Vx for my Mooney M20G

[Cam]

Hi all,

I just had a question for the group to see if this was a common 'feature' of the Mooney POH. I'm a new owner of a 1969 M20G and was reading the POH to find out what my Vx, Vy would be at KSLC (4200'). I looked in the performance section and found a table (in my POH, on pp 6-5) that listed various Rates of climb in Feet per Minute(FPM) and Miles Per Hour (MPH) against Altitude. It also had one column of Best Angle of Climb in MPH against Altitude.  It took me a while to notice but the best angle of climb speed was the same, at 82 MPH,  for every altitude from Sea Level (SL) to 13000 feet. In my head I thought that made sense as pressure decreases with Altitude and the speed(over the ground) for angle of climb would increase as my altitude increased etc. Not well thought out I know, but being lazy I didn't think about it further. 

Today I noticed that another section (under 'CLIMB' on pp 3-14, NORMAL PROCEDURES section) of the POH has this quote in it "The speed for maximum angle of climb (for obstacle clearance at full power, gear down, and flaps set at TAKEOFF) is a straight line variation from 81 MPH IAS at sea level to 72 MPH IAS at 10,000 feet."  Well, I thought to myself, that doesn't jive with what is in the performance table! What the <insert expletive of your choice here>. 

Went to google and started a little research on Vx and how to calculate etc. Found a good article (IMHO) here. (http://cospilot.com/documents/Why Vx and Vy Change with Altitude.pdf). SO my question - The only difference between the two recommendations for Vx is that in the performance table, the conditions are enumerated to be GEAR UP, FLAPS UP and the speed for Vx is a constant 82 MPH without variation for altitude, while in the NORMAL PROCEDURES the conditions are defined as GEAR DOWN, FLAPS SET FOR TAKEOFF, and then Vx is a variable speed between 81 and 72 MPH. - What  speed do I use? Is the performance table set to 82 for best cooling or something? Do different configurations affect the variable speed of Vx? Why is Vx variable in one configuration and not in another when it would seem to me that Vx should just vary with altitude. What am I missing here? 

 

Cheers

Cam

 

[Hank]

I only climb at Vx until obstacles are cleared, generally just a couple of hundred feet AGL. This is after I verify positive rate and raise the gear. Then I lower the nose to Vy and climb to altitude, if engine temps allow. Do whatever you normally do with flaps; when I use them, they also are raised when I can look down on the obstacle that encouraged their use. 

What possible scenario would have you climb several thousand feet at Vx? Try two takeoffs at your normal climb profile, and leave the gear down on one of them--that will answer your question about gear up or down. I do not know why your Owners Manual is confusing, but certainly by '69 they were printed and not typed (my 1970 Owners Manual is printed).

[jaylw314]

38 minutes ago, Cam said:

Hi all,

I just had a question for the group to see if this was a common 'feature' of the Mooney POH. I'm a new owner of a 1969 M20G and was reading the POH to find out what my Vx, Vy would be at KSLC (4200'). I looked in the performance section and found a table (in my POH, on pp 6-5) that listed various Rates of climb in Feet per Minute(FPM) and Miles Per Hour (MPH) against Altitude. It also had one column of Best Angle of Climb in MPH against Altitude.  It took me a while to notice but the best angle of climb speed was the same, at 82 MPH,  for every altitude from Sea Level (SL) to 13000 feet. In my head I thought that made sense as pressure decreases with Altitude and the speed(over the ground) for angle of climb would increase as my altitude increased etc. Not well thought out I know, but being lazy I didn't think about it further. 

Today I noticed that another section (under 'CLIMB' on pp 3-14, NORMAL PROCEDURES section) of the POH has this quote in it "The speed for maximum angle of climb (for obstacle clearance at full power, gear down, and flaps set at TAKEOFF) is a straight line variation from 81 MPH IAS at sea level to 72 MPH IAS at 10,000 feet."  Well, I thought to myself, that doesn't jive with what is in the performance table! What the <insert expletive of your choice here>. 

Are you sure?  Double check that.  For all aircraft, Vy DECREASES and Vx INCREASES with increasing altitude.  Vy generally decreases faster than Vx, but I don't know if that's with universal.  When they meet, that's near your service ceiling.

