Bob - S50

Climb Speed - Almost Always Faster than Vy

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This kind of came up in another thread, and rather than continue to drift further off topic I thought I'd start a new one.  This is a discussion starter with my take as the starting point.

I'm talking about normal climb to cruise, not an obstacle clearance climb.  No trees or hills off the end of the runway.  Just a normal everyday climb to cruise altitude but trying to do so in minimal time (best rate).

My hypothesis is that faster than Vy is better than Vy for several reasons:

1.  As speed increases the prop becomes more efficient and at least partially offsets the increase in drag.

2.  If I keep my speed up a bit (over 100 KIAS) I can close the cowl flaps to trailing and still keep the CHT's reasonable.  This actually decreases drag.

3.  Theoretical best climb speed happens at minimum drag so that we have maximum excess power for climb.  The drag curve is pretty flat near that speed (Vy).  Go a little slower or a little faster and you'll have less excess power which means a lower sustained climb rate.  Now let me use some completely made up numbers to illustrate why I think it is almost always better to climb at a speed faster than Vy

Let's assume Vy is 90 KIAS and I can get 1000 FPM climb at that speed.  Since the drag curve is pretty flat, let's pretend that if I climb at 80 KIAS or 100 KIAS, I can get 950 FPM.  In a perfect world, ignoring items 1 and 2 above, my best climb will happen at Vy.  However, I don't live in a perfect world.

Let's say I'm climbing at 90 KIAS and either due to a wind shift, a downdraft, or a moment of inattention my speed drops to 80 KIAS.  My climb rate capability has dropped from 1000 to 950 FPM.  If I try to maintain the previous climb rate my speed will continue to decay.  In order to get back up to 90 KIAS, I have to lower the nose to accelerate (this will happen automatically if the plane is trimmed for 90 KIAS).  Just lowering the nose to maintain 950 FPM won't work, I'd be stuck at 950 FPM and 80 KIAS.  I'll have to go even lower, say to 900 FPM.  And since the further I am from 90, the steeper the drag curve is, my speed will come back slowly at first, and then quicker as I approach 90.  On the other hand, If my moment of inattention caused my speed to increase, lucky me, I can simply zoom back to 90 KIAS and trade airspeed for altitude.

Now let's say I'm climbing at 100 KIAS and I once again get a speed decrease for one of the same reasons as above.  I'm now down to 90 KIAS which gives me the ability to climb better than I was.  I can just lower my nose to the same climb rate I had, and the plane will slowly accelerate back to my 100 KIAS.  And since my speed excursion put me closer to the flat part of the curve, my speed will come back more quickly at first and then more slowly as I approach 100 KIAS.  I haven't lost any average climb rate.  And as earlier, if my speed increased, I can trade airspeed for altitude.

It may turn out that my average climb rate is the same for both cases.  In the case of trying to fly Vy (90), I may spend time climbing at 1000 FPM and time at 900 FPM averaging out to 950 FPM.  In the case of flying at 100 I was able to maintain a steady 950 FPM.  Same time to altitude.  However, if my average speed was 85 in the first case and 95 in the second case, that's 10 knots faster for the climb.  If it took me 15 minutes to get to cruise altitude, I'd be a full 2.5 miles closer to my destination climbing at a target of 100 than I would be climbing at a target of 90.

Remember this was a theoretical, imaginary set of numbers used for thought only.  Your mileage may vary.

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Good morning, Bob!

How / where would I learn about prop efficiency and speed?

Something that seems somewhat important, yet gets lost in the overall efficiency discussions...

Any insight?

Best regards,

-a-

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I climb Vx until gear retraction, Vy until pattern altitude, and then cruise climb to altitude to keep the temperatures down.  I'm a new Mooney pilot, but this seems to work well for my M20C.

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I think i do this naturally as I follow CAS 115MPH as I climb. The TAS obviously is faster than my CAS as I creep into the sky. After reading countless threads on vintage “best practices”, I don’t touch anything except for the mixture as I climb. I never really thought about the cowl flaps as I do have a GEM. Good idea.


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1 minute ago, tigers2007 said:

I think i do this naturally as I follow CAS 115MPH as I climb. The TAS obviously is faster than my CAS as I creep into the sky. After reading countless threads on vintage “best practices”, I don’t touch anything except for the mixture as I climb. I never really thought about the cowl flaps as I do have a GEM. Good idea.


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I have the habit of pulling back to 25in/2500 RPM because I also fly a PA32 which is not rated for continuous full power.  I have a feeling I don't actually need to do that for the O-360 in the C.

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For me, normal cruise climb to altitude.......full power, 2700 RPM, cowl flaps open (if fitted on your plane), lean throughout climb, 120 mph/104k.

