M20J vs M20J Takeoff Performance

[kmyfm20s]

4 hours ago, Fry said:

Hello all,

I'm pulling up this old thread because I have looked for a explanation for something I do not understand, no matter how long I think about it.

The POH of a 1978 Mooney M20J 201 shows, under "normal takeoff", a ground roll of about 930ft (sea level, 15°C, no wind) . Liftoff speed 63 KIAS. Takeoff weight 2740lbs.

The POH of a 1996 Mooney M20J MSE shows a ground roll of 1500ft. Liftoff speed 59 KIAS. Of course, comparing apples to apples, so takeoff weight 2740lbs, sea level, 15°C, no wind.

That would mean 18 years later, the same engine with the same prop needed about 60% more distance to accelerate the same mass to even a lower liftoff speed.

That makes no sense to me. Can you resolve this puzzle? Which POH is right? What are your real-world ground roll distances?

I am asking because I am contemplating the purchase of a 1995 M20J MSE and I'm based at a paved 1800ft strip.

Thanks!

 

Are you sure you are reading the POH gross weight correctly? The 1996 J will have a gross weight of 2900.

[Fry]

@kmyfm20s: First of all, hello to Montgomery Field! I'm going there this week :-)

And thanks for your comment, but yes, I am comparing at the same weight, 2740lbs, as I have written above.

Unless someone offers an explanation, I will have to assume that the 1978 POH was written by the marketing department and the 1996 POH by the legal department. I'd just like to know what the engineers would have written :-) so again, unless someone can clear this up, I'll have to do some tests and collect my own numbers.

And before people say, see that your are airborne midfield, or add 50% just for precaution - thanks, I can do all that, but still like to know the real numbers for a start (and then add safety margin on top).

Edited July 3, 2019 by Fry

[mooneyflyfast]

You didn't ask for advice on the advisability of basing an M20J on an 1,800 foot strip but I am going to give it to you anyway FWIW.  I am sure that you are a highly experienced and proficient pilot but after 21 years and 2,300 hours in my 81J I would not feel comfortable doing it on a day to day basis, even if it was at sea level with clear approaches.  My home airport is 2,890 and that is adequate as  long as you are spot on altitude and speed.  After hundreds of landings there I did a go around on Monday because I was a little high on final.  1,800 feet would involve too much pucker factor for me.

As far as the performance of a later model 2,900 GW version vs a 2,740 GW earlier one, I don't know the number but it is going to be exactly the same given the same weight and technique.  I would use the pessimistic figure and add a good margin to that.

 

[Bob - S50]

6 hours ago, Fry said:

Hello all,

I'm pulling up this old thread because I have looked for a explanation for something I do not understand, no matter how long I think about it.

The POH of a 1978 Mooney M20J 201 shows, under "normal takeoff", a ground roll of about 930ft (sea level, 15°C, no wind) . Liftoff speed 63 KIAS. Takeoff weight 2740lbs.

The POH of a 1996 Mooney M20J MSE shows a ground roll of 1500ft. Liftoff speed 59 KIAS. Of course, comparing apples to apples, so takeoff weight 2740lbs, sea level, 15°C, no wind.

That would mean 18 years later, the same engine with the same prop needed about 60% more distance to accelerate the same mass to even a lower liftoff speed.

That makes no sense to me. Can you resolve this puzzle? Which POH is right? What are your real-world ground roll distances?

I am asking because I am contemplating the purchase of a 1995 M20J MSE and I'm based at a paved 1800ft strip.

Thanks!

 

I take everything in the POH with a grain of salt.  In our '78 J, there is no way I can get the nose off the ground at 63 KIAS.  I usually lift off at closer to 75 KIAS.  That also means a longer ground roll.  I've never really looked, but it feels like it takes me about 1200' or so to get airborne at SL.

Who cares what the book says.  Take your plane to nice long runway that has runway remaining markers and bring another pilot along with you.  Load it up as close to gross as possible then make some takeoffs using your normal procedures.  Have the other pilot note how much ground roll you had.  Make 4 or 5 of those takeoffs and use the longest one for your future computations.  Just for fun, compare your results with the POH.

[Fry]

@mooneyflyfast: Thanks for your comment.

Well come and fly in Europe, and you will soon get used to a little pucker factor. I guess all runways are shorter here than in Texas. There is a flight school based in Aachen EDKA using 3 M20Js for training. The strip in Aachen has a length of 520m (1700ft) at 623ft MSL.

But we digress... I agree with you that the performance of a 1978 M20J 201 and a 1996 M20J MSE at the same weight should be the same.

So... anybody can lighten up the contradiction of the POHs?

[KLRDMD]

6 hours ago, Fry said:

The POH of a 1978 Mooney M20J 201 shows, under "normal takeoff", a ground roll of about 930ft (sea level, 15°C, no wind) . Liftoff speed 63 KIAS. Takeoff weight 2740lbs.

The POH of a 1996 Mooney M20J MSE shows a ground roll of 1500ft. Liftoff speed 59 KIAS. Of course, comparing apples to apples, so takeoff weight 2740lbs, sea level, 15°C, no wind.