For perspective, my J POH says:

Normal procedures:

• Vy - 88 KIAS decreasing to 82 KIAS at 10,000 feet
• Vx - 69 KIAS increasing by 1 knot every 5,000 feet

The performance section only lists best rate of climb, and does not list any info on best angle.

@Hank, I think it's a valid question for obstacle clearance at high-altitude airports

Edited April 4, 2018 by jaylw314

[Cam]

My thought process, and I will admit it's not always the best , is that in the west here, you COULD get caught in an approach to a mountain pass, box canyon or something similar (I'll leave it to the reader to look up all of the NTSB reports on these particular scenario's and how to avoid, but ...). At that point, I would REALLY want to know my Vx and if it varies with altitude or configuration or if it is a constant. 

Hopefully this is a scenario that one would do your best to avoid and to get out of some other way (Turn around for example!) 

[skydvrboy]

Interesting differences between your '69 G and my '67 F.  My POH only shows best rate of climb in the performance section chart (113 mph @ 0', 102 mph @ 10,000').  Max angle of climb is only found in the flight procedures section (94 mph).  The flight procedures section also includes best rate of climb which matches the performance section.

I wouldn't have expected so much variation in the speeds since they both use the same air frame.

[Cam]

Jay - Yes I checked again and it says Vx DECREASING with Altitude. I've included a picture of the manual page. (Middle of the CLIMB section)

Skydrv - Yes the M20G and F have same body but a different engine. Mine is normally aspirated (hence the lower Gross Weight)  and your F is fuel injected is it not?  I'm not sure sure why the difference as I would have thought the same air frame would have the same flight dynamics. Maybe more HP allows a higher angle.   I've included my performance chart that says Vx is 82 mph as a constant with altitude in the GEAR UP and FLAPS UP configuration.

[jaylw314]

33 minutes ago, Cam said:

Jay - Yes I checked again and it says Vx DECREASING with Altitude. I've included a picture of the manual page. (Middle of the CLIMB section)

Skydrv - Yes the M20G and F have same body but a different engine. Mine is normally aspirated (hence the lower Gross Weight)  and your F is fuel injected is it not?  I'm not sure sure why the difference as I would have thought the same air frame would have the same flight dynamics. Maybe more HP allows a higher angle.   I've included my performance chart that says Vx is 82 mph as a constant with altitude in the GEAR UP and FLAPS UP configuration.

Weird.  I can't think of any aerodynamic reason why Vx should decrease with altitude.  Having gear/flaps down might change the numbers, but the trend should still increase with altitude. 

[Cam]

I know huh? - hence one of my questions!  

[kortopates]

Weird.  I can't think of any aerodynamic reason why Vx should decrease with altitude.  Having gear/flaps down might change the numbers, but the trend should still increase with altitude. 

Look for a table that shows CAS vs IAS and be careful with IAS vs CAS vs TAS. You should find that with altitude you will need a bit more IAS to maintain the same CAS.

If we stick to CAS, no error, then Vx will essentially not change with altitude yet Vy will decrease with altitude till it hits Vx at your DA ceiling.

For practical value you only use Vx to clear obstacles in airport/runway environment. You would never use Vx to clear a mountain pass miles away since you would use Vy to get to altitude in the least time and circle over lower terrain such as the airport if you couldn’t get high enough in a straight line to the pass.

Further, if you don't have immediate obstacles to clear there is no need to climb at Vx - it's actually adds considerable risk to pilots in an engine out because too often pilots don't push the yoke forward right away to glide speed and stall out spinning in with out any chance of recovery. ( just had such a fatality in my area a couple weeks ago) I much prefer to accelerate to a cruise climb airspeed above Vy (Vy+10-20) well before TPA when obstacles are not a concern. This gives you plenty of options in the form of time as well as improved sight picture in an emergency and without significantly decreasing your ability to make it back to airport if you do have sufficient altitude. Deakin has a excellent article on more details of why not to climb at Vx and Vy on Avweb.


Sent from my iPhone using Tapatalk

[Greg Ellis]

3 hours ago, Cam said:

Found a good article (IMHO) here. (http://cospilot.com/documents/Why Vx and Vy Change with Altitude.pdf).

You can find the video version of Rod Machado's article here if you wish.

