The pilot and the cockpit engine controls

[Richard]

We're going to learn together. This probably belongs in MX but I think HT is maximum exposure. If you are absolutely certain of the answers please hold back to let others respond first:

What does the throttle do?

If that, then why do we operate at reduced throttle settings in cruise?

 

[woodstock]

feeds gas to the engine?

for the same reason you don't floor your car the whole time while driving. (at least, not anymore, hahaha). you give it some good gas to accelerate it and when it's settled in you can ease off.

 

[T Bone]

We're going to learn together. This probably belongs in MX but I think HT is maximum exposure. If you are absolutely certain of the answers please hold back to let others respond first:

What does the throttle do?

If that, then why do we operate at reduced throttle settings in cruise?

I'm guessing you're referring to the use as referenced in Stick and Rudder? As well as what "that other" control does? Well then... it's really sort of an "either or" situation. The way my primary instructor phrased it, it depends on which of the two actions is the priority at that moment. The one with the priority is handled by "that other" control. There. Did I answer without giving it away? :yinyang:

 

[Richard]

I'm guessing you're referring to the use as referenced in Stick and Rudder? As well as what "that other" control does? Well then... it's really sort of an "either or" situation. The way my primary instructor phrased it, it depends on which of the two actions is the priority at that moment. The one with the priority is handled by "that other" control. There. Did I answer without giving it away? :yinyang:

Oh, gee. I'm just trying to keep it simple; no, I'm not even concerned with that other control at this point.

I haven't read S&R in years...don't make me read it again.

 

[ausrere]

feeds gas to the engine?

Doesn't the throttle control the amount of air, not the amount of fuel?

 

[gismo]

Doesn't the throttle control the amount of air, not the amount of fuel?

Of course the question is semantically vague as it could refer to the throttle control (on the panel) or the throttle plate in the induction system. With virtually every aircraft engine I know of, the throttle control affects both, although in some cases the effect on fuel is secondary, that is the fuel flow changes in response to the change in airflow. In many injected engines (including two of mine) there is a direct linkage from the throttle control to the fuel metering valve. In a carbureted system, the throttle plate is often located downstream of the fuel delivery nozzles and thus directly controls the volume of the air/fuel combination. In most if not all fuel injection systems on aircraft, fuel is added to the air well downstream of the throttle body so the throttle plate controls only air.

 

[Richard]

Okay, okay, UNCLE!, I give. The throttle control inside the cockpit of a normally aspirated single engine spam can (although it could be a lawn mower, for crying out loud)...it controls the volume of air into the engine.

For proper fuel/air mixture the rule of thumb is something like 6 lbs per hour per brake HP...regardless of altitude. So, why do we ever reduce the throttle control knob/lever in criuse??? Range, as listed in the POH? (Let's not get into whether the POH should be ignored or not)

It seems to me a fully advanced throttle control knob/lever should be left fully advanced throughout the flight up until descent for the approach. The 'other knob' is used for maintaining the proper mixture.

I recently received and am reading my copy of the, Sky Ranch Engineering Manual, by John Schwaner of Sacramento Sky Ranch, Inc. An FAA A&P I know recommended this book, he used it as the primary text when teaching future A&Ps. What Mr. Schwaner says in his book looks very much like what APS/GAMI has been saying. Even the graphs look like what I've seen from GAMI.

 

[Tom-D]

Okay, okay, UNCLE!, I give. The throttle control inside the cockpit of a normally aspirated single engine spam can (although it could be a lawn mower, for crying out loud)...it controls the volume of air into the engine.

For proper fuel/air mixture the rule of thumb is something like 6 lbs per hour per brake HP...regardless of altitude. So, why do we ever reduce the throttle control knob/lever in criuse??? Range, as listed in the POH? (Let's not get into whether the POH should be ignored or not)

It seems to me a fully advanced throttle control knob/lever should be left fully advanced throughout the flight up until descent for the approach. The 'other knob' is used for maintaining the proper mixture.

I recently received and am reading my copy of the, Sky Ranch Engineering Manual, by John Schwaner of Sacramento Sky Ranch, Inc. An FAA A&P I know recommended this book, he used it as the primary text when teaching future A&Ps. What Mr. Schwaner says in his book looks very much like what APS/GAMI has been saying. Even the graphs look like what I've seen from GAMI.

Answer is it simply controls manifold pressure, all engines, even the steam driven ones.

 

[Richard]

Answer is it simply controls manifold pressure, all engines, even the steam driven ones.

Ahhh, the words of a true master. You got me thinking now....

 

[Tom-D]

Ahhh, the words of a true master. You got me thinking now....

OK think about this,, what does the mixture control do.

 

[poadeleted20]

What does the throttle do?

In the standard carbureted normally aspirated light plane, it controls the position of the throttle valve in the carburetor, i.e., the amount of open area through which induction air can flow, thus controlling volume per time of air flow, which indirectly controls volume of fuel/air mix into the cylinders.

If that, then why do we operate at reduced throttle settings in cruise?

Because speed only goes up by the cube root of horsepower, but fuel flow goes up in direct proportion to horsepower. Thus, when you are in the 65-75% power range, an increase of 15% in power creates a 15% increase in fuel flow but yields only a 5% increase in speed. Economy suggests throttling back some at altitudes where full throttle is more than 65-75% power.

Also, heat created is in proportion to power, but cooling flow is in proportion to speed. Given the likelihood that folks will cruise their aircraft at more efficient power settings, manufacturers tend to develop their engine cooling budgets based on 65-75% power and the speeds expected at those power settings as well as leaning to the best power/best economy range. Flying at higher power settings creates a lot more heat but only a little more speed (and only a little more cooling), requiring richer mixtures (or, if you have them, open cowl flaps which increase parasite drag) to help cool the engine, thus creating even less economical flight.

 

[Richard]

OK think about this,, what does the mixture control do.

Oh no, I feel like I'm walking into the woods...alone...at night.

Okay, enriched mixture provides cooling and a cooler engine will be closer to best power? I'm talking managing engine temps here, is that the direction you're taking here?

 

[Richard]

Ron, I don't disagree with any of what you said. But... would you change any of that in consideration of LOP ops?


EX: You've just leveled off at 9,000 and settle in for that long cruise to, well, Gastons. Your normal aspirated carburetored non-turbo'd engine has one big cyl--so there is no question if all your cyl reach peak EGT close together. You aggressively lean until 50* LOP.

Does this EX influence what you posted?

 

[gismo]

Oh no, I feel like I'm walking into the woods...alone...at night.

Okay, enriched mixture provides cooling and a cooler engine will be closer to best power? I'm talking managing engine temps here, is that the direction you're taking here?

The mixture control adjusts the air/fuel ratio, usually by reducing the fuel flow rate. The mixture itself (air/fuel ratio) affects the burn rate, power produced, end gas composition, among other things, and at extremes it affects the engine's ability to run at all.

 

[gkainz]

The throttle control ... controls the volume of air into the engine.

Yep! Full throttle = loud air, less throttle = quieter air!

 

[poadeleted20]

Ron, I don't disagree with any of what you said. But... would you change any of that in consideration of LOP ops?


EX: You've just leveled off at 9,000 and settle in for that long cruise to, well, Gastons. Your normal aspirated carburetored non-turbo'd engine has one big cyl--so there is no question if all your cyl reach peak EGT close together. You aggressively lean until 50* LOP.

Does this EX influence what you posted?

No, it does not. The issue isn't leaning, it's power settings. Running LOP doesn't give you more power -- it gives you LESS. See the graphic under "Understanding the Relationships" in John Deakin's article on mixture (http://www.avweb.com/news/columns/182084-1.html). Note that peak EGT already eats about 3% of your best power output for that throttle setting, and 50 lean of peak eats about 10% of your best power output. The problem is that if you run 50 degrees LOP in a "normal aspirated carburetored non-turbo'd engine," it's going to be so rough you'll hate it. But none of this has anything to do with why "normal cruise" is in the 65-75% power range, which is all about cooling and economy.

 

[gismo]

Oh no, I feel like I'm walking into the woods...alone...at night.

Okay, enriched mixture provides cooling and a cooler engine will be closer to best power? I'm talking managing engine temps here, is that the direction you're taking here?

Close, but best power will have the CHT and oil temp pretty near the maximum you can get at any particular combination of MAP and RPM and in many cases hotter than you'd get at peak EGT (about 100 F higher EGT). On my engines (260 HP IO-470's) 20-30 LOP gives CHT's that are nearly 50 F cooler than "best" power mixture at 65% power output. During takeoff and climb (at least at flatlander altitudes) the engine is operated significantly richer than best power which does yield cooler CHT's mostly due to the slower burn at those ultra rich mixtures. BTW the extra fuel itself doesn't do much to cool the engine as the energy needed to heat that fuel plus the latent heat of vaporization for the excess fuel is a tiny, tiny fraction of the heat produced during combustion IIRC.