This has always provided good engine cooling, good forward visibility, and most overall efficient performance to my crushing altitude.

I’ve applied this in the C, E, F and R models.

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I was at a seminar where they talked about "Vz," or,  efficient climb speed. Essentially, it comes down to this profile after reaching a safe altitude:

1.32 x sea level Vy, WOT. lean and maintain good CHT.
When climb rate drops to 500 fpm, maintain it.
When speed drops to Vy, maintain it.

For the geeks, it's in part based on this paper submitted by an ERU rad student as part of his Masters requirement.

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Interesting discussion. It’s important, though, to identify what you’re optimizing for. My goal is always to outclimb my dead stick sink rate. This should permit a safe return should the engine go “kaboom!”

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2 hours ago, Bob - S50 said:

Theoretical best climb speed happens at minimum drag so that we have maximum excess power for climb.  The drag curve is pretty flat near that speed (Vy).  Go a little slower or a little faster and you'll have less excess power which means a lower sustained climb rate.

Vy occurs at the airspeed where the power available in excess of the power required is maximum. It's best glide that occurs at minimum drag speed. But, it's fair to say that for a high performance airplane, climb rate is relatively insensitive to airspeeds several knots above and below Vy. The VSI is too sensitive to turbulence to accurately determine steady state climb rate -- it's much more accurate to use multiple timed climbs from +/- 500' around the desired altitude and average the results.

Climb rate will depend on aerodynamics as well as engine power and propeller efficiency, so it's a bit complicated. Here are some interesting articles:

http://www.mooneypilots.com/mapalog/powersettings.html

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

Skip

 

 

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At a MAPASF PPP a few years ago Jack Napoli was my instructor. As we were climbing out I was close to Vy at 85 knots and 700 fpm . Jack asked me to lower the nose and increase speed to 120 knots. Climb rate re- stabilized at 700 fpm also. Jack explained the Mooney wing was more efficient at that speed in the climb. 

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Alan,

Siri respelled you’re fpm with rpm... :)

 


 

.....

Quickly reading through Skip’s MAPA article above... 

Bob Kromer refers to an ‘old Mooney Test Pilot’ saying the wing is most efficient at 120mias...

 

Now we can combine wing efficiency and prop efficiency in the climb discussion... (?)

Mr. Kromer has also given an excellent presentation on Mooney climb out at the Mooney Summit... :)

Best regards,

-a-

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With a K model, I have found that OAT determines minimum climb speed.  In summer I must climb at 110 to 120 KIAS to keep CHT at or below 380 F on the hottest cylinder (#5).  Winter gives some flexibility on climb speed.

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2 hours ago, amillet said:

At a MAPASF PPP a few years ago Jack Napoli was my instructor. As we were climbing out I was close to Vy at 85 knots and 700 fpm . Jack asked me to lower the nose and increase speed to 120 knots. Climb rate re- stabilized at 700 fpm also. Jack explained the Mooney wing was more efficient at that speed in the climb. 

I'm not actually sure what is meant by "the wing being more efficient." Aerodynamicists use Oswald's efficiency (e) factor to correct for non-ideal lift distribution. But e is a fixed value independent of airspeed. Efficiency might best be represented by CL/CD max. But that's a fixed characteristic also. One thing that does change with airspeed is the propeller efficiency. Propellers designed to maximize cruise speed are most efficient at a certain rpm at cruise speed, so a higher climb speed will be more efficient than a lower one. 

Skip

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I would expect...

As the AOA increases...

The point of separation moves forward...

And the drag of the wing vs. the lift it produces, increases...

So the efficiency being discussed is more about the increase in drag...?

PP thinking out loud... definitely not an aerodynamicist.

Best regards,

-a-

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I rotate at 65, then accelerate to 80 then take the gear up and keep accelerating to 100 - 120 where my climb is around 750 or so.  Perfectly happy with that, can close half cowl flaps keeps engine cool, climbing happily, keeps the controllers happy as im moving forwards and clmbing at a decent rate.  

All,speeds in knots.

Andrew

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6 hours ago, carusoam said:

Good morning, Bob!

How / where would I learn about prop efficiency and speed?

Something that seems somewhat important, yet gets lost in the overall efficiency discussions...

Any insight?

Best regards,

-a-

Here's a good place to start:

Professor Rogers

 

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3 hours ago, PT20J said:

Vy occurs at the airspeed where the power available in excess of the power required is maximum. It's best glide that occurs at minimum drag speed. But, it's fair to say that for a high performance airplane, climb rate is relatively insensitive to airspeeds several knots above and below Vy. The VSI is too sensitive to turbulence to accurately determine steady state climb rate -- it's much more accurate to use multiple timed climbs from +/- 500' around the desired altitude and average the results.

Climb rate will depend on aerodynamics as well as engine power and propeller efficiency, so it's a bit complicated. Here are some interesting articles:

http://www.mooneypilots.com/mapalog/powersettings.html

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

Skip

 

 

That's true, but since power is constant at any given altitude, maximum excess power will also occur at minimum drag.

And I believe best glide speed is actually a bit faster than Vy.  Vy will give you minimum sink rate or maximum TIME aloft.  Best glide DISTANCE will require a bit more speed.

Here's another good article to read:

See How It Flies

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For piston powered airplanes, there are two sets of curves necessary to define performance speeds: thrust required/thrust available, and power required/power available. One way to remember which set to use is that if a time element is involved, the power curves are apropos.

Below are the formal definitions of a number of common performance speeds. Anderson, John D., Aircraft Performance and Design is a good reference. (Also, Anderson's Introduction to Flight is excellent because he explains concepts in a very straightforward manner). Less technical explanations can be found in Eckalbar, John C., Flying High Performance Singles and Twins.

Vx = best angle of climb = speed where there is maximum excess thrust available above the thrust required for level flight.

Vy = best rate of climb = speed where there is a maximum excess of power available above the power required for level flight.

Best glide = speed for greatest glide distance with no wind = L/Dmax speed = minimum drag speed (since lift is a constant approximately equal to weight). At this speed the parasite drag and induced drag are equal and each comprise 1/2 of the total drag. This is also the speed for maximum range.

Maximum endurance speed = speed for lowest fuel flow = [CL3/2/CD]max which works out to be L/Dmax x 0.76. At this speed, induced drag is twice the parasite drag and 2/3 of the total drag. This is also the minimum sink speed.

Carson speed is minimum fuel flow per unit of velocity = [CL1/2/CD]max and works out to L/Dmax x 1.32

If you look at the thrust available and power available curves you will notice that the maximum thrust occurs when the airplane is stationary and thrust actually decreases with airspeed. However, the power available is zero when stopped (because no work is being done since the airplane is not moving). The power available increases with velocity.

Skip

 

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11 hours ago, cferr59 said:

I climb Vx until gear retraction, Vy until pattern altitude, and then cruise climb to altitude to keep the temperatures down.  I'm a new Mooney pilot, but this seems to work well for my M20C.

In my C, the gear goes up once I verify positive rate (~treetop height), and i climb at Vx to assure obstacle clearance; when I use Takeoff Flaps, I try to remember to raise them around here, too, when I'm looking down on the obstacle that worried me.. Then I transition to Vy to altitude, lower the nose and accelerate. Up high, I set power and start leaning at about 130 mphi; around 3000msl on short runs, it may happen somewhere above 145mphi.

There's no benefit to climbing with gear down any longer than necessary. If "something happens" and you need to land, it moves down pretty fast. 

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1 hour ago, Hank said:

There's no benefit to climbing with gear down any longer than necessary. If "something happens" and you need to land, it moves down pretty fast. 

Yep. The bunch that won't retract the gear for fear of needing it again drive me a little batty. It comes down pretty fast. Don Maxwell had an engine failure during a test flight, decided he didn't want to land straight ahead in the junk yard, did the impossible turn (made the tower controllers duck ;-) and was just about to touch down when he remembered the gear. Said it went clunk, chirp.

Skip

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1 hour ago, PT20J said:

Yep. The bunch that won't retract the gear for fear of needing it again drive me a little batty. It comes down pretty fast. Don Maxwell had an engine failure during a test flight, decided he didn't want to land straight ahead in the junk yard, did the impossible turn (made the tower controllers duck ;-) and was just about to touch down when he remembered the gear. Said it went clunk, chirp.

Skip

And those of us lucky enough to have the Johnson Bar it goes up and down even faster. 

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23 minutes ago, Skates97 said:

 the Johnson Bar it goes up and down even faster. 

should this be moderated?

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13 hours ago, Hank said:

In my C, the gear goes up once I verify positive rate (~treetop height), and i climb at Vx to assure obstacle clearance; when I use Takeoff Flaps, I try to remember to raise them around here, too, when I'm looking down on the obstacle that worried me.. Then I transition to Vy to altitude, lower the nose and accelerate. Up high, I set power and start leaning at about 130 mphi; around 3000msl on short runs, it may happen somewhere above 145mphi.

There's no benefit to climbing with gear down any longer than necessary. If "something happens" and you need to land, it moves down pretty fast. 

Next time you fly check VS at Vy, then transition to 120 and check VS. I believe you’ll find they are close.

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