Do you really trust your POH performance numbers

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pelmet
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Do you really trust your POH performance numbers

Post by pelmet » Sun Aug 18, 2019 2:46 pm

An interesting video that makes a comparison....

https://www.youtube.com/watch?v=IMRafSuzkQ8
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photofly
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Re: Do you really trust your POH performance numbers

Post by photofly » Sun Aug 18, 2019 4:02 pm

For the 182, the technique used in this demonstration doesn't match the instructions in the POH, so the performance doesn't match the POH performance. Big surprise.

Takeoff: the shortest takeoff ground roll is achieved with 20° flaps, and this pilot only used 10°. This pilot also waits until the aircraft flies itself off the runway: there's no positive rotation. The POH requires "Lift off at 49KIAS", not "wait until aircraft leaves the runway".

Landing: the minimum landing ground roll technique requires HEAVY braking (verbatim from the POH) which means right on the edge of a skid (maximum tyre traction occurs with the wheel turning about 10% slower than the ground speed, I believe) and retracting the flaps and applying full up elevator immediately upon touchdown. The pilot didn't do this, either.

Moral: if you're going to fly like a muppet, expect muppet-like performance.
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JasonE
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Re: Do you really trust your POH performance numbers

Post by JasonE » Sun Aug 18, 2019 4:55 pm

I recently tested myself in a 172 vs book numbers. I was considering going into a fairly short private strip with an obstacle at 1 end on a hot/humid day. I did much better than they experienced in that video with only about 10-15% over book numbers, but the margin was too slim for my comfort and opted for alternate/safer plan.
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goingnowherefast
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Re: Do you really trust your POH performance numbers

Post by goingnowherefast » Sun Aug 18, 2019 5:58 pm

I believe a brand new aircraft flown by a test pilot with recent several hundred hours on type is capable of matching book numbers. Give me (or anybody) a brand new 172, lets practice for 3 hours a day for 2 months and you'll get much closer to book numbers, possibly even beat them slightly.

I bet the pilots who compete in STOL contests routinely beat the book performance numbers.

Also, expect to burn through brakes and tires like they're free.

Now if you want to fly comfortable, rolling take-offs, easy on brakes after touch down, etc., then yes, you'll be well beyond the maximum performance capability of the aircraft.
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Dry Guy
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Re: Do you really trust your POH performance numbers

Post by Dry Guy » Sun Aug 18, 2019 6:02 pm

I wonder what we won't be really trusting next?
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photofly
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Re: Do you really trust your POH performance numbers

Post by photofly » Sun Aug 18, 2019 7:08 pm

goingnowherefast wrote:
Sun Aug 18, 2019 5:58 pm
I believe a brand new aircraft flown by a test pilot with recent several hundred hours on type is capable of matching book numbers.
Why does it take several hundred recent hours on type to use 20° of flaps and raise the nose at 49 KIAS?

If a not-brand-new aircraft is 30% deficient in performance compared to a new one then there's something wrong with it and it needs maintenance. And for the record, the aircraft in the video was in pretty good shape.
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goingnowherefast
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Re: Do you really trust your POH performance numbers

Post by goingnowherefast » Mon Aug 19, 2019 4:25 am

Well yeah, you need to use the right flap settings and speeds. That's inexcusable. An old aircraft flown as specified by the book will be much closer to book numbers.

Takeoff is easier than landing, How does one practice maximum performance braking without doing it several times first. An important factor in landing distance is also the touchdown point. Doesn't help to float 50' past the intended touchdow point if you want to stop in 900'.

I say new airplane because texture of the paint matters, maybe the engine is tired with a couple cylinders just barely above minimum compression, the prop is filed down significantly with a couple fresh rock dings, tire is perhaps a little soft, bearings don't spin like they used to, shimmy dampener allows a little wiggle. Still might meet all the minimum airworthy standards, nearly worn out and far from peak performance.

The aircraft was capable of meeting the book numbers when it was brand new and flown as specified. An old aircraft flown as specified might meet what JasonE experienced:
JasonE wrote:
Sun Aug 18, 2019 4:55 pm
I did much better than they experienced in that video with only about 10-15% over book numbers....
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photofly
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Re: Do you really trust your POH performance numbers

Post by photofly » Mon Aug 19, 2019 6:29 am

goingnowherefast wrote:
Mon Aug 19, 2019 4:25 am
Doesn't help to float 50' past the intended touchdow point if you want to stop in 900'.
That's quite true, but in these tests they were measuring from the actual point of touchdown, so it didn't matter where the aircraft contacted the ground.
I say new airplane because texture of the paint matters, maybe the engine is tired with a couple cylinders just barely above minimum compression, the prop is filed down significantly with a couple fresh rock dings, tire is perhaps a little soft, [ don't spin like they used to, shimmy dampener allows a little wiggle. Still might meet all the minimum airworthy standards, nearly worn out and far from peak performance.
I think this bears examination. Let's look at each in turn:

Paint: I don't think the paint texture matters very much (there's certainly no restriction on flying with dirty or rough paint, like there is for textured frost). Moreover, worn paint would cause extra parasite drag. Parasite drag matters very little at low speeds such as occur during the takeoff and landing roll. To the extent that it would matter, it would actually shorten the landing roll, not lengthen it.

