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07 Aug 2022, 20:40 [ UTC - 5; DST ]


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 Post subject: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 09:40 
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Folks, I recently test-flew an MU-2K in very good shape and got 253 ktas at 25,000' vs 289 ktas flight manual prediction for identical weight/alt/temp at max cruise power.

I know there are a thousand pitfalls in measuring performance and I promise to come back and report when, as I expect, this data point is confirmed/refuted. But for now, rather than present all the data for in-depth analysis, I'd really like to know from MU-2-experienced persons: do MU2s generally fly on book speeds, or a few knots slow, or all over the place..... or even if no one ever really checks them against the flight manual, so we don't really know.

For reference -- I hear that Beech flight manuals are pretty accurate, and I can verify that a Mooney 252 that I owned flew dead on book, within 1 knot and 0.1 gph.

Thanks in advance --

Charley Brown


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 10:00 
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Username Protected wrote:
Folks, I recently test-flew an MU-2K in very good shape and got 253 ktas at 25,000' vs 289 ktas flight manual prediction for identical weight/alt/temp at max cruise power.

Something is wrong. Is this equipped with the original -6 engines, or upgraded to -10 engines?

How was KTAS measured? Need to make sure that data point is valid.

Can you provide the tail number and date of test flight? From that, I can determine the KTAS from the GPS ground speed and winds aloft. This will double check your measurement.

Do you have fuel flow, RPM, ITT, KIAS, OAT, weight data during the speed run test.

I would not buy that airplane without a thorough borescope. I have seen a -6 K model with eaten up hot section, so it does happen if it has been flown improperly.

If this is -10 equipped, and it truly only did 253 KTAS, then something is seriously wrong.

It is also possible there is some other problem. and the engines are fine.

Mike C.

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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 10:13 
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If your numbers are correct, Boroscope the motors. They are likely sick. What’s the time since overhaul and hsi?


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 10:23 
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Mike^2, thanks for the quick replies.

I purposefully masked the airplane identity ... for now ... because the owner is equally surprised and is promising to look into the situation. It is a completely stock MU-2K with -6 engines.

You both reacted the same way that I did -- something's wrong here. And it may very well be my measurements.

I promise that I will provide my complete data eventually because there should be an interesting story here, one way or the other.

Meanwhile, I'm still interested in any verifiable data or well-informed assertions indicating that MU-2s do/not fly close to book.

Charley


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 10:28 
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On a -6 K I don’t think you will get book numbers.
The book numbers on our -6 M (SA TCDS) were fantasmical, in some cases well higher than what a -10 converted plane would actually do.

Airspeed indicators could be off?

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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 10:41 
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My K-model with -6's and mid time engines was consistently 275-285ktas depending on temp.. That was cruising in high teens to FL220 typically.


Please login or Register for a free account via the link in the red bar above to download files.


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 10:47 
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Thanks Tom, that's interesting about the M model.

Two airspeed indicators agreed well, but just to indicate ONE of a THOUSAND possible pitfalls, what if the pitot or static source were off? Also, since I was not actually planning a cruise speed check, I did NOT turn around and check GPS groundspeed in the opposite direction.

So again, I'll resist the temptation to prejudice the question with incomplete or maybe even erroneous data, and momentarily keep asking for actual info on flight manual veracity, which should be of general interest to everybody.


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 10:51 
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My former F Model with wimpy -1s had a TAS of 260kts religiously so something is off if all you are getting is 253 kts with -6s. I'm pretty sure JD's has -6s and he is getting 285 usually I think.

What's your torque and ITT gauges say? That doesn't lie either and might support the sick engine theory.


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 10:56 
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Steve, thanks, very helpful. As a quid pro quo, I'll send you my cockpit instrument photos and the flight manual page for comparison. I'll share them with everyone else later when I have my full story together.

PS -- is there a way to download those pictures and look at them in larger scale? (I'm a total noob at BeechTalk).


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 11:02 
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Charley,

How did you set cruise power at 25,000’?

It has been decades since I flew a MU2, so my memory is very fuzzy.

I am assuming ITT was limiting at that altitude.

Were fuel flows right for that altitude?

Were temps and fuel flows the same for both motors?

GPS (accounting for wind) will answer the question as to how fast it was going.

If I was owner I’d take it up and fly 360deg, 120deg & 240deg long enough to see what the average ground speed is at that power setting before calling my mechanic.

I’m hoping the problem is just a pitot static issue.

Forrest


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 11:20 
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Mike T, thanks. Kirk Zastrow reported 265 ktas on 60 gph in his F model that sold this year, which is pretty consistent with your datum.

If I spot Kirk's TAS/FF on the MU-2K cruise page, it corresponds to 22,000 feet which is about where you'd probably go for best cruise performance, I'm thinking.

BTW, my impression is that all MU-2 engines are pretty similar except for the temp limits? That a -1 and -10 pull the same at low altitude?


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 11:45 
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Here is an article on TAS testing. The 3 heading method is very simple. Find smooth air.
Stabilize on a random magnetic heading and note the GPS GS, turn left 90 degrees stabilize and not the GS, turn left, and note the GS. Plug those numbers into the Spreadsheet link in the article below and you have a very accurate TAS number.

https://www.kitplanes.com/flight-testing-finding-tas-from-gps-data/

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Ground B25 GS, P-51 GS DPE renewal


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 12:23 
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Username Protected wrote:
Here is an article on TAS testing. The 3 heading method is very simple. Find smooth air.
Stabilize on a random magnetic heading and note the GPS GS, turn left 90 degrees stabilize and not the GS, turn left, and note the GS. Plug those numbers into the Spreadsheet link in the article below and you have a very accurate TAS number.

https://www.kitplanes.com/flight-testing-finding-tas-from-gps-data/

Doug, how many minutes on each leg for a reasonably accurate measurement?

Thanks,

John


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 12:57 
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Username Protected wrote:
Doug, how many minutes on each leg for a reasonably accurate measurement?

Thanks,

John


That would depend on how high you are, and how much energy was lost, turning onto each leg, but if IAS has not changed in 60 seconds, it is probably not going to change much more. A turbine airplane that is at a very high altitude, high angle of attack, and lower indicated airspeed, is going to take much longer to stabilize than a B-55 at low angle of attack.

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Ground B25 GS, P-51 GS DPE renewal


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 27 Dec 2021, 13:08 
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On a normal IFR Pitot Static check do they run the Airspeed up to verify accuracy?
I had them do it on my new Garmin panel just to make sure everything was reading right.

I guess I recall them doing it on our M-model, but we had the A/P disconnect plumbing in the pitot lines and they didn't match left to right.

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