 

[gismo]

No, it does not. The issue isn't leaning, it's power settings. Running LOP doesn't give you more power -- it gives you LESS. See the graphic under "Understanding the Relationships" in John Deakin's article on mixture (http://www.avweb.com/news/columns/182084-1.html). Note that peak EGT already eats about 3% of your best power output for that throttle setting, and 50 lean of peak eats about 10% of your best power output. The problem is that if you run 50 degrees LOP in a "normal aspirated carburetored non-turbo'd engine," it's going to be so rough you'll hate it. But none of this has anything to do with why "normal cruise" is in the 65-75% power range, which is all about cooling and economy.

You are very correct in that with the same MAP/RPM you will get less power LOP than at best power. OTOH, I think that technically you could run higher power (actual power output) at 50 LOP than at best power if thermodynamics (getting rid of the heat) was the only issue (assuming you could actually flow enough air to get that much power).

Also FWIW there are three factors in the efficiency equation. Airspeed (AOA really), RPM (affects propeller efficiency), and BSFC (engine efficiency). And to complicate matters BFSC is affected by both mixture and RPM although the RPM effect is generally swamped by the relationship between RPM and prop efficiency. At typical cruise power settings most normally aspirated engines have an efficiency peak somewhere near peak EGT (usually on the lean side by 20-30 degrees). Running richer or leaner than that with RPM and IAS constant will decrease your MPG.

That said IAS has the biggest effect on MPG of anything you have control over.

 

[Tom-D]

Close, but best power will have the CHT and oil temp pretty near the maximum you can get at any particular combination of MAP and RPM and in many cases hotter than you'd get at peak EGT (about 100 F higher EGT). On my engines (260 HP IO-470's) 20-30 LOP gives CHT's that are nearly 50 F cooler than "best" power mixture at 65% power output. During takeoff and climb (at least at flatlander altitudes) the engine is operated significantly richer than best power which does yield cooler CHT's mostly due to the slower burn at those ultra rich mixtures. BTW the extra fuel itself doesn't do much to cool the engine as the energy needed to heat that fuel plus the latent heat of vaporization for the excess fuel is a tiny, tiny fraction of the heat produced during combustion IIRC.

Actually the simple answer is, all the mixture control does is position a mixture control valve.

Mixture control valve, controls fuel flow. (even works equally well in a diesel, pressure injected system, and a carburator)

 

[gismo]

Actually the simple answer is, all the mixture control does is position a mixture control valve.

Well if you want to get way down to what pushes what, all the red knob does in most cases is move the center of the mixture control cable.

 

[Tom-D]

Well if you want to get way down to what pushes what, all the red knob does in most cases is move the center of the mixture control cable.

There ya go. Let the heavy thinkers sweat the theory. We'll just go fly!

 

[poadeleted20]

Actually the simple answer is, all the mixture control does is position a mixture control valve.

Right.

Mixture control valve, controls fuel flow. (even works equally well in a diesel, pressure injected system, and a carburator)

Not so right. It controls the valve. Valve opening plus pressure drop across the carb throat (which is a function of throttle position) controls fuel flow. I guarantee that if you change throttle setting without changing mixture, fuel flow will change. Same for changing mixture without moving throttle. Fuel flow is a function of both.

 

[Tom-D]

Right.

Not so right. It controls the valve. Valve opening plus pressure drop across the carb throat (which is a function of throttle position) controls fuel flow. I guarantee that if you change throttle setting without changing mixture, fuel flow will change. Same for changing mixture without moving throttle. Fuel flow is a function of both.

""(which is a function of throttle position)"

Nope not so right too, at idle the drop is not enough to cause a flow,(That is why we have an idle circuit) at wide open throttle plate position the cruise jet is not rich enought at MAP/atmospheric pressure, so we have an enrichment valve. all these circuits gain fuel from the mixture control valve.

If they did not, you would not be able to secure the engine at all throttle positions.

"" I guarantee that if you change throttle setting without changing mixture, fuel flow will change. ""

Not true, When the fuel flow is controled by the mixture valve, as in lean side of best RPM, and you open the throttle, the airflow will change, not the fuel flow. When the mixture becomes too lean to run, the engine will quit.

The fuel will not stop flowing until the airflow thru the venturi becomes to low to cause a transfere, and that will happen as soon as the throttle plate is opened with out corresponding RPM increase, which is allowed by the increase in fuel flow, which you have stopped by closing the mixture valve.

