Vy For Science - Why are Mooneys Special

[Flow]

Hello folks,

I posted this on PPrune about why a TN J seems to have its indicated Vy INCREASE (backwards from what you would expect) as it climbs. No one really seems to have cracked it.

https://www.pprune.org/tech-log/633530-factors-cause-vy-increase-altitude.html

So are there any Turbo drivers here that notice their best rate of climb tends to occur at a faster IAS as the get into the flight levels. That is before the wastegate shuts. If so does anyone know why this happens?

 

[ArtVandelay]

I think it has to do with the turbo becoming more efficient at higher altitude because of the cooler air. Here’s a good article where they talk about Vy and the power curve, I think the power curve rises with a turbo, before falling.

 

 

https://www.boldmethod.com/learn-to-fly/performance/vx-vy-altitude-and-where-they-meet/

[Mooneymite]

A different perspective.  Useless speeds:

https://www.advancedpilot.com/articles.php?action=article&articleid=1842

[Flow]

So this comment is the closest answer?

"As to why it would increase in this case maybe the power plant is more efficient as we have dramatically reduced the OAT, added ~8psi to the pressure differential over the turbine wheel and the waste gate is now closed. With a Turbo Normalized installation with automatic waste gates you should be wide open throttle the whole way up so maybe we are seeing more power dragging Vy north, but by 5% total and maybe over 10% further stretched away from min sink? Possibly?"

Is there any data on TN performance increase with altitude?

[Flow]

9 minutes ago, Mooneymite said:

A different perspective.  Useless speeds:

https://www.advancedpilot.com/articles.php?action=article&articleid=1842

Yep thanks, aware and good article but the question is about POF not airmanship.

[N201MKTurbo]

It could be that Sandman just made up the numbers. I don’t believe half of the performance numbers published for this setup.

FWIW. This setup has a fixed wastgate just like a stock 231 (exactly the same parts). It has a pop off valve on the compressor side that regulates the MP  up to critical altitude, which is FL210 on mine.

[Austintatious]

5 hours ago, Mooneymite said:

A different perspective.  Useless speeds:

https://www.advancedpilot.com/articles.php?action=article&articleid=1842

Good article.

I was just mentioning in another thread how overall, what speed you climb at will not drastically change the fuel burn for the trip. Reason being that if you climb at a slower speed, you cover less ground and run the engine longer (thus burn fuel longer) which offsets the better climb rate.

Point being that the difference between climbing at Vy vs a higher "cruise climb speed" makes vary little difference in total energy for the trip (it does make a difference but not a massive one)

This is similar to what this guy points out with Vx vs Vy.  there is a difference, but not a massive one.   I would however point out that even this small difference becomes important, especially in the FAA's eyes when it comes to instrument flying, as they allow/require SHOCKINGLY small margins for obstacle clearance.  IIRC about 35 feet per NM.  That  is insane and any of us only clearing an obstacle by that much would cause soiled britches.   THe point being that when you look at what the FAA requires, the small difference between climbing at Vx vs Vy can literally mean the difference between meeting that small margin or not.

[gsxrpilot]

I'm not an aeronautical engineer, and not even a professional pilot. So I might be way off base here...

I do fly a turbo Mooney with an autopilot that includes an altitude pre-select feature with rate of climb setting. I always climb full throttle, full prop, and full rich. So I'm basically running 100% HP all the way to my chosen cruise altitude. I regularly set my climb to be 800 ft/min and then leave it to the autopilot to maintain that rate.

I haven't paid too much attention to IAS, but I know my TAS rises significantly throughout the climb. I regularly see TAS in excess of 150 knots during an 800 ft/min climb in the mid to high teens.

[Ibra]

(Joking) as you burn fuel while you climb ceiling and VY increase 

[Flow]

16 minutes ago, gsxrpilot said:

I'm not an aeronautical engineer, and not even a professional pilot. So I might be way off base here...

I do fly a turbo Mooney with an autopilot that includes an altitude pre-select feature with rate of climb setting. I always climb full throttle, full prop, and full rich. So I'm basically running 100% HP all the way to my chosen cruise altitude. I regularly set my climb to be 800 ft/min and then leave it to the autopilot to maintain that rate.

I haven't paid too much attention to IAS, but I know my TAS rises significantly throughout the climb. I regularly see TAS in excess of 150 knots during an 800 ft/min climb in the mid to high teens.

That is interesting, thanks. What the IAS is doing here in the real world is the key, vertical speed also being a TAS.

[RLCarter]

Thinner air, less cooling, lower the nose ?

[Flow]

6 hours ago, N201MKTurbo said:

It could be that Sandman just made up the numbers. I don’t believe half of the performance numbers published for this setup.

FWIW. This setup has a fixed wastgate just like a stock 231 (exactly the same parts). It has a pop off valve on the compressor side that regulates the MP  up to critical altitude, which is FL210 on mine.

I don't know what to say, I see that is true but why would they do that? Simplicity I guess. That puts paid to the idea that the power plant is more efficient at 20,000ft and shows why the climb rate falls away so much on a 231. Maybe with a 30" max TN style application rather than 40" it is able to hold MAP up that high with the same very leaky design. But I am still not understanding the factors in play that could generate additional additional stretch north in Vy away from unpowered (trust 0) V (AoA) min sink. TAS yes IAS, not getting it.