 

[Cam]

HI Kortopates,

Thanks for thoughts. I totally agree that there is usually no reason to climb at Vx, and climbing at Vy +10 or +20 is better for many reasons.  Also agree on you should never depend on Vx in the mountains. 

All that being said though, there should be a specific value for Vx. As a pilot I should know that speed. I should know that speed and the factors that influence it, and I should especially know that speed if I fly in the mountains even as I do every thing to avoid having to use it. That Vx may change depending on configuration or altitude, and in fact all of the literature I can find says that the speed of Vx 'should' increase as altitude increases. (I think I have that right). My problem is that I seem to have the POH giving 1. A conflicting DECREASING trend on one section, and 2. A constant in another. Both of these instructions/values seem to fly in the face (see what I did there?  ) of the Vx explanations and other POHs that I've read so I'm wondering why? 

As well, I did actual check the CAS vs IAS chart and the speeds are the same  except when using 15 degrees of flap and then 79 MPH CAS reads 80 MHP IAS

I'm just totally curious now.

[Cam]

Thanks Greg!

 

 

[Cam]

Interesting note - Thanks to Kortopates I did the calculations for TAS at various altitudes and found (assuming Standard lapse rates ) ....

if you take a straight line DECREASE in IAS from 81 MPH to 72 MPH from SL to 10000, you get a (close) constant of 82 MPH TAS . This might make you think that the text and the performance chart match, but the performance chart says CAS is the constant 82 MPH, and since the CAS and the IAS don't vary (significantly - 1 MPH at 80 MPH), I'm still left with the same questions.  

[jaylw314]

45 minutes ago, Cam said:

Interesting note - Thanks to Kortopates I did the calculations for TAS at various altitudes and found (assuming Standard lapse rates ) ....

if you take a straight line DECREASE in IAS from 81 MPH to 72 MPH from SL to 10000, you get a (close) constant of 82 MPH TAS . This might make you think that the text and the performance chart match, but the performance chart says CAS is the constant 82 MPH, and since the CAS and the IAS don't vary (significantly - 1 MPH at 80 MPH), I'm still left with the same questions.  

I wonder if the POH writer had a brain fart and assumed Vx must be a constant TAS?

 

[skydvrboy]

Yes, my F is fuel injected giving it 20 more hp.  I just didn't expect that big of a difference in Vx and Vy.  I think the difference is due more to the greater allowable gross weight in the F, which requires a higher air speed to provide the same lift with the same wings.  FWIW, the speeds listed in my POH are IAS in both sections.  If I had to guess, I'd say you have a misprint in your manual, but I would do a LOT more research before I acted on that assumption.

[kortopates]

1 hour ago, Cam said:

Interesting note - Thanks to Kortopates I did the calculations for TAS at various altitudes and found (assuming Standard lapse rates ) ....

if you take a straight line DECREASE in IAS from 81 MPH to 72 MPH from SL to 10000, you get a (close) constant of 82 MPH TAS . This might make you think that the text and the performance chart match, but the performance chart says CAS is the constant 82 MPH, and since the CAS and the IAS don't vary (significantly - 1 MPH at 80 MPH), I'm still left with the same questions.  

My mistake on expecting to find a CAS correction based on Altitude - such a chart does not exist.

Just like @jaylw314 has indicated above, all the more modern NA Mooney POH will show you that Vx IAS goes up with altitude. The closest modern example to your G is the J model which indicates only going up 1.0 kts for every 5000'. More horsepower and Turbo aircraft go up at faster rate but still relatively a small amount(e.g. 1.5 kts per 5000' for K Encore). The longbody's don't even publish a change in Vx based on altitude - presumably since its pretty negligible. But I sure can't explain your POH verbiage that says the opposite wrt to IAS speeds.

So I would go by the chart you on page 6-5 that shows a constant Vx of 82 CAS.

My MAPA numbers show 70 as a VX takeoff config speed, but this isn't specifically called out in your POH and I believe this stems from the midpoint of the recommended Vref speed in your POH of 65-75. The recommended MAPA Vref uses the higher number of the range or 75 mph.

