Fuel Mixture Control & Condition Levers

[your_dreamguy]

Hey,

Does anyone know what the mixture control in the cockpit actually changes in GA airplanes? Does it just fine tune the throttle plate in the carb slightly or does it just reduce fuel metering somehow or does it do something else? So, when you move the mixture control lever in the cockpit, what is actually moving in the engine?

Also, in turboprop planes, there are condition levers. Can someone explain the difference between conidition levers and mixture control in GA planes? My thought is that condition levers adjust fuel flow and kind of act like mixture control in GA planes. If that is the case, why doesn't the AC manufacturer of turboprops call those levers mixture control instead of condition levers?

Look forward to your responses.

Oh yeah, how do you become a millionaire in aviation?....
Start with two million dollars

 

[skyking1976]

Mixture controls regulate the ratio of fuel/air entering a reciprocating engine by regulating the amount of air allowed into the carb/injection system. I'm not exactly sure what the mixture control manipulates on the engine. I'm sure that someone knows.

Condition levers regulate the flow rate of fuel into the burner can in a turbine engine. The reason condition levers are not called mixture controls is that the fuel/air mixture tends to remain constant. As you introduce more fuel into a turbine, it in turn sucks in more air.

Take the $2 mil and buy a couple of Krispy Kreme franchises. Get rich. Buy an airplane.

 

[SkyWestCRJPilot]

On some airplanes like the King Air or BE-1900 there are three engine control levers for each engine. I've never flown a King Air so I'll let someone else explain about it. See the photo

On the Brasilia (and most other regional turboprops, ATR, Saab 340, Do-328) there are just two engine control levers for each engine. Power levers and condition levers. See the photo

On the Brasilia the condition levers regulates the propeller RPM by talking to the PCU or propeller control unit (basically your propeller governor). In flight with the condition levers full forward or "Max" the propeller RPM stay at 100%. The prop RPM can be reduced all the way to a minimum of 80% by pulling the conditions levers all the way back to "Min". On the ground with the power levers at ground ilde the prop rpm goes from 65% at Max to 50% at Min. The condition levers have a detent if you pull them back beyond Min. You have to lift up on them over this detent and you then bring the prop into feather (this is done on shutdown or when feathering an engine that quit inflight). Then there is a small lever behind the condition lever that you pull on and bring it past the final detent to fuel shutoff which deprives the fuel to the engine. So the condtion lever gets it's name because it helps you determine what kind of condition you want to fly in, max (takeoff, landing, climb) min (cruise, descent) feather (prior to engine shutdown).

 

[Be200pilot]

To put it very simply, condition levers control the idle of the turbine (n1) when the power levers are at flight idle. As soon as you move the power levers up past where the condition levers have the N1 set at, the condition levers do nothing. During flight, you can move the condition levers from low idle and back to high idle all you want and it wont change a thing. Condition levers also immediately cut fuel to the engine if necessary. They do not change the fuel/air mixture in any way as do mixture controls.

 

[SkyWestCRJPilot]

During flight, you can move the condition levers from low idle and back to high idle all you want and it wont change a thing.

On a King Air perhaps but not on a Brasilia. Moving the condition levers in flight changes the prop RPM speed.

 

[Vladimir Lenin]

Mixture controls regulate the ratio of fuel/air entering a reciprocating engine by regulating the amount of air allowed into the carb/injection system

I am pretty sure mixture regulates the amount of fuel that enters the carb, rather than air. There's a little needle in a metering jet that you move with the red knob, and in turn it leans out the mixture (less fuel, so as to keep up with lessening density)

got me on the condition levers, I thought they were for the same purpose, to regulate fuel, with power lever on the left and prop just right of it

 

[skyking1976]

I am pretty sure mixture regulates the amount of fuel that enters the carb, rather than air. There's a little needle in a metering jet that you move with the red knob

My bad.... you are correct, sir.

 

[great cornholio]

I forgot what exactly moves when you move the mixture I used to know but not anymore. Just dont let a student pull it out on landing thinking its the throttle.

In the BE 1900 the condition levers pretty much just control the N1 speed that you will get when you put the throttles to the idle detent. Usually around 68% for low idle and 72% for high idle. Then of course if you go below the low idle detent then the fuel gets cut off and the engine quits. So if the engine is above 72% N1 you can move the condition lever all day and nothing will happen unless of course you pull it down into the fuel cut off pos. The prop levers control properller rpm of course. But the condition levers dont. The FCU takes care of the fuel/air mixture.

