A Car's Manual Transmission and RPM/MP Analogy

[211º]

All, feel free to correct my logic.

To better understand my engine and its operation, I'd been searching for a solid analogy between a manual transmission car and the Mooney's RPM/MP.  While reading Fly the Engine by Kas Thomas, I found the attached highlighted text. I'm posting this here in case it is of interest to anyone else.

• RPM is to the gearshift as
• MP is to the accelerator

So,  

• trying to climb a steep hill in 3rd gear at either a slow speed or without enough accelerator input is going to cause the car to "lug lug lug"; 
• trying to ascend a thousand feet at (say) 2500 RPM and 18" MP is going to cause the Mooney to "lug lug lug"

And,

• On the interstate, I cruise in 5th Gear at a lighter push on the accelerator; but I could cruise in 4th Gear with a heavier push on the accelerator to get the same speed and that would make the engine hotter
• On a cross country cruise, I can cruise at 2,700 RPM and 20" MP; but I could also cruise at (about) the same IAS at 2,400 RPM and 23" MP and that would make the engine hotter.

[Ibra]

I think it is a good analogy, for variable propellers but works pretty well for "variable Pitch" than "variable RPM" with CSUs/Governors, although there are few intuitions:

- Low gear is fine pitch (high rpm speed) and High gear is high pitch (low rpm speed)

- Full gas is high MP and Low gas is low MP

At slow ASI speeds you need low gear (high RPM) and at high ASI speeds you need high gear (low RPM)

Obviously, one can drive his car at cruise 50mph in many forms ("over-square" on 6th gear, right at 5th gear, "under-square" on 4th gear) all should be ok and healthy but surely, not on 6th gear and full gas at 10mph uphill (too much "over-square") nor 1st gear and full gas at 70mph downhill (too much "over-speed"), with no gas you can drive in any config as long as it is not 6000rpm 

Analogy fails when you talk about which one goes first gearbox or gas VS MP or RPM and the range of RPM (max rated 8000rpm vs 2700rpm )

At specific % power you get a desired ASI in cruise, if that can be achieved by various prop RPMs then there is probably no "correct RPM setting" but one can argue about various operation constraints (e.g. fuel flow, cylinders temps, decibels, over-square, making TBO...)

[jaylw314]

2 hours ago, 211º said:

All, feel free to correct my logic.

To better understand my engine and its operation, I'd been searching for a solid analogy between a manual transmission car and the Mooney's RPM/MP.  While reading Fly the Engine by Kas Thomas, I found the attached highlighted text. I'm posting this here in case it is of interest to anyone else.

• RPM is to the gearshift as
• MP is to the accelerator

So,  

• trying to climb a steep hill in 3rd gear at either a slow speed or without enough accelerator input is going to cause the car to "lug lug lug"; 
• trying to ascend a thousand feet at (say) 2500 RPM and 18" MP is going to cause the Mooney to "lug lug lug"

And,

• On the interstate, I cruise in 5th Gear at a lighter push on the accelerator; but I could cruise in 4th Gear with a heavier push on the accelerator to get the same speed and that would make the engine hotter
• On a cross country cruise, I can cruise at 2,700 RPM and 20" MP; but I could also cruise at (about) the same IAS at 2,400 RPM and 23" MP and that would make the engine hotter.

You're pretty much correct, although to be fair, running a motor a high MP (open throttle) and low RPM (high gear) is what will cause it to lug. 

Practically, thought, if your intention is to use it as a teaching tool for people who have driven a car, it's not very helpful.  Nobody drives a car the way we run aircraft motors in cruise and climb, with almost full throttle all the time.  Even in descents we're using 50-60% throttle.  Drivers have a tough time understanding why you go FASTER if you increase your constant speed prop RPM, since they generally experience higher gears allowing them to go faster than lower gears.

A more accurate analogy would be driving a car with a trailer uphill.  You need almost the full power of the car's motor, so now the analogy applies.  If you shift to a lower gear to be near the rev limit, you will go faster than if you're in a higher gear, because you can't MAKE it to the rev limit.  Unfortunately, most drivers never encounter such a scenario

[Vance Harral]

There are a number of reasons why a constant speed prop isn't analogous to a manual gearbox transmission in a car, and I try to dissuade people from using that analogy.

First, the propeller has a continuous range of pitch, not discrete pitch points.  An automatic CVT transmission of the type found in certain Subarus and other modern automobiles is a little more analogous to a constant speed prop; but most drivers barely understand how automatic transmissions work to start with, much less the concept of a continuously variable transmission.

Second and more importantly, in a manual gearbox vehicle, the gearshift lever sets the gear ratio directly.  A constant speed propeller has no control that directly sets the propeller pitch.  Rather, it is constantly and automatically "shifting" the propeller pitch to maintain a selected RPM.  A helpful thing to explain to students is that if you were to safety wire the propeller control full forward such that it couldn't be moved at all, you'd still get about 90% of the benefit of the system.  In fact, Cirrus SR22 Turbo 5th gen and later aircraft are designed exactly this way: they have constant speed propellers, but there is no propeller control lever, and the selected RPM is effectively hardwired at 2500 RPM.

Third, the sweet spots for power and fuel efficiency in cruise on a typical car vs. a typical airplane are so different as to be almost completely unrelated.  In almost all cars, you want to be in the highest gear during all cruise operations, downshifting only for unusual conditions like climbing a hill.  Airplanes have more complicated and sometimes counter-intuitive RPM relationships.  For one thing, selecting the highest "gear" (most coarse pitch) may result in undesirably slow cruise speeds that are not necessarily fuel efficient.  Another interesting example from the cruise tables in my POH: 7500', 71% power, 10.4 GPH, and 165 MTAS cruise speed can be achieved at both 23"/2350RPM and 21"/2600RPM.  There is no difference in fuel used and (AFAIK) no meaningful difference in operating temperate at these two settings.  The reason to select one vs. the other has mostly to do with passenger comfort (noise and vibration), rather than any difference in speed or fuel efficiency.  2360 RPM will usually be less noisy, but may cause greater vibration and therefore passenger fatigue in certain airframes.

I've spent hours across years poring over the performance tables of my airplane and thinking about how to best manage the propeller control.  And after all that, I've settled on a trivial strategy: prop full forward for takeoffs, climbs, and landings (in case of go-around), full back for engine-out scenarios to minimize drag, and 2500 RPM everywhere else.  Again, 90% of the benefit is the governing control system that automatically increases propeller pitch as load decreases, to maintain a constant RPM.  Exactly what RPM is selected in cruise is almost an afterthought, and dramatically less important than the operation of the gearshift knob in a manual gearbox automobile.

In summary, if you're trying to understand or explain how a constant speed prop works, I actually think the automobile manual transmission analogy hurts more than it helps.  That's especially true in the modern era, where many automobile drivers aspiring to be pilots have never driven a manual gearbox automobile in the first place.

[Ragsf15e]

1 hour ago, jaylw314 said:

You're pretty much correct, although to be fair, running a motor a high MP (open throttle) and low RPM (high gear) is what will cause it to lug. 

Practically, thought, if your intention is to use it as a teaching tool for people who have driven a car, it's not very helpful.  Nobody drives a car the way we run aircraft motors in cruise and climb, with almost full throttle all the time.  Even in descents we're using 50-60% throttle.  Drivers have a tough time understanding why you go FASTER if you increase your constant speed prop RPM, since they generally experience higher gears allowing them to go faster than lower gears.

A more accurate analogy would be driving a car with a trailer uphill.  You need almost the full power of the car's motor, so now the analogy applies.  If you shift to a lower gear to be near the rev limit, you will go faster than if you're in a higher gear, because you can't MAKE it to the rev limit.  Unfortunately, most drivers never encounter such a scenario

Wait, you pull back your power to descend??  

[Hank]

9 minutes ago, Ragsf15e said:

Wait, you pull back your power to descend??  

In my C, I maintain cruise MP and EGT all the way until I start slowing for pattern entry [usually a mile or two after leveling off at TPA]. That means inching throttle back and nudging mixture forward as I come down and the air thickens up. But reducing power to descend does not make up time or fuel used in the long, slow climb to altitude at the beginning of the flight.

I've heard of people who climb at one speed, then cruise and descend at the same other speed; I climb at Vy, cruise at 145-148 KTAS and descend with only a push on the yoke and nudge of trim--this usually takes me from 140-145 mphi in cruise 165-170 mphi in the descent, and I enjoy the extra speed. But it's not "free," because I paid for it in the climb.

[Ragsf15e]

2 hours ago, Hank said:

In my C, I maintain cruise MP and EGT all the way until I start slowing for pattern entry [usually a mile or two after leveling off at TPA]. That means inching throttle back and nudging mixture forward as I come down and the air thickens up. But reducing power to descend does not make up time or fuel used in the long, slow climb to altitude at the beginning of the flight.

I've heard of people who climb at one speed, then cruise and descend at the same other speed; I climb at Vy, cruise at 145-148 KTAS and descend with only a push on the yoke and nudge of trim--this usually takes me from 140-145 mphi in cruise 165-170 mphi in the descent, and I enjoy the extra speed. But it's not "free," because I paid for it in the climb.

I do close to that, but I keep wide open throttle in the descent until slowing for pattern entry.  If ROP, I have to slightly increase mixture during descent.  If LOP, I can just leave it alone.  I plan for 500 fpm descent, so this puts me upper middle of the yellow arc.  Obviously only in smooth air.

Its definitely a thing in early training to be taught to reduce power to descend, but other than to keep a fixed pitch prop below redline or to remain below Vne, you’re just giving up the 30” earlier you could have arrived!

[Hank]

35 minutes ago, Ragsf15e said:

I do close to that, but I keep wide open throttle in the descent until slowing for pattern entry. 

You, sir, have fuel injection! Those of us with carburetors have to operate slightly differently. 

[RobertGary1]

I wouldn’t talk about discrete gears, that part of the analogy doesn’t really equate. It’s more like a CVT transmission. 
 

-Robert 

[jlunseth]

There are major differences between the auto and its drive system, and the prop aircraft. The tires on a car are directly connected to the pavement, so a car engine told to accelerate at too low a speed may lug. We are talking about normal operation and setting aside such things as slippery pavement or tires spun on initial acceleration by a high powered engine.  A constant speed propeller in an aircraft is itself a type of transmission or fluid coupling. It is constantly adjusting pitch to maintain propeller efficiency. If you operate an aircraft with a constant speed prop at two different power settings, one low and one high, but at the same RPM, the blade pitch is going to be different.

[Bob - S50]

Regardless of how far you try to take the analogy, telling someone that running low RPM and high MP is like lugging the engine on your car, is still valid.

[Ibra]

The other "limited analogy" is aircraft wing itself, on the same % power it can fly on two speeds or angle of attack (or wing pitch), high one on cruise side and one low on back of drag curve, I am sure everybody likes the former when they are on high power regimes rather than "dragging aircraft wing with high engine power"

The same for propeller pitch, you can fly on two or many angle of attack (geometric pitch & prop speed slip) while being able to deliver the same % power, but flying fine pitch is better for high power regimes than "dragging coase prop with high engine power"

Obviously, this analogy is also limited as it depends on aircraft speed, propeller speed and having a perfect CSU unit that replicates max efficiency curve of prop under various engine power, prop rpm & aircraft speed settings, when this is not perfect one has to assume that under high power % regimes there is no other better ways than flying fine pitch and high prop/aircraft speeds...

Edited June 11, 2020 by Ibra

[jaylw314]

7 hours ago, Ragsf15e said:

Wait, you pull back your power to descend??  

 In all fairness, at altitude you're only making 50-60% MP at sea level WOT anyway.   But yes, I reduce power to descend.  Flying faster (than best glide) always equals more fuel burned, and I always prefer getting there with more fuel reserve in the tank than a couple minutes early.

Most drivers don't understand CVT's or even automatic transmissions, since the gearing is usually decided by a computer based on a table or schedule based on a number of factors, so still not a good teaching example. 

Bicycle gearing might be a better analogy, since most biking is done at near maximum sustained effort.  Well, not me, perhaps, but that's what I've heard

 

[PT20J]

I’ve never liked that analogy. I think it’s confusing because it isn’t really accurate.

Internal combustion engines develop maximum torque and horsepower at fairly high rpm. Automobiles travel at relatively slow speeds. The engine cannot be be matched to the drivetrain without a gearbox and clutch.

Airplane engines run (mostly) at constant high rpm. They work perfectly well with a fixed pitch propeller (no “transmission”).

The purpose of the constant speed prop is to broaden the range of airspeeds over which the propeller is most efficient. In cruise, lower rpm and higher MAP tends to be more efficient (and quieter) but in a NA engine, you run out of MAP as you climb and have to increase rpm as you climb if you want to maintain power. 

”Lugging” is probably the place where the analogy makes the most sense.

Skip

 

[RobertGary1]

4 minutes ago, PT20J said:

The purpose of the constant speed prop is to broaden the range of airspeeds over which the propeller is most efficient.

 

I’d argue that’s the only reason a car has a transmission too. 
 

-Robert

[carusoam]

Great...

it’s summary time...

1) The analogy has value...

2) it gets MSers thinking...

3) The analogy falls down technically, because setting the rpm doesn’t set the prop’s pitch.

4) Setting the prop’s pitch would be more like changing gears.

5) So... Analogy is technically inaccurate, and outdated at the same time... and goes in the file where over-square is bad... 

6) No need to be specific... it would be equally inaccurate if an automatic trans was used...

7) Oddly enough... we left out down shifting into the corners... 

8) when you come Screamin’ into the traffic pattern...  Dropping the gear to help slow down... throttle pulled all the way back... speed brakes deployed..... when you push the blue knob all the way in.... the engine revs to max rpm....   aerodynamic braking at its finest.... just Add flaps before cornering...

9) Lots of air gets compressed during this braking maneuver... Similar, but not really like a Jake brake...
 

10) the O’s TopProp... has max certificated power at the allowed 2700 rpm... In the experimental world... crank up the rpm, add more FF, and calibrate the tach to go higher...500+HP is possible... cylinders and TBO may need some adjustment....

11) The O’s TopProp... has a recommended rpm for efficiency as well... 2550rpm... but it will take a whole bunch of experimenting to to prove that detail...
 

Our governor of our constant speed props are so cool, and highly developed over time.... the only thing they have changed is the manufacturing precision... They work really well until they are completely worn out...
 

PP thoughts only, not a mechanic...

Best regards,

-a-

[Ragsf15e]

5 hours ago, Hank said:

You, sir, have fuel injection! Those of us with carburetors have to operate slightly differently. 

I believe you, and I have no doubt that you know how to run your engine best, but why does the carb make you slowly pull throttle to keep the cruise manifold pressure vs just leaving it wide open?

[RobertGary1]

7 minutes ago, carusoam said:

Great...

it’s summary time...

1)

6) No need to be specific... it would be equally inaccurate if an automatic trans was used...

Most modern automatics now are cvt. The analogy works well with a cvt transmission. 
 

-Robert 

[Hank]

4 minutes ago, Ragsf15e said:

I believe you, and I have no doubt that you know how to run your engine best, but why does the carb make you slowly pull throttle to keep the cruise manifold pressure vs just leaving it wide open?

She doesn't like LOP, and at 2500 I'm limited to 21" (my Key Number is 46).

[Hank]

Just now, RobertGary1 said:

Most modern automatics now are cvt. The analogy works well with a cvt transmission. 
 

-Robert 

I like my wife's CVT, but in mine they added fake "shift points" complete with Drive and DriveSport which changes the shifting points for the gears that aren't in the tranny . . . . ????    

[carusoam]

Where are the Tesla drivers...?

They didn’t get a transmission, do they?

Max torque at 0 RPM... makes for a fun launch...! 
 

Regenerative braking is cool... like putting gas back in the tank...

Regenerative braking wouldn’t work for a CB Mooniac... When no braking is ever used....   

Best regards,

-a-

[PT20J]

1 hour ago, RobertGary1 said:

I’d argue that’s the only reason a car has a transmission too. 
 

-Robert

No, the car does not have a propeller.

[Ragsf15e]

42 minutes ago, Hank said:

She doesn't like LOP, and at 2500 I'm limited to 21" (my Key Number is 46).

Isn’t that about 65% power?  I definitely run my engine between about 60-75% in cruise, but during climb and descent, why wouldn’t you use the extra power available at lower altitude? Are you concerned with possibly abusing the engine at higher power?

[Hank]

2 minutes ago, Ragsf15e said:

Isn’t that about 65% power?  I definitely run my engine between about 60-75% in cruise, but during climb and descent, why wouldn’t you use the extra power available at lower altitude? Are you concerned with possibly abusing the engine at higher power?

I climb WOT / 2700, but there's no need to descend that way. I just keep the level cruise MP and EGT as I descend. Do you really add power to descend??? If I'm cruising at 20" /2500 up nice and high (70% at 10,000), left alone MP will rise to ~26" by my home pattern (TPA = 1200 msl). 26" / 2500 is 82%, not a place I want to be for very long. So I back off on throttle to hold 20" (20" / 2500 at sea level is nit on my Performance Table, but 23" / 2500 is 73%, so I can extrapolate it to be ~64%, a nice place to be).

[Ragsf15e]

4 minutes ago, Hank said:

I climb WOT / 2700, but there's no need to descend that way. I just keep the level cruise MP and EGT as I descend. Do you really add power to descend??? If I'm cruising at 20" /2500 up nice and high (70% at 10,000), left alone MP will rise to ~26" by my home pattern (TPA = 1200 msl). 26" / 2500 is 82%, not a place I want to be for very long. So I back off on throttle to hold 20" (20" / 2500 at sea level is nit on my Performance Table, but 23" / 2500 is 73%, so I can extrapolate it to be ~64%, a nice place to be).

My tpa is 3,000’ so there’s a little of it.  But yeah, I leave wot until it’s time to slow down and enter the pattern.  Does the engine go above 75% power? Sure, but I maintain a safe mixture. 
I don’t think there’s anything wrong with either way, I just wondered if it was something specific to the carburetor engines.  Honestly the best reason I’ve heard for pulling power in descent is to remain in the green arc, but the F has higher limits and if the air is smooth, I’m comfortable in the yellow.