Is one with and one without flaps ?

[Fry]

@Bob: that's what I will do anyway. It would just be nice to know what made Mooney write those numbers in the POH.

Because, seriously, if the airplane performs significantly worse than the POH says, I would assume some technical problem.

And if it performs significantly better than the POH says, I would lose trust in the POH.

Both would be bad.

[Bob - S50]

1 minute ago, Fry said:

@mooneyflyfast: Thanks for your comment.

Well come and fly in Europe, and you will soon get used to a little pucker factor. I guess all runways are shorter here than in Texas. There is a flight school based in Aachen EDKA using 3 M20Js for training. The strip in Aachen has a length of 520m (1700ft) at 623ft MSL.

But we digress... I agree with you that the performance of a 1978 M20J 201 and a 1996 M20J MSE at the same weight should be the same.

So... anybody can lighten up the contradiction of the POHs?

I don't know if it would make much difference, but the '78 might make a little more power for takeoff than the '96 because of a different magneto setup.  The '78 has the 'D' magneto timed to 25 BTDC.  I'm guessing the '96 has two magnetos timed to 20 BTDC.

[Fry]

@KLRDMD: Both are with flaps 15.

[kmyfm20s]

In 1987 they changed to enclosed landing gear I believe that would increase the drag with the extended gear. I'm would bet there have also been prop changes that would optimize top speed rather than takeoff roll since that's what buyers were after.

[Fry]

@Bob: 50% longer ground roll means ~25% less acceleration. Again, same engine, same power rating, same prop.

@kmyfm20s: Intuitively, I don't think the aerodynamic drag of the inner gear doors plays a significant role at those low speeds in the takeoff roll (zero to 60 knots).

Any other ideas?

[Danb]

Fry that’s asking for quite a bit of performance and high degree of skill allowing for nothing but perfection from you, the plane and weather conditions. 

No offense that’s ripe for a nice 172 or Cherokee 

[Fry]

@Danb: I agree, that's why I am thinking about this. But it is doable! - there are planes other than a 172 or Cherokee (e.g. a Cirrus SR22T, and a Mooney M20C) based on that field, and what I wrote above about that flight school is also true. There is a video on Youtube where someone makes a precautionary landing in an older M20J at my base, and it looks absolutely not marginal, quite the contrary. But we are talking takeoff here, not landing. So, should the performance at MTOW (to be tested at a longer field) turn out not to be sufficient, that simply means I will have to save on weight.

Any idea why the POH values for (apparently) the same plane vary by 50% before and after 1990? Seems Money decided to put in the larger figures about at the same time they went to 2900lbs MTOW. (of course I am comparing figures AT THE SAME TOW) What I'm curious about, if the older numbers are "marketing-driven", i.e. too optimistic, and the latter ones are "legal-driven", i.e. very conservative, then what are the real-world figures?

The reason I am focusing on ground roll here is (i) there is not much aerodynamics or technique involved in getting a rolling a/c up to a certain speed, so that highlights the discrepancy of the numbers, (ii) there are no close obstacles at either departure end.

And BTW, I would not want to own a Cherokee or 172. I guess I don't have to explain why in this forum. No offense :-)

Edited July 3, 2019 by Fry

[gsxrpilot]

I think you'd be fine on that runway. If you can't get a J off the ground in 1800 ft. something's wrong with it.

[jaylw314]

41 minutes ago, Fry said:

@Danb: I agree, that's why I am thinking about this. But it is doable! - there are planes other than a 172 or Cherokee (e.g. a Cirrus SR22T, and a Mooney M20C) based on that field, and what I wrote above about that flight school is also true. There is a video on Youtube where someone makes a precautionary landing in an older M20J at my base, and it looks absolutely not marginal, quite the contrary. But we are talking takeoff here, not landing. So, should the performance at MTOW (to be tested at a longer field) turn out not to be sufficient, that simply means I will have to save on weight.

Any idea why the POH values for (apparently) the same plane vary by 50% before and after 1990? Seems Money decided to put in the larger figures about at the same time they went to 2900lbs MTOW. (of course I am comparing figures AT THE SAME TOW) What I'm curious about, if the older numbers are "marketing-driven", i.e. too optimistic, and the latter ones are "legal-driven", i.e. very conservative, then what are the real-world figures?

I have an '86 J, and my POH shows "takeoff distance" as about 950 feet at gross 2740 lbs, paved surface, no wind, normal takeoff.  I find that to be a little optimistic, and estimate wheels off the ground is more like 1100-1200 feet at gross, but not 1500 feet.

That being said, I don't know if "takeoff distance" is always the same as "ground roll."  Somewhere there's probably a definition for "takeoff distance", but I'm guessing that it does not apply to our single-engine light aircraft.  I'm also guessing that the definition of "takeoff distance" varies liberally depending on who you ask.   That may account for the differences in the POH over time.