 

[jlunseth]

First I have a question. What is the basis for stating that Vy increases? Is it stated in the POH (or STC), or are you using your VSI and noticing an actual condition of your aircraft in flight?

I don't know that I have a great answer for you, but I have a clue. My aircraft is a 231 with an intercooler, Merlyn, and a JPI 930. The 930 has an algorithm for percent HP when Rich of Peak. I have noticed that, on takeoff at my home airport (910' MSL), 100% HP is around 2700 RPM's, full rich, and 36" of MP, or maybe a little more, say 36.5".  However, as I ascend into the flight levels, 100% HP can be achieved at lower MP's. It is somewhat OAT dependent, but I have seen situations where the algorithm reports 100% HP in the 32" range.  

I don't know how JPI's algorithm is structured, and in my aircraft it comes up with a percent HP that is probably about 8% low most of the time. Nevertheless, at least according to JPI's algorithm, if power is set (ROP) at 36" on takeoff and that 36" is maintained up into the flight levels, the algorithm says that the engine output actually increases to somewhere in the vicinity of 110%.

Notwithstanding the Merlyn, MP is pilot controlled in the 231, but not in turbo aircraft with fully automatic controllers.  In those aircraft it is set and forget - firewall the throttle on takeoff, the engine makes, say 36" (depends on the engine), and at FL210 or any altitude less than the critical altitude, the MP will still be 36".  The JPI algorithm, however, seems to say that power output has increased by as much as 10% even though MP (and RPM and fuel flow) has remained unchanged.

So that could explain why Vy is observed to increase.

[Flow]

Just now, jlunseth said:

First I have a question. What is the basis for stating that Vy increases? Is it stated in the POH (or STC), or are you using your VSI and noticing an actual condition of your aircraft in flight?

I don't know that I have a great answer for you, but I have a clue. My aircraft is a 231 with an intercooler, Merlyn, and a JPI 930. The 930 has an algorithm for percent HP when Rich of Peak. I have noticed that, on takeoff at my home airport (910' MSL), 100% HP is around 2700 RPM's, full rich, and 36" of MP, or maybe a little more, say 36.5".  However, as I ascend into the flight levels, 100% HP can be achieved at lower MP's. It is somewhat OAT dependent, but I have seen situations where the algorithm reports 100% HP in the 32" range.  

I don't know how JPI's algorithm is structured, and in my aircraft it comes up with a percent HP that is probably about 8% low most of the time. Nevertheless, at least according to JPI's algorithm, if power is set (ROP) at 36" on takeoff and that 36" is maintained up into the flight levels, the algorithm says that the engine output actually increases to somewhere in the vicinity of 110%.

Notwithstanding the Merlyn, MP is pilot controlled in the 231, but not in turbo aircraft with fully automatic controllers.  In those aircraft it is set and forget - firewall the throttle on takeoff, the engine makes, say 36" (depends on the engine), and at FL210 or any altitude less than the critical altitude, the MP will still be 36".  The JPI algorithm, however, seems to say that power output has increased by as much as 10% even though MP (and RPM and fuel flow) has remained unchanged.

So that could explain why Vy is observed to increase.

Thank you, we do keep coming back to reduced exhaust back pressure due to reduced ambient pressure and therefor greater pressure drop over the turbine, cooler induction charge, and a net boost increase so at sea level you maybe getting a 6" boost above ambient while at 20,000 you are 24" above ambient or 20" if your MAP has dropped back to 32.

I have added the pictures from the Pprune post to the OP above.

[N201MKTurbo]

Everybody keeps drifting to climb rate. Vy is an airspeed that produces the maximum rate. What that rate is shouldn't matter. Is there a power component to Vy? I thought it was just the wing and maybe the CG.

[carusoam]

Is this a serious question?

A theoretical question?

Or something else...?

 

Realistically the Vx and Vy are air speeds used to get away from the ground....
 

Once safely away from the ground there are better choices...

For theory recognize how the aerodynamics change with DA...

Combine with your power plant performance...

Don’t forget your prop is part of your power plant...

Your wallet will probably speak up after a while... climbing slowly for extended periods of time can have some inherent CHT challenges...

If you are Interested in a great discussion... say that up front... 

Otherwise you may have a situation of Theoretical tail chasing... 

Lots of complexity without the ability to use in a less than ideal atmosphere...

PP thoughts only, not a CFI...

Flow, I think you have stumbled upon a topic that is related to Vz... aka Carson’s speed... which is ultimate efficiency and very slow...

The slowness loses interest for everyday type flying... but if you are flying from CA to NZ... this would be interesting... 