Try this, get your engine above the idle circuit at about 1300 RPM, lean to 1250 RPM and open the throttle, see what happens.

Now if you CLOSE the throttle, the engine will continue to run normaly, because the carb transitions to the idle circuit and the mixture valve is not closed far enough to prevent idle circuit fuel flow.

There are only two devices in a MS MA3-SPA carbs found on light civil aircraft,

1. is the cruise jet, it will meter fuel as you describe at all power setting by feeling the drop in Hq across the venturi.

2. is the mixture valve, it controls ALL fuel leaving the float bowl. When the setting of the mixture valve is below the fuel flow allowed by the cruise jet, the cruise jet is basically out of the picture.

 

[PoAdeleted5]

Answer is it simply controls manifold pressure, all engines, even the steam driven ones.

Not diesels.

No throttle.

Diesels run wide open all the time. Power is controlled by fuel input, thus the engine is always running far over-lean.

 

[Tom-D]

Not diesels.

No throttle.

Diesels run wide open all the time. Power is controlled by fuel input, thus the engine is always running far over-lean.

yep, thinking mixture

 

[Dave Siciliano]

One can see from these posts why engineers and theorists aren't teaching beginning students...haa. Talk about asking a watch maker for the time!!

What happened to the wheels on the plane go round an round (and in the case of a retract -- up and down!!) Geesh guys..Richard wasn't trying to do a pre-buy, just 'xplain when you push the red knob, the fuel flow increases. I think!!!

You even got me confused and I actually understand some of this stuff--reminds me of a cost accounting prof. I couldn't listen to in grad school. Got an A by reading the book and not attending his class.

I'm wondering how you guys would explain how women work to a young man going on his first date

 

[poadeleted20]

Nope not so right too,

While your statements are true in certain limited circumstances, about the only time this occurs is on the ground, not in flight. Your points, while correct in theory, are not relevent to cruise power settings in a normally aspirated, nonturbocharged engine.

 

[Richard]

One can see from these posts why engineers and theorists aren't teaching beginning students...haa. Talk about asking a watch maker for the time!!

What happened to the wheels on the plane go round an round (and in the case of a retract -- up and down!!) Geesh guys..Richard wasn't trying to do a pre-buy, just 'xplain when you push the red knob, the fuel flow increases. I think!!!

You even got me confused and I actually understand some of this stuff--reminds me of a cost accounting prof. I couldn't listen to in grad school. Got an A by reading the book and not attending his class.

I'm wondering how you guys would explain how women work to a young man going on his first date

Righto, Dave. Nonetheless, it is quite informative to listen to the big boys talk theory. My original point was most pilots typically have two knobs/levers to control how their flight will go. Pilots really, really need to understand what is happening when they move the knob/lever. Can a pilot ever know too much about the plane he flies?

You'd probably believe it, but I've been astounded by what I've seen or heard from some pilots--including CFIs--WRT engine operation. That's a whole 'nother thread...

 

[Henning]

We're going to learn together. This probably belongs in MX but I think HT is maximum exposure. If you are absolutely certain of the answers please hold back to let others respond first:

What does the throttle do?

If that, then why do we operate at reduced throttle settings in cruise?

It controls the total BTU output of the engine by limiting the caloric medium (fuel). The thermal expansion of the fuel is converted to linear mechanical motion by the piston which is then converted to rotary motion by the connecting rods and crankshaft. This is done at a thermal efficiency rate of about 25%-28% on most reciprocating gas engines, the remainder is displaced into the exhaust and through the cooling system, whatever that may be. In an aircraft it is radiated through the fins on the heads and barrel as well as the oil cooler.

We pull back the throttle for cruise for a variety of reasons. From a structural view, the engine components may not be strong enough to sustain full power ratings for extended periods of time (duty cycle) so we rate them for Take Off Power with a time limitation, typically 5 minutes, with a max continuous power of 85%.

Another reason we pull back the throttle for cruise is efficiency. Since drag increases geometrically with speed, we can reduce the throttle considerably with a dissproportionaly lower reduction in speed, thereby conserving fuel and extending our range.

 

[Henning]

Doesn't the throttle control the amount of air, not the amount of fuel?

Both actually, directly in a FI engine by opening a valve in the fuel servo and indirectly in a carburated engine by passing more air through the venturi tube which creates a greater vacuum and sucks more fuel, in more complex carbs, it also physically pulls metering pins out of the jets.

