Will the real M20J Vy Please Stand up?

[testwest]

Hi everyone

I am doing some research in the subject area of the posts I made for the "CHTs on climb out" thread that I just edited and bumped.

The reason for the edit is a disparity in Vy (best rate of climb) speeds in the various M20J POHs. For our airplane, a 1977 M20J s/n 42, the POH lists Vy as 88 KIAS at sea level standard day gross weight (2740 lb). There is very little position error correction at this speed so the calibrated airspeed for Vy is about the same.

In later versions of the POH, Vy under the same atmospheric conditions (at 2900 lb gross weight vice 2740) is listed as 86 KIAS (KCAS virtually the same, again). This change was first published when the gross weight was increased for M20J s/n 24-3218 and subsequent, in POH p/n 3202B.

Ordinarily, as weight increases the speed for Vy increases, but from the new POH, a 2740 lb. M20J would have a SL standard Vy of 84 KIAS.

On the M20J drag polar that I have, Vy consistently calculates out at 91 KCAS under the same conditions and weight. Hmmm.

Does anyone out there happen to know why the Vy speed was reduced in the later models? Especially any insight to the flight test program that was re-done for the gross weight increase?

[Jeff_S]

Seriously? You're trying to differentiate between essentially 3-5 knots indicated? You're lucky if your ASI is within that margin of error. I'm jesting a little of course, but wondering what the end goal is of your analysis.

[Shadrach]

I do not think that the POHs of the earlier years were that reliable. For my 67' F  Vy is 98Kcas and Vx is 82Kcas. In the POH it's in MPH (113mphcas) and they subtract 1mph from Vy for every 1000 feet of alt...

[Shadrach]

Quote: Jeff_S

Seriously? You're trying to differentiate between essentially 3-5 knots indicated? You're lucky if your ASI is within that margin of error. I'm jesting a little of course, but wondering what the end goal is of your analysis.

[jelswick]

I found the same thing with my 1981 J when prepping for my BFR recently.  I always use 90 knots, but the POH was in the aircraft and me at home, so looked at many generic checklists on line that varied quite a bit, not so much in speed range, but just differing numbers.  One point seeing all of those did remind me of though is that the number is going to vary by weight and it's actually a range based on that rather than a fixed number.  The range again though is as pointed out by someone above not a substantial swing.

[DaV8or]

It is my super limited understanding and I really don't know anything about drag polars, but isn't it just a mathmatical calculation based on an airfoil shape and dimensions? Couldn't the Vy number in the real world be effected by other parts of the aircraft, like the fuselage and horizontal stabilizer? I'm guessing that Mooney derived the Vy numbers in the POHs by actuall flight testing.

If it was me, I'd call Vy 90kts and go fly. Nice kind of average between the books and your acedemic findings. It's easy to remember and easy to see on the ASI.

[Skywarrior]

The reason Norman is this interested in the discrepancies is obvious to me, based on my college Flight Testing classes. The only people who mention "position error" are current or former test pilots.

Chuck M.

[Shadrach]

Quote: DaV8or

It is my super limited understanding and I really don't know anything about drag polars, but isn't it just a mathmatical calculation based on an airfoil shape and dimensions? Couldn't the Vy number in the real world be effected by other parts of the aircraft, like the fuselage and horizontal stabilizer? I'm guessing that Mooney derived the Vy numbers in the POHs by actuall flight testing.

If it was me, I'd call Vy 90kts and go fly. Nice kind of average between the books and your acedemic findings. It's easy to remember and easy to see on the ASI.

[Jeff_S]

I guess my point was that while I understand the concept of computing the theoretical drag polar, and maybe doing that just for fun (well, THAT I couldn't understand, but hey, I was a French major) in the real world, our planes are all so NOT fresh of the factory floor, and have all been modified up one side and down the other, that it really just that...theory. But if that is the engineer's version of me doing a crossword puzzle, then I totally get it.

[testwest]

Thanks for all the replies, everyone!

For Shadrach, is there any way you could email me a copy of the climb performance section of your POH/Fight Manual? My email is on my profile.

For my research project, I am trying to refine the drag polar (mathematical model of M20J performance) so that the model accurately predicts data that aligns with previously gathered and published data for a limited set of conditions. This way the model then outputs accurate data for any input condition.

And a 4 knot difference in published data (88 KIAS for Vy versus 84 KIAS for the exact same conditions and weight) is significant. I do know the model is off right now because it is giving me 91 KIAS for Vy.

