Are GAMI Useful for M20J IO-360 Engines?

[Pinecone]

55 minutes ago, Shadrach said:

ISo, If speed is proportional to the square of power (I've been taught this is true) and given that a propeller blade is a wing, why is power directly proportional to RPM?  Is the prop not subjected to the same drag properties as the airframe?

Interestingly both the the Mooney M20F POH and the Cherokee 140 show proportional increases in speed across the power range. both POHs suggest that 10% increases in power correlate to 10mph increases in speed.

The PA28 POH says 110mph at 55% (82.5hp) 120mph at 65% (97.5hp) and 130mph at 75% (112.5hp).

The M20F POH says 156mph a 59% (118hp) 166mph at 69% (138hp) and 176mph at 79% (158).

BTW, if you read John Deakin's Pelican's Perch Articles on LOP operation, he also talks about increasing the throttle (if not WOT) to recover the % power when operating LOP.

Yes, but that is part of the comparison.  It is not 1 to 1.  So going from 2000 RPM to 2500 RPM is not exactly a 25% increase in power.  But it is a specific increase in power that is repeatable.

Interesting.  But drag increases as the square of the speed.  So as you go one increment faster, it takes more power to do another increment faster.

[Pinecone]

9 minutes ago, AH-1 Cobra Pilot said:

No.  Power is proportional to velocity cubed.  P∝FV, and F∝V².

Since P=Tω, you can use RPM with the torque, but torque can be hard to determine.  Turboprops usually have PROP RPM and Torque gauges, making the calculations easy., but on a recip, since you do not have a torque gauge, you need RPM, MP, altitude, temperature, and Fuel Flow to determine approximate power.

I once had a CWO trying to tell everyone to "set cruise power as soon as reaching altitude, as that gets you to cruise speed fastest."  I pointed out that higher acceleration is produced by higher power, and reducing the RPM from 1900 to 1700 is a greater percentage reduction than the increase in torque it produces.  He kept trying to argue that the props "take a bigger bite of the air" at the lower RPM.  (Of course, I pointed out, that that bigger bite is reflected in more torque?)  He would not believe the Math, and had to test the idea in the aircraft on his next flight before he would believe me.

OK

Yes, and with a fixed pitch prop, to turn it X RPM requires Y power.  That is fixed.  So RPM tells you power.

With constant speed prop, the same RPM can be different pitches, so we don't have that relationship.  If we had a prop pitch indicator, we could infer power from RPM and Pitch.

[Shadrach]

19 minutes ago, Pinecone said:

BTW, if you read John Deakin's Pelican's Perch Articles on LOP operation, he also talks about increasing the throttle (if not WOT) to recover the % power when operating LOP.

Yes, but that is part of the comparison.  It is not 1 to 1.  So going from 2000 RPM to 2500 RPM is not exactly a 25% increase in power.  But it is a specific increase in power that is repeatable.

Interesting.  But drag increases as the square of the speed.  So as you go one increment faster, it takes more power to do another increment faster.

I've read and linked them more times than I can count...

[Shadrach]

19 minutes ago, Pinecone said:

OK

Yes, and with a fixed pitch prop, to turn it X RPM requires Y power.  That is fixed.  So RPM tells you power.

With constant speed prop, the same RPM can be different pitches, so we don't have that relationship.  If we had a prop pitch indicator, we could infer power from RPM and Pitch.

yes but doesn't prop efficiency change with pitch and RPM? 

[Shadrach]

27 minutes ago, Pinecone said:

BTW, if you read John Deakin's Pelican's Perch Articles on LOP operation, he also talks about increasing the throttle (if not WOT) to recover the % power when operating LOP.

Yes, but that is part of the comparison.  It is not 1 to 1.  So going from 2000 RPM to 2500 RPM is not exactly a 25% increase in power.  But it is a specific increase in power that is repeatable.

Interesting.  But drag increases as the square of the speed.  So as you go one increment faster, it takes more power to do another increment faster.

I never suggest it was 1 to 1, only that the POHs shows it as a constant. For the PA18 the POH says it takes an additional 15hp accelerate from 110mph to 120mph. It takes an additional 15hp to accelerate from 120mph to 130mph.  M20F POH say it takes 20hp to go from 156mph to 166mph and it takes an additional 20hp to get from 166mph to 176mph.  

Honestly, I think most POH performance numbers offer a just good enough framework for fuel planning but don't really hold up to much scrutiny beyond that.

[PT20J]

I don’t know about the older models, but I understand that beginning with the J Mooney used a torque meter during engineering test flights to verify power. LoPresti wanted >200 mph on 200 hp with the fewest and least costly mods possible and he knew that he couldn’t cook the books because everyone would be out to prove him wrong. So, accounts from that period describe meticulous flight testing measurements. At MooneyMAX, Bob Kromer showed a picture of a later model (don’t remember which) during his tenure with a torque meter mounted between the engine and prop.

I would not be surprised if earlier model handbooks contained a bit of wishful thinking. A handling qualities consultant once told me he saw flight test data at Beech with a marginal note “add 5kts for the handbook.” Everyone fudged a bit in the 70’s I think.

Skip

[Pinecone]

1 hour ago, Shadrach said:

Honestly, I think most POH performance numbers offer a just good enough framework for fuel planning but don't really hold up to much scrutiny beyond that.

Agreed

[AH-1 Cobra Pilot]

2 hours ago, Pinecone said:

OK

Yes, and with a fixed pitch prop, to turn it X RPM requires Y power.  That is fixed.  So RPM tells you power.

With constant speed prop, the same RPM can be different pitches, so we don't have that relationship.  If we had a prop pitch indicator, we could infer power from RPM and Pitch.

No.  Think of it this way:  You have your C-152 WOT in a climb at 4000' DA.  Then you put it in a dive with WOT at 4000' DA.  Will the RPM s be the same?  Will the engine power be the same?

[Shadrach]

2 minutes ago, AH-1 Cobra Pilot said:

  Will the RPM s be the same? 

No

3 minutes ago, AH-1 Cobra Pilot said:

  Will the engine power be the same?

Yes

[jaylw314]

3 hours ago, Shadrach said:

So, If speed is proportional to the square of power (I've been taught this is true) and given that a propeller blade is a wing, why is power directly proportional to RPM?  Is the prop not subjected to the same drag properties as the airframe?

Because the power is not (just) being used to overcome the drag of the blades, it's being used to perform work on pushing the air.  I recall reading GA props can be up to 90% efficient in terms of motor power being turned into useful work.

So while that remaining 10% is presumably used to overcome drag (and that presumably goes up to the cube of RPM), the other 90% simply reflects the amount of power the motor is putting into moving air.  I suspect if you crunch such numbers, the final power output will be proportional to RPM, minus a few percentage points as it increases.

[jaylw314]

10 minutes ago, AH-1 Cobra Pilot said:

No.  Think of it this way:  You have your C-152 WOT in a climb at 4000' DA.  Then you put it in a dive with WOT at 4000' DA.  Will the RPM s be the same?  Will the engine power be the same?

We're talking constant airspeeds here.  When you start your climb or descent, RPM and power are unchanged until airspeed changes.  That's a whole other variable.

Edited September 21, 2022 by jaylw314

[Shadrach]

17 minutes ago, jaylw314 said:

Because the power is not (just) being used to overcome the drag of the blades, it's being used to perform work on pushing the air.  I recall reading GA props can be up to 90% efficient in terms of motor power being turned into useful work.

So while that remaining 10% is presumably used to overcome drag (and that presumably goes up to the cube of RPM), the other 90% simply reflects the amount of power the motor is putting into moving air.  I suspect if you crunch such numbers, the final power output will be proportional to RPM, minus a few percentage points as it increases.

The prop is a wing flying perpendicular to the thrust line. that creates lift parallel to the thrust line. I am not understanding what "pushing the air" means.

[jaylw314]

34 minutes ago, Shadrach said:

The prop is a wing flying perpendicular to the thrust line. that creates lift parallel to the thrust line. I am not understanding what "pushing the air" means.

"The prop creates lift forwards" and "the prop pushes the air backwards" say the same thing, that action/reaction thingey

[Shadrach]

20 minutes ago, jaylw314 said:

"The prop creates lift forwards" and "the prop pushes the air backwards" say the same thing, that action/reaction thingey

You're just talking parasitic vs induced drag. It's no different than any other wing.