 

[great cornholio]

I think I just rememberd what moves when you move the mixture lever. I believe that there is a needle valve that is in the carb or just prior to the carb. (Im sure someone will correct me if it is somewhere else) anyways that valve gets moved by the mixture control. So as it was said earlier the mixutre control adjusts the amout of fuel and then amount of air always stays constant. So you have x amount of air and the mixture is rich the needle valve in is full open letting max amount of fuel into the fuel/air charge and so you have a rich fuel/air ratio. Now you have the same amount of air but you move to mixture to a lean settting this closes the needle valve some letting lets say 1/2 of the fuel from max rich in. Now if you look at the ratio of air to fuel there will be a lot more air in this given charge. So now the mixture is lean.

 

[Vladimir Lenin]

so what's N1 then? the speed of the compressor that squeezes diesel and air before it ignites and shoots out back? so then, how do you control it?

 

[skyking1976]

so what's N1 then?.... how do you control it?

The N1 stage of a turbine compresses air before it gets burned. Fuel is introduced in the combustion chamber (or can), mixes with the compressed air, burns, and exits through the power turbine section. The N1 speed, therefore, is regulated by the amount of fuel injected into the burner can.

On the PT-6 the amount of air that is allowed to enter the can is regulated by a bleed air valve known as P2.5 (between the 2nd and 3rd compressor stages). If less air is needed in the can, the P2.5 valve will open and vent some air back into the engine inlet area. If more air is needed, the valve will close and allow more air to enter the can.

 

[metrodriver]

Of course everything above is correct except.... in a Garrett. Only power levers and speed levers, no condition levers. Power levers control the fuel control unit forward of flt idle and the underspeed governor in ground idle / reverse. Speed lever controls prop rpm between 71 and 97% with the speed lever in the low position and between 94 and 100% in high. The lower numbers are with the power lever at flt idle, the higher numbers with the power levers forward.
Of course everything is on a fixed shaft so no N1 or so. There is a rpm gauge that gives you at what percentage of max rpm the engine (and via the gearbox the prop) is running unlike a PT6 where N1 is the speed of the gasturbine (controlled by the fuel control unit and condition lever at idle) and Np is the prop rpm.
For those that don't know; a PT6 has a compressor driven by a couple of turbine wheels, the propellor has its own turnine wheel(s). There is no connection between the gas generator (engine) and propeller unlike the Garret where everything is mounted on the same shaft. That's why you see props on a PT6 feathered when shutdown or at 0 pitch (flat) on a Garrett, there is to much prop blade resistence during start if the props are not flat on a Garret.
On a piston the throttle only allows air into the engine and it sucks fuel out of the carburator (in the venturi). The amount of fuel is controlled by the mixture control that operates a needle valve inside the carburator making the opening to the venturi smaller or bigger

 

[Kingairrick]

And we have a winner!!!

 

[skyking1976]

Of course everything above is correct except.... in a Garrett.

I forgot about those gol-durn Garretts. What keeps them at flat pitch when they are shut down? Do they have low pitch locks like, say the Rolls Royce Dart?
SK

 

[varicam]

I forgot about those gol-durn Garretts. What keeps them at flat pitch when they are shut down? Do they have low pitch locks like, say the Rolls Royce Dart?
SK

Actually, the Garrett start locks serve a different purpose from the Rolls-Royce (and Allison) low pitch stops. The aforementioned low-pitch stops mechanically prevent blade angle from going below the low-pitch setting while the power levers are in the inflight (alpha) mode. Bringing the power levers into the ground (beta) mode hydraulicly releases the locks and allows the blade angle to decrease for fine pitch and reverse. The Garrett start locks are nothing more than pins that are operated thru centrifigal force. The pins retract during normal operation, and extend when the prop slows down during shutdown. The procedure is to hold the power levers in reverse during shutdown. Prop oil pressure will hold the blades in reverse until it diminishes, and by then prop rpm is low enough to permit the start lock pins to extend. With no prop oil pressure, a spring tries to drive the blades toward the feather posirion. But, the start locks prevent them from going higher than zero pitch.

 

[Cardinal]

I've always wondered this, how screwed are you if you forget to bring the power levers into reverse as your Garrett spools down? Will the props feather and leave you stuck indefinetly in BFE?

 

[varicam]

I've always wondered this, how screwed are you if you forget to bring the power levers into reverse as your Garrett spools down? Will the props feather and leave you stuck indefinetly in BFE?

Not usually a problem. It happens occasionally even when the pilot does everything right. There are electric unfeather pumps that can be used to get the props on the start locks before engine start.

 

[enigma]

Who cares about condition levers, Props are for boats.


Just kiddin, carry on.

enigma