It's important because the focus of the research is the postulation of an efficient climb speed based on the Carson's speed number that I wrote about here: http://www.mooneyspace.com/index.cfm?page=2&mainaction=posts&forumid=3&threadid=1784

The relevant text from that post:

"Vx, best angle, is the best foot of climb per foot forward. [distance based].

Vy, best rate, also propeller (not jet) power-on L/D max, is the best foot of climb per second. [time based].

Carson's speed climb (at WOT), the one you never heard of, {about 1.32 times Vy} is the best foot of climb per forward velocity [speed based]."

My paper is going to introduce a new V speed, called Vz, based upon a Carson's speed climb until the point a performance limit is reached, currently postulated to be 500 fpm.

And yes, Skywarrior "outed" me, I am currently an experimental test pilot at Boeing.

More later..........

[Parker_Woodruff]

Hi Norman,

My 1990 M20J-AT listed Vy as 86 KIAS (2740 lb).  I figured the difference between the earlier and later M20Js must have been because of the slightly higher amounts of drag on the older airframes.  My stock 1990 model was just cleaner than a stock 1977 model.

I'm not a flight test engineer, so I don't know if that's true.  But it's just a guess.

[Shadrach]

Norman, I'm happy to do that for you, but my POH is for a 67F model. I only referred to it to illustrate the lack of accuracy in the factory manuals back in the day. If you still think it would be of use let me know. 

[John Pleisse]

88KIAS. The earlier J's are lighter AND have less drag.

[testwest]

So, I think I am going to go with 86 knots as Vy for the J, at least for refining the drag polar.

For Jim R, if you have copies of those Aviation Consumer articles by Brent Silver, I'll pay ya for 'em!!

[DaV8or]

Quote: testwest

And yes, Skywarrior "outed" me, I am currently an experimental test pilot at Boeing.

More later..........

[testwest]

Not yet, that is on today's to-do list. I think Bill Wheat may still be hanging out at Dugosh.....?

[edit] I did get ahold of Bill Wheat through the (new and improved) Dugosh web site http://dugosh.com/

Here is what I sent:

Hi Bill

I am working on a research project, and was wondering about the change in Vy
(best rate of climb) speeds as published in the various flight manuals. The
67 M20 F lists Vy as 98 knots, the early M20J is 88 knots (our airplane is
24-0042, N201JX) and the late (2900# gross weight) Js list Vy as 86 knots,
and the chart in the late books leads one to believe a 2740# M20J now has a
Vy of 84 knots.

Can you fill me in on a little background? Was the flight test program to
certify the J for the amended type certificate under a lot of schedule
pressure back in 1976?

And here is what he sent back....he sure answers e-mail fast!!

Norman,

The M20K came along soon after the M20J.  This required structural changes to the tubular cabin structure due to a heavyier engine and higher airspeeds.  The early M20J, due to changes and additional fairings, increasing the operating loads retracting the gear, there was a operating actuator change to a more powerful unit. This allower hogher operating speeds for the gear.  Also there was an increase in gross weight.

Since both the M20J and M20K were made on the same production line at the same time it was more practical to make both models the same except for the engines ,equipment, and fuel system.

Bill Wheat

Although he does not state so explictly, it sounds like the MAC flight test department probably took the same strategy and tried to combine as much work as possible and not re-fly tests if the FAA did not require those tests.

I definitely think the Vy for our airplane is closer to 86 knots than 88 (certainly not 91). An 86 knot Vy gives a Carson's Speed of 113 KIAS...91 knots Vy gives 121 knots for Carson's, and when I tried a 121 knot cruise-climb (actually a practice Vz schedule) the airplane reached its performance limit of 500 fpm climb far too early in the profile.

My AFM (77 M20J) says a Vy climb from sea level to 8000', standard day, gross weight (2740#) gives:

Time                       Fuel                           Distance

10 minutes               16 lb (2.7 gallons)       14 nm

So the velocity made good over that ground distance is 84 knots (D/(T/60))....and the MPG is 5.2 (nautical) miles per gallon.

After the drag polar is refined, I'll try to predict time-fuel-distance for a 8000' climb, SL stnd, MTOGW using the postulated Vz schedule and the drag polar.

More later..........

[testwest]

Bumping this thread, I have made huge progress in modeling the Mooney M20J in Benchmark. I think I'll be able to publish some charts before too long. Had to get used to this new MacBook Air laptop.....wow. Starting to think I've crossed the river and ain't going back....