck
Compressors mush the air into combustion chamber...squeeze
Ignition takes place in combustion chamber...boom
Burning fuel/air mixture expands and goes out the back of engine...blow

= "air-breathing rocket that blows on a windmilling set of fans that helps it breathe." (just keeping it simple)

Edit: btw. unlike in a piston engine the "suck, squeeze, boom, blow" is not separate actions or "strokes". A jet is constantly sucking, squeezing, booming and blowing. If fact I would call it "burning" rather than "blowing". Burning like a rocket.

Originally posted by Golfer
Turboprops don't leech energy from the turbines to turn the prop, they're connected via a gearbox.  So the turbine speed is controlled by the throttles, the prop speeds are controlled by prop levers and there are also condition levers that regulate fuel flow into the turbine.

Umm ... if the props are connected to the turbines via a gearbox then ... they do leech energy from the turbines. Also, unless there is some sort of variable gearing the prop and turbine speeds are linked at a fixed ratio, so both changing prop pitch and fuel flow will affect prop/turbine speed.

Originally posted by Golfer
The turbine engines on turboprops aren't really big enough to power anything, they're essentially little APU motors.

Well, they better be big enough to power the prop.

[Golfer]

Umm ... if the props are connected to the turbines via a gearbox then ... they do leech energy from the turbines. Also, unless there is some sort of variable gearing the prop and turbine speeds are linked at a fixed ratio, so both changing prop pitch and fuel flow will affect prop/turbine speed.

I'd been going about this conversation under the assumption that there was a basic working knowledge of jet engines.

When you said "leeching power" from the turbines, it seemed as though you were trying to define Bleed Air, where air is taken from the compressor section to power airplane systems such as anti-ice and environmental systems.  

"Leeching power" and "directly connected" seemed to clash as definitions in my mind.

Well, they better be big enough to power the prop.

I meant if they were mounted as straight turbojet motors...they can spin a prop but they don't produce enough thrust to get my car fast enough to get a speeding ticket.

Umm ... if the props are connected to the turbines via a gearbox then ... they do leech energy from the turbines. Also, unless there is some sort of variable gearing the prop and turbine speeds are linked at a fixed ratio, so both changing prop pitch and fuel flow will affect prop/turbine speed.

Nope.  The prop pitch is controlled by a prop governer which regulates pitch of the blades and thus RPM.  So you move the throttle to create as much power as you want, and move the prop levers to the desired setting for noise reduction, better cruise ect ect.


This is the same as the airplanes in game.  You have throttle which controls Manifold Pressure, the prop control regulates prop rpm (+/-) keys.  The propellers use a governer to pump oil into the hub, and based on the position of flyweights, the settings of the set spring and a number of other things, the props regulate themselves to maintain a constant RPM.

In turboprops, if you have the prop lever full forward (max rpm) and pull the power levers back to idle, the prop turns into a big drag inducing disc because the pitch flattens out to maintain RPM and its just a big airbreak.

[hitech]

jigsaw: Read item 1 in the none counter rotating props. That is what I am describing. Torque does not change with engines on wings instead of center line.

Also in statments 2 and 3 it sais when the engine is lost the plane will tend to roll. What is causing this roll? because in the same statement they are saying torque is either working with or against that roll. Therefore it isn't the torque which has always been there but somthing else.

Once again that is exactly what I am describing about lift generated by slip stream.


So I am in complete agreement with that page.


HiTech

[GScholz]

Originally posted by Golfer
I'd been going about this conversation under the assumption that there was a basic working knowledge of jet engines.

Umm... will "I think so" do?

I'm basically trying to work out where all the forces of a turbo-prop go, so I can figure out if it creates torque.

Originally posted by Golfer
"Leeching power" and "directly connected" seemed to clash as definitions in my mind.

I meant "leeching" as in "stealing" power from the turbines. Power that the turbines otherwise would use to do more sucking and squeezing.

[jigsaw]

HiTech: I think I see where we're both having some confusion. Hopefully I can clear it up. I've been discussing this from the standpoint of having only one engine working, which is probably the root of confusion.

Torque itself, indeed does not change. But, because the engines are no longer on the longitudinal axis, if you lose an engine, torque now wants to rotate about the CG.

What causes the roll is a combination of "A" and "T" in PAST.  "P" and "S" are yaw. The diagram is discussing only how torque is contributing.  I've got diagrams for the other three as well. This is why I suggested talking to a MEI. Vmc is much easier to explain in person.

Let me know if this makes sense.

[MiloMorai]

Originally posted by Golfer
The turbine engines on turboprops aren't really big enough to power anything, they're essentially little APU motors.


 

The 4 Allison T56-A-15 turboprops @ 4,300 horsepower in the C-130 are a bit large for use as CPUs, would you not say?

[Golfer]

The 4 Allison T56-A-15 turboprops @ 4,300 horsepower in the C-130 are a bit large for use as CPUs, would you not say?

Gack, I need a lawyer to approve all my posts from now on to ensure no loopholes in what I'm saying.

The turboprops I've been using as examples all have smaller (1015 or less SHP) and those engines aren't very big at all.

[hitech]

jigsaw: Realy no need to talk to an MEI, I do understand the terms and there function.

But torque always rotates about the CG no mater that the torque is applied on a wing or center line. Acctual when 1 engine stops there is less torque on the plane being caused by the  "T". It just that the "A" now has a differential between the wings.


HiTech