Is there confusion with Vx on our vintage M20s?

[PT20J]

Here’s all CAR3 has to say on the subject

[Shiny moose]

My POH 67f states gear up for climb, doesnt say anywhere to leave gear down until no usable runway 

if your on a 10K foot runway do you wait until your at 4K AGL before the gear goes up?

How would you know where  that point is. Is it 1000 feet remaining or how about 700 if your light,

what about a 2500 foot strip with real trees not 50 ft make believe, when is that point of no usable runway?  

on a high pitch attitude can you see that runway 

could you identify your usable runway spot  if you loose the engine during a VX climb on a 4K runway

Tailwind after take off at 50 ft 

 

 

[AIREMATT]

1 hour ago, PT20J said:

Here’s all CAR3 has to say on the subject

How about takeoff at 1.3 Vs1 vs. Vx+5. Does that mean the Perf data should be reflected at Vx+5 as the target speed? And we can infer that it is safe to fly at that speed based on CAR3 certification?

 

[David Lloyd]

22 hours ago, Browncbr1 said:

The reason why it is taught to leave gear down until clear of the obstacle is because some planes have gear doors that open up and create drag in order to stow the gear before closing back up.     I don’t know of any complex singles that have that kind of gear door… it’s just larger jets I know of like that.   

Bonanzas do have the inner gear door that opens at the beginning of the gear retraction.  Supposedly, clear the obstacle before retracting the gear as that door opening increases drag.  Don't remember seeing that in the POH nor was ever apparent to me flying a Bo.  The high gear speeds did make for a very effective speed brake however.

[PT20J]

10 hours ago, AIREMATT said:

How about takeoff at 1.3 Vs1 vs. Vx+5. Does that mean the Perf data should be reflected at Vx+5 as the target speed? And we can infer that it is safe to fly at that speed based on CAR3 certification?

No, the requirement allows a speed less than 1.3 Vs1 only if it “can be shown to be safe.” The POH/AFM or operating manual for the airplane is the final authority. Here are the pertinent pages for an M20J. Gross weight Vs1 stall speed is 61 KIAS and the 50’ speed is 76 KIAS which is 1.25 Vs1. 

 

[AIREMATT]

That’s a much nicer chart than for the 60’s C and E models.
Our 80MIAS Vx as listed in Owners manual is about 1.25 Vs1 and a Vx+5 is about 1.3 Vs1.

CAR3 Climb detail seems to be a worst case minimum requirement not calculated optimum requirement. It allows for gear retracted but is still with takeoff flaps— maybe in case they don’t come up. It only requires a minimum climb of 1:12 (500’ per mile) in that configuration.

That may not be the optimum climb configuration or performance depending on angle or rate. The balked landing performance seems similar in that the minimum required is gear down/flaps landing configuration. It isn’t optimum (1:30) or desired configuration but worst case, you can still go up.

[steingar]

Mine is 80 but I try not to maintain it long.  Vintage Mooney's are known for poor engine cooling and I want to keep mine in a high power/low airflow situation for the minimum amount of time possible.

[PT20J]

I think it is important to realize that there is a speed for crossing the 50' obstacle and there is a speed for best angle of climb. They are different speeds developed for different purposes. There is a tendency to conflate the two perhaps because two common training aircraft (the C-152 and the PA28) list the same speed for both. Also, the FAA Airplane Flying Handbook suggests using Vx for the 50' crossing speed in the absence of other direction from the manufacturer.

Vx, like Vy, is a performance climb speed used after takeoff in the clean configuration, unless noted. Cessna makes this clear by listing both speeds in its AFMs under the heading ENROUTE CLIMB, FLAPS UP.

So, Vx is really unrelated to the short field takeoff maneuver except when it is the speed listed in the manufacturer's handbook, or when no speed is listed in the handbook.

Skip

Edited February 25, 2022 by PT20J
Corrected C-172 to C-152

[carusoam]

Examples of data for Vx and clearing the trees… from the latest POH (1976) for the M20C….

Part of it is written in one section…

The other half is in graphic form….

The notes on the chart give flap position…

The pic on the chart shows gear down, clearing the 50’ tree…

 

-a-

[Hank]

These are easier to read, and are why I'm not co fused about Vx in my 1970 C. These are from the 1970 Owners Manual; I also have a copy of the 76 version, very similar except for location of some controls on the panel. 

Vy is 100 mph - Altitude in thousands.