In terms of the video, notice how the pilot was nailing 67 KIAS all the way down to when the camera changes.   If you're confident you can do that under pressure, it's certainly doable, but the margin for error is very small.  If he was a couple knots faster, he'd be slamming on the brakes

[midlifeflyer]

Part of the difference may be related to the difference in target airspeed. While  the tables adjust for lower gross weight, they do not reduce the target airspeed accordingly. It's only 4 kts but that's an almost 10% increase.

[Fry]

@jaylw314 you are absolutely right. Speed control is of the essence - and pretty much standard in any flying.
You just need to know exactly _which_ speed to maintain, and that's one of the reasons I am studying these POHs.

[Fry]

@midlifeflyer: well the target speed is actually lower for the later, higher MTOW M20Js...

@all: thanks for your replies!

[midlifeflyer]

Just now, Fry said:

@midlifeflyer: well the target speed is actually lower for the later, higher MTOW M20Js...

@all: thanks for your replies!

How about the 50 ft target airspeed?

BTW a number of folks commented on reality vs tables. Someone recently did a video comparing the new ForeFlight tables with actual performance and can up with quite a wide variation.

 

[Fry]

That is all correct and the reason I am just looking at ground roll here.

Ground roll is actually not really influenced much by technique, aerodynamics etc. It just means "how long does this engine and prop need to accelerate that mass to the target speed".

As far as I understand it, at least.

[jaylw314]

So a hypothesis on what might play a role here is that the FAA defined "takeoff distance" as 1.15x the distance required to reach a height of 35'.  That definition only has to do with aircraft certified under part 23, which does NOT apply to M20's, but I wonder if Mooney started using that definition at some point in the last 20 years?  Anybody know?

Edited July 3, 2019 by jaylw314

[PT20J]

Here are the pertinent pages from the POH/AFM for my '94 J (TOP) and my '78 J (BOTTOM). The newer version doesn't specifically say "ground roll" but that can be inferred by an obstacle height of 0'. For 20-deg C at sea level and 2740 lbs. the '78 version shows a ground roll of 965' and 1831' over the 50' obstacle whereas the -94 version shows a ground roll of 1440' and 2200' over a 50' obstacle. That's a whopping 49% increase in ground roll and a 20% increase in distance to clear a 50' obstacle. All the listed test conditions are the same except that the '78 version notes to lean for smooth operation (which shouldn't make a difference at sea level) and the '94 version references 80% relative humidity. Using an online calculator (https://wahiduddin.net/calc/calc_da.htm), 29.92 in-Hg, 20 deg-C, 80% RH is a DA of 826' whereas the same conditions at 50% RH yields a DA of 737' -- hardly significant. The stated liftoff speeds are about 5 KIAS lower for the later version which I would expect to decrease the ground roll all else being equal. Maybe @mike_elliott or @donkaye or one of the other Mooney instructors has an explanation. 

Skip

 

 

 

 

[mike_elliott]

1 hour ago, PT20J said:

Here are the pertinent pages from the POH/AFM for my '94 J (TOP) and my '78 J (BOTTOM). The newer version doesn't specifically say "ground roll" but that can be inferred by an obstacle height of 0'. For 20-deg C at sea level and 2740 lbs. the '78 version shows a ground roll of 965' and 1831' over the 50' obstacle whereas the -94 version shows a ground roll of 1440' and 2200' over a 50' obstacle. That's a whopping 49% increase in ground roll and a 20% increase in distance to clear a 50' obstacle. All the listed test conditions are the same except that the '78 version notes to lean for smooth operation (which shouldn't make a difference at sea level) and the '94 version references 80% relative humidity. Using an online calculator (https://wahiduddin.net/calc/calc_da.htm), 29.92 in-Hg, 20 deg-C, 80% RH is a DA of 826' whereas the same conditions at 50% RH yields a DA of 737' -- hardly significant. The stated liftoff speeds are about 5 KIAS lower for the later version which I would expect to decrease the ground roll all else being equal. Maybe @mike_elliott or @donkaye or one of the other Mooney instructors has an explanation. 

Skip

 

 

 

 

I dont, the question might have to be directed at the authors, Skip. 

[M20F]

Didn’t see the next page of posts ignore this.  

Edited July 4, 2019 by M20F

[carusoam]

Fry,

All the data for Mooneys improved its accuracy each year...

The charts show the older data sets, the graphs show the more complete data...

It is wise to compare all of the details... MGTW changed after some serial number...

Making sure your plane complies with the data is critical...

Comparing actual T/O roll to book values is pretty easy with a portable WAAS source, and the CloudAhoy app...

Practice your launch technique on a nice long runway... collect data... compare...

Amazing how good the data can be with no guessing, or forgetting, or other common human errors...

A simple change of 200rpm is 10%. and can stretch the T/O roll 50%, M20R data... M20J will be similar...  adding weight or higher temps can all do the same thing.... All stuff to become familiar with first...

Grass details also come in long and short, dry and wet...

HP has a tremendous effect on ground roll.  Some things like mag timing, prop governors, exhaust systems, and intake systems, can add to the challenges...

PP thoughts only, not a CFI...

Best regards,

-a-