[jaylw314]

1 minute ago, Shadrach said:

You're just talking parasitic vs induced drag. It's no different than any other wing.

Sure?  What additional conclusions can you draw by looking at it that way? 

[Shadrach]

Just now, jaylw314 said:

Sure?  What additional conclusions can you draw by looking at it that way? 

My conclusions are the same and that conclusion is that power is not really proportional to RPM but it's close enough for POH work.

When I suggested that a prop blade operated under the same physics as any other wing with the rhetorical question "Is the prop not subjected to the same drag properties as the airframe?" You said:

1 hour ago, jaylw314 said:

Because the power is not (just) being used to overcome the drag of the blades, it's being used to perform work on pushing the air.  I recall reading GA props can be up to 90% efficient in terms of motor power being turned into useful work.

Which is to say some other "work" besides the drag (parasitic and induced) of the blades. 

[jaylw314]

31 minutes ago, Shadrach said:

My conclusions are the same and that conclusion is that power is not really proportional to RPM but it's close enough for POH work.

When I suggested that a prop blade operated under the same physics as any other wing with the rhetorical question "Is the prop not subjected to the same drag properties as the airframe?" You said:

Which is to say some other "work" besides the drag (parasitic and induced) of the blades. 

Thanks for clarifying, that encourages further discussion much better than curt and dismissive responses!

I'd point out that my response was in reference to your comment about "[drag] increasing with the square of speed."  You were clearly taking about parasitic drag, not induced drag, so when I mentioned "drag", I was couching it in terms of parasitic drag.  You're right, though, I could have been more clear in making that distinction.

Edited September 21, 2022 by jaylw314

[Pinecone]

3 hours ago, AH-1 Cobra Pilot said:

No.  Think of it this way:  You have your C-152 WOT in a climb at 4000' DA.  Then you put it in a dive with WOT at 4000' DA.  Will the RPM s be the same?  Will the engine power be the same?

No and no.

 

[Shadrach]

35 minutes ago, jaylw314 said:

Thanks for clarifying, that encourages further discussion much better than curt and dismissive responses!

I'd point out that my response was in reference to your comment about "[drag] increasing with the square of speed."  You were clearly taking about parasitic drag, not induced drag, so when I mentioned "drag", I was couching it in terms of parasitic drag.  You're right, though, I could have been more clear in making that distinction.

Was I curt and dismissive? I certainly have that capability but it was not my intent.

[Shadrach]

1 minute ago, Pinecone said:

No and no.

 

Why would power be different? 

[EricJ]

11 minutes ago, Shadrach said:

Why would power be different? 

If both cases are WOT the case with higher RPM will be making more power for most engines.   For the C152 case with an O-200 that'll likely be true even well past "redline", which is why the Reno formula guys put little props on them and spin them faster.

[Shadrach]

4 minutes ago, EricJ said:

If both cases are WOT the case with higher RPM will be making more power for most engines.   For the C152 case with an O-200 that'll likely be true even well past "redline", which is why the Reno formula guys put little props on them and spin them faster.

Does this mean my fuel burn goes up during descent?

[EricJ]

4 minutes ago, Shadrach said:

Does this mean my fuel burn goes up during descent?

If it is WOT and the RPM goes up it likely would, I'd think.    Regardless, if an engine is breathing normally and makes flat  (constant) torque vs rpm (which is pretty typical for many n/a motors), then power would increase linearly with rpm.   Once the intake gets to be a choke point and can't flow more air to meet the demand as rpm increases, torque and power start dropping off.   The Reno formula guys have demonstrated that the O-200 intake is not an air flow restriction well past normal redline, for the context of the C-152 example.

[jaylw314]

34 minutes ago, Shadrach said:

Was I curt and dismissive? I certainly have that capability but it was not my intent.

It came across that way to me, yes, but it relieves me hearing it was not your intent.  It's easy to get these sort of things wrong online  

[Pinecone]

Until the flow in the intake, at its narrowest point, goes supersonic, the flow will increase.  But it will not be linear beyond some point.