Vx is "about 80 mph." Things have improved since 1970 . . . . I generally make my initial climb at 85 mph if anything tall is in front of me,.and raise Takeoff flaps if I used them when I'm looking down at whatever it was.

[AIREMATT]

41 minutes ago, Hank said:

These are easier to read, and are why I'm not co fused about Vx in my 1970 C. These are from the 1970 Owners Manual; I also have a copy of the 76 version, very similar except for location of some controls on the panel. 

Vy is 100 mph - Altitude in thousands.

Vx is "about 80 mph." Things have improved since 1970 . . . . I generally make my initial climb at 85 mph if anything tall is in front of me,.and raise Takeoff flaps if I used them when I'm looking down at whatever it was.

So the chart says Wing flaps - take off position

Text in takeoff section says Vx about 80Mias Flaps and gear up for obstacle clearance. This only apply to taller than 50’ obstacles? Like terrain around valleys?

[Hank]

1 hour ago, AIREMATT said:

So the chart says Wing flaps - take off position

Text in takeoff section says Vx about 80Mias Flaps and gear up for obstacle clearance. This only apply to taller than 50’ obstacles? Like terrain around valleys?

I would say that Vx applies regardless of why you need to climb in a short distance.

Vx = best climb rate over distance covered

Vy = best climb rate by time

If you need to clear terrain, antennas, tall buildings, etc., that are a fixed distance from you, whether you're starting on the ground or approaching something while in cruise, if the distance is short and you can't dodge, fly Vx.

[Blue on Top]

I have to say that @Hank is nailing this one!

My "The Mooney Flyer" article this month (March) is on this exact topic.  Thanks for the great idea MooneySpace!.

On a strange note, the takeoff and landing data are NOT in the FAA/DOA approved sections in the manual I used.  

[Hank]

1 hour ago, Blue on Top said:

On a strange note, the takeoff and landing data are NOT in the FAA/DOA approved sections in the manual I used.  

These are the only sections in my Owners Manual. None of what I posted above came from here:

[Blue on Top]

@Hank  The C, E and F manuals I looked at are the same way - those same sections are FAA/DOA approved in my versions.  Btw, DOA stands for Delegation Option Authorization.  This means that the FAA had given Mooney the authorization to act on their behalf.

[Hank]

Just now, Blue on Top said:

@Hank  The C, E and F manuals I looked at are the same way - those same sections are FAA/DOA approved in my versions.  Btw, DOA stands for Delegation Option Authorization.  This means that the FAA had given Mooney the authorization to act on their behalf.

Mine above says "FAA APPROVED." The only DOA is in your posting, which to me always meant "Dead On Arrival."

[Ragsf15e]

4 hours ago, Hank said:

I would say that Vx applies regardless of why you need to climb in a short distance.

Vx = best climb rate over distance covered

Vy = best climb rate by time

If you need to clear terrain, antennas, tall buildings, etc., that are a fixed distance from you, whether you're starting on the ground or approaching something while in cruise, if the distance is short and you can't dodge, fly Vx.

Still confused though… the chart says wing flaps t/o position and gives speeds for best rate of climb (Vy), not Vx, but it does give 50’ obstacle clearance.  
 

The text says best angle of climb “about 80mph” gear and flaps up, and the checklist calls for flaps up approximately 80-90mph.  
So if 80 is Vx clean (say climbing out of a valley), should we get both gear and flaps up quickly after a short field t/o and hold 80mph?  If not, what speed do we use with some other combination of gear/flaps for obstacle clearance?  
 

Other airplanes i fly have an obstacle clearance airspeed for use in takeoff configuration.  We just don’t have it spelled out quite as clearly.

[Hank]

1 hour ago, Ragsf15e said:

Still confused though… the chart says wing flaps t/o position and gives speeds for best rate of climb (Vy), not Vx, but it does give 50’ obstacle clearance.  
 

The text says best angle of climb “about 80mph” gear and flaps up, and the checklist calls for flaps up approximately 80-90mph.  
So if 80 is Vx clean (say climbing out of a valley), should we get both gear and flaps up quickly after a short field t/o and hold 80mph?  If not, what speed do we use with some other combination of gear/flaps for obstacle clearance?  
 

Other airplanes i fly have an obstacle clearance airspeed for use in takeoff configuration.  We just don’t have it spelled out quite as clearly.