Best regards,

-a-

[GeeBee]

Rate of climb is dictated by excess hp. That is (Pa minus pr) divided by weight x 33,000 = ROC in FPM. If you look on figure 2.4 of Aerodynamics for Naval Aviators you will note that at high altitude the drag curve shifts forward on the X axis, which is velocity. While Vy always occurs at the same L/D ratio, the velocity required to achieve that ratio on the forward side of the curve in TAS increases. While TAS will increase for the same given airspeed it cannot increase sufficiently to get to the same L/D ratio without increasing the IAS. Thus indicated Vy will often increase with altitude depending on the shape of the drag curve.

 

[RLCarter]

I agree with @N201MKTurbo, my 1st post wasn’t very clear. Vy is 102, then at or above 20,000 Vy is increased to 108, It’s not that your gaining anything other than airspeed by lowering the nose. I think this is help with cooling at the higher altitudes

[N201MKTurbo]

https://www.boldmethod.com/learn-to-fly/performance/vx-vy-altitude-and-where-they-meet/

 

http://cospilot.com/documents/Why Vx and Vy Change with Altitude.pdf

 

[GeeBee]

18 hours ago, N201MKTurbo said:

Everybody keeps drifting to climb rate. Vy is an airspeed that produces the maximum rate. What that rate is shouldn't matter. Is there a power component to Vy? I thought it was just the wing and maybe the CG.

There is a power component. The rate of climb is dependent on the excess horsepower. To achieve maximum rate the horsepower in excess of the drag has to be at a maximum.

 

[Flow]

Guys guys guys, great comments and the Bold method and Machado articles are a good start but this is a POF question about why a particular TN Mooney's Vy INDICATED INCREASES  with altitude (the opposite of what the texts explain) I have edited the OP for emphasis.

No other aircraft I know of does this. Maybe Mooney's are special, maybe just this one TN J. I understand, especially the latter Mooneys, have pretty much the best turbo installations in GA aircraft of the era so maybe they actually are. What special characteristic is it that allows them to defy the textbooks is the question. What the actual answer is will be very interesting.

The best I can see so far is that turbo power-plant efficiency increases (a lot) with altitude, but does it? Anyone with a late(ish) model turbo want to go and play test pilot and report real world Vy at 5000 - 6000 and 18,000 - 19,000... for science...

 

[jlunseth]

Well, but rate does raise a little bit of an issue doesn't it. "Best rate" does not mean that the rate goes up when the aircraft is at, say, 20,000, versus when it is at 10,000. "Best rate" only means, at what airspeed can the best rate of climb be achieved at a given altitude.  If the rate goes down at 20 versus at 10, then the pilot must have found it necessary to fly faster to keep the rate up as high as possible, even though that rate has declined.

Hypothetically, at an altitude above critical altitude, the pilot may need to fly at, say, 160, in order to keep the climb rate at 100 fpm, and the pilot in my hypothetical will find that 100 is best rate.

So isn't that the answer? The increase in the airspeed required to make "best rate" - whatever that is for a given aircraft at a given altitude - must increase because the wings generate less lift in the thinner air, and must pitch higher to achieve that lift. Not too long after critical altitude the aircraft will reach an altitude where it cannot fly fast enough to achieve any climb at all, and if it flies slower it will descend. Classic coffin corner math.

[N201MKTurbo]

Sadly my turbo J has never been near the coffin corner.

I've only had it in the flight levels twice and I don't recall what climb speed I was using. I remember it was still climbing at about 500 ft/min. next time I'm up there I will try to determine Vy, but you may have to wait a few years.

It should perform very similarly to an early 231. The power down low and the power up high are about the same. So why doesn't the Vy of the 231 increase?

BTW, I'm sticking with my original premise that Sandman just made up the numbers.

[Shadrach]

On 7/1/2020 at 1:30 PM, gsxrpilot said:

I'm not an aeronautical engineer, and not even a professional pilot. So I might be way off base here...

I do fly a turbo Mooney with an autopilot that includes an altitude pre-select feature with rate of climb setting. I always climb full throttle, full prop, and full rich. So I'm basically running 100% HP all the way to my chosen cruise altitude. I regularly set my climb to be 800 ft/min and then leave it to the autopilot to maintain that rate.

I haven't paid too much attention to IAS, but I know my TAS rises significantly throughout the climb. I regularly see TAS in excess of 150 knots during an 800 ft/min climb in the mid to high teens.

One would expect TAS to increase.  Vy should decrease with altitude by ~ .6 KIAS for every 1000ft.  TAS increases by ~2% per 1000ft.  

[GeeBee]

2 hours ago, Flow said:

Guys guys guys, great comments and the Bold method and Machado articles are a good start but this is a POF question about why a particular TN Mooney's Vy INDICATED INCREASES  with altitude (the opposite of what the texts explain) I have edited the OP for emphasis.

No other aircraft I know of does this. Maybe Mooney's are special, maybe just this one TN J. I understand, especially the latter Mooneys, have pretty much the best turbo installations in GA aircraft of the era so maybe they actually are. What special characteristic is it that allows them to defy the textbooks is the question. What the actual answer is will be very interesting.

The best I can see so far is that turbo power-plant efficiency increases (a lot) with altitude, but does it? Anyone with a late(ish) model turbo want to go and play test pilot and report real world Vy at 5000 - 6000 and 18,000 - 19,000... for science...

 

Boeing 737NG for one.