So in order - our MAPA V speeds numbers for the G are:

Vref/Liftoff 75 mph

Takeoff at 50' agl 80 mph (based on your POH suggesting by 80-90 mph IAS you should have cleared all obstacles  and raised gear and flaps by this range)

Initial Climb=Vy clean 101 mph

Cruise climb 110 mph

Lastly going back to Vx takeoff config - as mentioned your POH doesn't specifically call out the 70 mph, but just list a range of Vref at 65-75 mph (based on weight). I would interpret this as Vx takeoff config being around 70-75 mph with Vx clean being 82 as stated in the table on page 6-5. But I would not worry about the POH note regarding Vx decreasing with altitude given that is in direct conflict with the POH table 6-3 and more modern Mooney POH including the J with the same airframe, just your G is less HP. But if you could also email Frank Crawford at Mooney, whom is responsible for the POH's & documentation and ask if he has any information on that. Good chance he probably does somewhere.

Edited April 4, 2018 by kortopates

[MikeOH]

Ok, interesting discussion.  I'm going to be the heretic and suggest BOTH sets of numbers in the POH are correct!

NOT an aeronautical engineer, but here's my reasoning:

To state the obvious, it's excess horsepower over what is required to maintain level flight that allows the plane to climb.  The IAS numbers that are decreasing are with the aircraft DIRTY, whereas the constant IAS figures are with a CLEAN configuration.  My reasoning is that a lot of horsepower is being used to overcome the drag of the flaps and, especially, the gear.  Drag depends linearly on the co-efficient of drag (Cd) but increases with the square of airspeed.  I suspect the Cd of the gear is HUGE, compared to the Cd of a clean airframe alone.  Thus, since you are already losing horsepower at higher density altitudes you don't want to waste all of it overcoming the drag of the gear, so a SLOWER IAS actually results in more 'leftover' horsepower for climb.

To put it another way, if you tried to climb at 82 mph IAS when 'dirty' at high DA you might be using all the horsepower to overcome gear drag and would barely be climbing.

Probably something flawed with my 'logic' here....Blast away!

[carusoam]

Summary...

1) Vrotate is pretty slow... about 65, you are accelerating as you go through it...

2) Vx comes up next about 75, while the gear is still down and flaps are still in the T/O position... still accelerating...or climbing steeply... 

3) 50’agl, the tree is clear, the gear is away, the POH has said move on to Vy...

4) Past 1000’ agl, the airframe is cleaned up Vy goes to Vy + CHT concerns...

5) Continuing on... cruise climb is a blend of Vs and CHT management 120 on the ASI... common for MSers...

6) Early POHs and owners manuals are known to be either missing data, acquired data from other sources, or were contradictory.... they improved each year...

7) Get a copy of the latest POH for your airframe. Especially when it comes to performance charts....

8) Read a copy of the newest POH of a similar airframe M20J.  This will indicate how much data can be found or generated for your airframe.  Recognize the missing sections in your own POH...

9) Vx is a small piece of the puzzle. Important but small, compared to density altitude (DA) and T/O performance numbers...

10) Can’t climb out over a 50’ tree, if you didn’t get off the ground in time...  once the AOA is raised to a high level the efficiency of lift is terrible, as the drag approaches a very large number, compared to the available thrust...

11) Another thing to consider... Best engine out speeds... in Modern Mooney POHs the efficiency of the wing  is measured in its ability to go the farthest forward for each foot of altitude.  This number is very weight and balance dependent... get a copy of that graph/chart as well...  it opens the door to why and how things work...

12) thanks to Paul, our MS resident instrument guru, for pointing out the importance of CAS vs IAS.  Our OP is noting speeds to the nearest whole number.  These numbers aren’t on my ASI.  Also... See the discussion on accuracy vs precision....

13) I left out the knots/mph discussion because this post is just a theoretical discussion...  the values are very similar between Long and short body...  the LB speeds are in KIAs... SB in MPHs... (approx. a 15% difference overall)

 

Even if you had an exact recommended speed from your POH... how and when would YOU use it?

The more I have flown, the less Vx has become a deciding factor... Gobs of extra runway and hp have become stronger deciding factors.

MS has lost two planes in the T/O portion of the flight (not an engine out situation).  One a departure stall where the highly experienced pilot recounted the details for us... the other was a DA miscalculation with a mush into trees... Good fortune was the difference between being able to elaborate on the situation and not...

Please, think a lot regarding the details and the use of Vx...  like short runway performance numbers... practice on long runways, a lot, first... 

Pp thoughts only, not a CFI...

Best regards,

-a-