Does this help? You wrote it above:

• Vx is the best angle of climb, which is altitude gain per unit of distance over the ground.
• Vy is the best rate of climb, which is altitude gain per unit of time.

The two concepts are not the same. You defined them yourself above. Are you still confused, just because the book doesn't call them "Vx" and "Vy"?

 

[Ragsf15e]

1 hour ago, Hank said:

Does this help? You wrote it above:

• Vx is the best angle of climb, which is altitude gain per unit of distance over the ground.
• Vy is the best rate of climb, which is altitude gain per unit of time.

The two concepts are not the same. You defined them yourself above. Are you still confused, just because the book doesn't call them "Vx" and "Vy"?

 

No, I’m with you on those.  
I’m strictly talking about a short field takeoff over an obstacle.  What airspeed and config?  Personally I pull up the gear, leave t/o flaps and fly “about 90”, but that’s purely my technique.  

[Hank]

2 hours ago, Ragsf15e said:

No, I’m with you on those.  
I’m strictly talking about a short field takeoff over an obstacle.  What airspeed and config?  Personally I pull up the gear, leave t/o flaps and fly “about 90”, but that’s purely my technique.  

It all depends: length of field; grass / asphalt & condition; height of obstacle; distance to obstacle; departure weight. There's not one answer.

One or two people, half tanks or so, 3000' to a 50' obstacle, just a normal takeoff--no flaps, gear up soon, then hold Vx 80-85 briefly to establish climb, then lower the nose and maintain normal Vy climb.

2000' wet grass, close obstacles--takeoff flaps, soft field takeoff, gear up, climb at Vx until above obstacles, flaps up and resume normal climb.

Obstacles shouldn't surprise you during cruise, but pulling up to 80-85 mph regardless of flap position will create significant amount of climb in our Mooneys. A climbing turn would often be preferable in that situation. 

[Blue on Top]

6 hours ago, Hank said:

Mine above says "FAA APPROVED." The only DOA is in your posting, which to me always meant "Dead On Arrival."

@HankIt's been changed now to ODA (more than a decade ago). Maybe they got the hint. :)) It's Organization Designation Authorization (ODA) now.  

[Blue on Top]

The table used for the article has both airspeeds (V_X, best angle, and V_Y, best rate) versus altitude) identified.  See table below.  These values are for gear UP (enroute). Takeoff distances for small, single-engine, GA airplanes are always gear DOWN ... until clear of the 50' obstacle ... especially for electric gear.  Note: The airplane will be above 50' before the gear is fully up.

The chart is somewhat confusing in that boxed columns 1, 2 and 4 go together as V_Y, best rate of climb, information. Column 3 is V_X, best angle of climb airspeed. Wind will not change V_Y information (airspeed to fly or rates of climb), nor will it change the V_X airspeed to fly. V_X rates of climb are not published because they change a lot with multiple conditions/factors.  The V_X angle (gradient) will also change with those conditions/factors ... especially wind.  Hope this helps.  If not, email me at solutions@blueontop.com or call (316) 295-7812.  I'd be glad to help.

PS. If this chart were to be updated, column 3 should be removed and that information should be added as a general note below.  

[carusoam]

Interesting that the chart is in CAS as well….

CAS has some large differences from KIAs at high AOA and slower speeds…

Bigger differences with older ASIs vs more modern ASIs… might be airframe placement changes as well, not just the instruments themselves…

 

Thanks for the additional details Ron/Blue!

Best regards,

-a-

[Blue on Top]

19 hours ago, carusoam said:

Interesting that the chart is in CAS as well….

CAS has some large differences from KIAs at high AOA and slower speeds…

-a-

@carusoam Good observation! As you pointed out, my educated guess is that this chart would look strange being in IAS.  In addition, I think that there is an AOA factor on the static ports. At the higher AOAs and with power, the curves are unique.  Maybe the static ports should have been lower and/or further forward.  They didn't have the CFD that we have today. 

[MikeOH]

2 hours ago, Blue on Top said:

@carusoam Good observation! As you pointed out, my educated guess is that this chart would look strange being in IAS.  In addition, I think that there is an AOA factor on the static ports. At the higher AOAs and with power, the curves are unique.  Maybe the static ports should have been lower and/or further forward.  They didn't have the CFD that we have today. 

Per another page in the M20F POH, below, it doesn't look like there is an appreciable difference in IAS vs CAS (<1 mph) for the range of airspeeds we are discussing: