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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 28 Dec 2021, 09:09 
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Attachment:
MU-2K Spec Range ISA+10C 9000 lb.JPG

This is the relevant airplane flight manual (AFM) page. I don't expect a real-world stock MU2K to go 300 ktas, but Steve Bankston's did 284 at 16,000 ft on 81 gph.

I plotted fuel flows assuming 6.7 lb/gal, this is very simple on speed and S.R. axes.

Comparing two data points -- airplane flight manual (afm) and test (test) at the same altitude, temp, weight, rpm, fuel flow, and prop efficiency, the speeds (V) depend on drag coefficient (CD) and engine efficiency (ETA), translating fuel flow into torque, as follows:

(Vtest/Vafm)^3 = (ETAtest/ETAafm) x (CDafm/CDtest)

You can estimate the ETA ratio by looking at the torque you got versus the torque for the same fuel flow and altitude in the flight manual -- that's how much your engines are "down" compared to the AFM airplane. The rest is drag. This is all very approximate but should yield consistent comparisons.

In the photos that Steve Bankston kindly provided, I assume his airplane was around 9000lb (a mid-weight for a K model) and the gauges show 16,000 feet, ISA+11C, 284ktas, 81gph and 70% torque. I spotted the letter B on my chart at 81gph at 16,000 feet. The AFM shows 78% torque at that fuel flow, so the ETA ratio is (70/78) or 90%, or about 10% down. The speed ratio is 284 (Steve's speed) vs 296 (AFM). Cube that, divide by 0.90, and you have about 2% left for the drag deficiency, i.e., Steve's plane was only 2% off book drag.

Note, Steve's rpm setting was min cruise, rather than max, so I bet his engines were better than 90%, and his drag was a little more than 2% off perfect.

For the airplane I tested, it was right at 9000 lb, ISA+7C (including TAT correction to OAT), and 25000', average torque (with a bad split between the gauges) 58%, 253 ktas, 65gph, max rpm. So if it was at max cruise power, it should be where the letter C is spotted on the chart (the corner point where 25000 foot line hits the ITT limit line), which incidentally matches the observed fuel flow very well. At that point, the torque is 62% so ETA is (58/62) or 93.5%, and the engines don't look too bad at 7% down. The velocity ratio is 253/289, cube that and divide out 0.93 engine defect and you get a drag ratio of 1.39 -- a 39% drag increase... which I take to be dang near impossible unless you left the flaps down, hung a spoiler, or maybe left the gear out.

The gear was retracted, the control wheel was centered, the roll trim was nearly centered and the spoilers were observed on the ground to be properly rigged. I can't absolutely swear that the flaps were retracted but the other pilot points out that we would have noticed if they had been left extended, when we went to extend them during deceleration. Rate of climb was zero and we'd been on speed for about 2 minutes -- no acceleration. Yaw trim was approx 2deg. I examined the plane carefully after flight and the seals all looked very good including the doors and the tank-to-tip seals; all the deice bladders were smooth and well bonded, the gear doors that close on the ground closed well. The props looked perfect and the visible compressor and turbine blades looked perfect. The engines started normally and the airplane accelerated well on takeoff. Pressurization was working properly. I never saw the flaps retracted on the ground so it's possible that they don't stow perfectly at flaps 0.

So I'm stumped. It's an interesting conundrum. Should have more data in a few weeks.


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 28 Dec 2021, 09:55 
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If the engines are sick, ITT, FF, and torque won't line up. You will end up with max ITT and FF, but low torque.

The fact there was a large split in torque could be an indicator the torque gauges are miscalibrated. Or it could be the engines are differently sick.

Before I would buy this airplane, I would do the following:

1. Borescope

Doesn't matter if you find some other explanation, get a borescope. The -6 engines can be abused more easily than the -10 and an engine can make remarkable amounts of power with internal damage. This step saved a friend of mine from buying a K model with badly eaten first stage wheels, about $200K to fix.

2. Torque calibration

Given the gauges are split, you need to know if this is real or miscalibrated. The simplest is to set the gauges to the DSC (datasheet customer which gives torque pressures from last engine stand run), or the latest Lebow run (if there is one). It would be interesting to note what they were set at versus the DSC numbers.

If you really want to do this, do a Lebow run. This involves replacing the prop with a torque sensor prop and doing some ground runs. Then you will get real torque values. A Lebow run is the gold standard in torque check.

3. Pitot/static checks

You *may* just have a pitot leak or static leak. A small static leak from cabin can lead to elevated altitudes and reduced indicated airspeed, for example. A pitot leak can lead to under reading airspeed.

4. Compensation check

The DSC also lists the compensation setting, and adjustment of the ITT reading to normalize engines. This can be adjusted. You can get the unadjusted ITT by pulling the ENG CTRL breaker. Then you can see how many degrees and which way it moves and compare it to the DSC. If the compensation is excessive, someone has been trying to hide the condition of the engines which leads to near or over temp operation without indication.

You really can't buy the airplane until you understand why it underperforms.

Again, you need external verification of the airspeed. This can be from ADSB data and winds aloft soundings.

I can probably find which airplane this is. There are not that many K models, and one you eliminate the known -10 ones and the ones owned by Air 1st, it is a pretty short list of candidates to find ones that have recently flown at FL250, which isn't all that common for -6 K models.

Mike C.

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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 28 Dec 2021, 11:00 
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Realize that the pre SRL models have no overtemp protection for the engines. A less than diligent pilot could easily over temp an engine due to misreading the chart or getting distracted.


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 28 Dec 2021, 12:33 
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I’d start looking for a static system leak. Would make it appear slower at altitude than it really is. Easy to occur.

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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 28 Dec 2021, 13:30 
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Username Protected wrote:
Realize that the pre SRL models have no overtemp protection for the engines. A less than diligent pilot could easily over temp an engine due to misreading the chart or getting distracted.

I think they may have had over-temp protection, but I’m not sure if all of them did or if mine has an optional feature. My K model has the toggle switches that provide over-torque and over-temp protection. I was advised that when the -10 conversion was made and I lost my ITT gauges (the -10 conversion switches to EGT probes from ITT but has no SRL) the over-temp protection system is lost, but the torque protection remains (mine works very well and limits fuel flow as necessary to keep TQ at 100% or less). So it is quite possible that this stock -6 K model with ITT sensors had both torque and temp protection…but again, I don’t know if my plane has an optional system or if it was stock.

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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 28 Dec 2021, 14:04 
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Ok. I thought temp protection was only available on the SRL models. I trained some on a Merlin with -3 to -10 converted engines and it had no temp protection system in it.


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 28 Dec 2021, 14:40 
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No temp protection on my -1's.

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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 28 Dec 2021, 16:19 
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Username Protected wrote:
Ok. I thought temp protection was only available on the SRL models. I trained some on a Merlin with -3 to -10 converted engines and it had no temp protection system in it.

I don’t think any stock MU2 came with either torque or temp protection. I believe both systems were STC add ons.

Also, SRL is not “protection”. It merely computes a Single Red Line value for all conditions so the pilot doesn’t have to look it up in a table. So I can always be assured that if my temp is below the red line on the gauge, I’m ok. But it doesn’t prevent me from pushing well past the red line if I keep moving the levers past it.

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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 28 Dec 2021, 16:50 
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Jon,

They did, most operators removed them in the mid 80/90’s; -5/6 engines had a habit of torching the stators when left in ITT mode. They were capped off and 880 was used as cruise temp limit for the 135 airframes I flew. Took our P model in to Tulsa for work and the shop was amazed we still had functional limiters.

Nigel


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 28 Dec 2021, 17:50 
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Username Protected wrote:
Jon,

They did, most operators removed them in the mid 80/90’s; -5/6 engines had a habit of torching the stators when left in ITT mode. They were capped off and 880 was used as cruise temp limit for the 135 airframes I flew. Took our P model in to Tulsa for work and the shop was amazed we still had functional limiters.

Nigel

Interesting, I thought I recalled that all the limiters were after market. Thanks for the correction. A lot changes and evolves over the 40-50 years these things have been in use.

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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 28 Dec 2021, 19:12 
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Thanks all, esp Mike C for suggestions. The airplane has a power limiter but I've heard from a guy with a sense of humor that "only Pat Cannon ever uses those". And I don't really know where these engines came from (who was using them). The info is available so I can dig it up, along with their recent maintenance history. At any rate, it should be easy enough to borescope the first stage turbine through an igniter hole and curl around through the combustion chamber exit?


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 28 Dec 2021, 19:28 
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I'm not sure of the MU2 version, but -10 in a 441 the temp numbers at cruise are arbitrary numbers. You'll have a DSC sheet that specifies a range of temperature trim values your shop sets to get the torque to line up. If you are getting exactly the same torque when the temps match it is likely someone made it that way, and not that your engines are exactly the same. Essentially on your better (more efficient) engine, they tell the gauge it's at a higher temp than it is so when both engines are at "redline temp" the torque matches.

In any case, note at what altitude your engines temp out and the torque at each engine when that happens. Your engine torque was lebow calibrated at factory, and at every overhaul since so that should be correct and is not adjustable by a typical A&P or shop. I expect that is accurate.

If you are that far off of book airspeeds you are possibly running your engines much too cool, which is easy to fix compared to the opposite. It's so far off though it'd say you have a TAS issue.

Again this is for a 441 -10 installation with a toque and temp limiter and FCU and the SRL system so not sure what applies.

I would never buy any turbine aircraft without a borescope on both engines. It is relatively cheap, doing an early hot section is not.


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 29 Dec 2021, 23:32 
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Username Protected wrote:
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.



Yep 285-290 all day in the very high teens and low 20s. Following this thread, very curious what the result ends up being. Hopefully temp gauges are just off and you’re running too cool. Have I mentioned today how much i love my MU-2? Speed and high wing loading are so addicting. Love hearing Delta or PC12s cry on the freq about turbulence while my passengers sleep away in comfort :-)

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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 03 Jan 2022, 09:30 
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I’ve had three MU2s, an “N”, a Marquise and now a Solitaire. They all made/make book numbers.

There are two Power Assurance Charts in the AFM (I’m assuming that it’s in the older AFMs as well). The number on the gauges should match the chart. This is your starting point. I have attached the charts for a Solitaire. Other models are different

The pitot systems are separate and the static system was checked within 24 months. If both ASIs agree, it’s not a pitot static system problem

Where are the numbers on or pictures of the gauges?


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 Post subject: Re: MU2 Pre-buy speed 36 ktas below book
PostPosted: 03 Jan 2022, 09:44 
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Username Protected wrote:
The pitot systems are separate and the static system was checked within 24 months. If both ASIs agree, it’s not a pitot static system problem


I used to think that as well, until : I chased a low reading airspeed indication for over 2 years on my (then) newly acquired Aerostar 600A (non-pressurized), despite that 2 pitot/static tests showed no problem.
Then I had a Malibu that was in my shop that had the same problem as soon as the cabin was pressurized, this despite that the pitot/static test was fresh and showed no leaks whatsoever.

In BOTH instances it was a B nut on the back of the T on the airspeed indicator that was not tight, just slightly loose, so that no leak was detected during ground tests, but positive cabin pressure would leak into the airspeed indicator enough to push the IAS down .

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