 

[Tom-D]

It controls the total BTU output of the engine by limiting the caloric medium (fuel). The thermal expansion of the fuel is converted to linear mechanical motion by the piston which is then converted to rotary motion by the connecting rods and crankshaft. This is done at a thermal efficiency rate of about 25%-28% on most reciprocating gas engines, the remainder is displaced into the exhaust and through the cooling system, whatever that may be. In an aircraft it is radiated through the fins on the heads and barrel as well as the oil cooler.

We pull back the throttle for cruise for a variety of reasons. From a structural view, the engine components may not be strong enough to sustain full power ratings for extended periods of time (duty cycle) so we rate them for Take Off Power with a time limitation, typically 5 minutes, with a max continuous power of 85%.

Another reason we pull back the throttle for cruise is efficiency. Since drag increases geometrically with speed, we can reduce the throttle considerably with a dissproportionaly lower reduction in speed, thereby conserving fuel and extending our range.

KISS= push the big knob in, houses get little, pull the big knob out, houses get big again.

 

[Greebo]

When I push the really BIG knob in, the ground gets closer, very fast...

When I pull the really BIG knob back really really far, the ground gets closer even faster.

 

[Len Lanetti]

We pull back the throttle for cruise for a variety of reasons. From a structural view,

...as full throttle, full rich and max RPM at pressure altitudes of about 3,000 feet has a certain M20C zipping along at almost Vne in level flight.



Len

 

[Richard]

It controls the total BTU output of the engine by limiting the caloric medium (fuel). The thermal expansion of the fuel is converted to linear mechanical motion by the piston which is then converted to rotary motion by the connecting rods and crankshaft. This is done at a thermal efficiency rate of about 25%-28% on most reciprocating gas engines, the remainder is displaced into the exhaust and through the cooling system, whatever that may be. In an aircraft it is radiated through the fins on the heads and barrel as well as the oil cooler.

We pull back the throttle for cruise for a variety of reasons. From a structural view, the engine components may not be strong enough to sustain full power ratings for extended periods of time (duty cycle) so we rate them for Take Off Power with a time limitation, typically 5 minutes, with a max continuous power of 85%.

Another reason we pull back the throttle for cruise is efficiency. Since drag increases geometrically with speed, we can reduce the throttle considerably with a dissproportionaly lower reduction in speed, thereby conserving fuel and extending our range.

Huh? Are you saying the throttle control lever directly controls fuel flow? And why do we reduce throttle in cruise when we can simply lean the mixture which would yield a reduced HP? For the typical NA NT GA single the limitation of max continuous power is not an issue, just stay below RPM redline.

Don't hurt me.

 

[Henning]

Huh? Are you saying the throttle control lever directly controls fuel flow? And why do we reduce throttle in cruise when we can simply lean the mixture which would yield a reduced HP? For the typical NA NT GA single the limitation of max continuous power is not an issue, just stay below RPM redline.

Don't hurt me.

On a FI engine, yes it has a direct effect on the fuel servo. As to the "typical" (not typical for the planes I fly though) Normally Aspirated non Supercharged engines, you didn't put that in there earlier On an N/A engine above a certain altitude, you'll be full throttle anyway. The reason, especially on a carburated engine to use the throttle is to keep the manifold pressure low (high vaccuum) which keeps the flow rate of the airfuel mixture higher which in turn holds the fuel particals in suspension better allowing for a better and more efficient burn. With FI it's not as critical as the injector nozzle sprays near the valve, but it does have an effect on the swirl of the fuel/air into and scavenging of the exhaust gasses out of the cylinder. BTW, to me, a much more critical issue is "Why don't most aircraft have a properly tuned exhaust?" This is THE most critical thing on a multi-cylinder recip engine, and is the starting point for an increase in performance and efficiency, and if you don't do it, you are wasting a good part of the benefit of any other modifications to your engine.

 

[PoAdeleted5]

BTW, to me, a much more critical issue is "Why don't most aircraft have a properly tuned exhaust?" This is THE most critical thing on a multi-cylinder recip engine, and is the starting point for an increase in performance and efficiency, and if you don't do it, you are wasting a good part of the benefit of any other modifications to your engine.

Think about it:

Big bore 4/6 cylinders, low RPM, tuned exhaust would make for very long headers with narrow tubes and the size/weight would be impractical on light aircraft.

 

[Henning]

Think about it:

Big bore 4/6 cylinders, low RPM, tuned exhaust would make for very long headers with narrow tubes and the size/weight would be impractical on light aircraft.

Not true. Look at the experimental market, there are some nice tuned 'headers' available.