Engine: The story about Continental running engine tests with incrementally increased leakage - until they ran an engine with no piston rings at all and still got 90% performance on a dynamometer - is probably just a story. But the tiny leakage needed to reduce static compression to 40/80 will not measureably affect engine performance. Given that a minimum ground roll landing is carried out with the engine at idle, it could not affect the landing roll at all.

Also note that engines with a fixed pitch propellor should have their static RPM assessed each takeoff run as a broad gauge of engine power. I have flown many worn engines and never had to reject a takeoff because the RPM was below limits due because of age or wear. For a typical static RPM of 2200, each 100 RPM represents about 4.5% power, so being within the permitted tolerance gives an indication that the engine is producing at least 95% of rated power.

Prop:
A worn prop may reduce engine power to thrust conversion and therefore negatively impact takeoff performance. There are strict dimensional tolerances for propellors, which (one imagines) are set such that any excessive loss of efficiency will mean a prop is out of limits and unairworthy. It will not affect landing performance.

Bearings:
A dragging bearing is a maintenance issue and needs to be fixed. Once replaced the bearing will function as new, because it is new. (I have just replaced both bearings on my 1975 airplane.) While such a defect will lengthen a takeoff roll, it will also shorten a landing roll.

Tires:
This is a maintenance issue. An aircraft with soft tires will have a longer takeoff roll, but a shorter landing roll.
minimum airworthy standards
I'm not sure what these are, or where one finds them. If a defect measurably impacts performance it has to be fixed. If a defect negatively affects performance to the extent of lengthening a takeoff or landing roll by 30% then in no sense is the aircraft airworthy. We don't have a protocol for reducing the maximum takeoff weight by 30% because the aircraft is "tired", do we?
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Kirk: This is a dangerous mission. Likely, one of us will die. The landing party will be me, Spock, McCoy, and Ensign Ricky.
Ensign Ricky: Aw, crap.

goingnowherefast
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Re: Do you really trust your POH performance numbers

Post by goingnowherefast » Mon Aug 19, 2019 9:58 am

A good example of minimum airworthy standards is a model of Hartzell propeller. It comes in 74" and 72" varieties. The 74" version can be filed down to dress prop tip damage until 72". The 72" version has no such allowance. The 74" prop, when new, has about 5% better takeoff performance compared to a brand new 72" version. It's reasonable to assume that a 74" prop, worn and dressed down to the minimum standard of 72" would perform similar to the 72" propeller.

Minimum airworthy standards are found in the specific aircraft or part manufacturer's maintenance manual or the AC 43.13

A brake disk rotor might be brand new, or very near the worn out thickness. The nearly worn out rotor will be more susceptible to heat and brake fade, especially on a warm day.

Are the tires brand new, or is there almost no tread left? Both are still legal and airworthy.

To discredit the video, you also have to use the brakes to the limit of tire traction or significant brake fade.

Point to ponder. When they flight test new transport category aircraft, they install worn out brakes and tires for the rejected takeoff data collection. It's because they want the worst case scenario and are required to by transport category certification standards. Utility aircraft are not required to meet transport category standards, so they install good parts and give the sales team something to brag about.

Yes, poor technique is probably responsible for a majority of their performance difference, but don't discount "non-perfect" aircraft mechanical condition either.
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Re: Do you really trust your POH performance numbers

Post by photofly » Mon Aug 19, 2019 10:10 am

I agree the prop is one place where wear can conceivably impact takeoff performance, and brakes (and tire condition) can do so for the landing.

goingnowherefast wrote:
Mon Aug 19, 2019 9:58 am
To discredit the video, you also have to use the brakes to the limit of tire traction or significant brake fade.
I didn't detect any attempt at "MAXIMUM" braking in the video, and the voiceover even says "they avoided wear on the brakes...", did you?

I don't think anyone should expect POH "maximum" performance during everyday flying, and if that's the moral of the video and it needs to be hammered home to weekend pilots, so be it, but I'm naturally suspicious of excusing poor performance because the airplane is just "old" or "tired" in some kind of generic way.
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Kirk: This is a dangerous mission. Likely, one of us will die. The landing party will be me, Spock, McCoy, and Ensign Ricky.
Ensign Ricky: Aw, crap.

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JasonE
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Re: Do you really trust your POH performance numbers

Post by JasonE » Mon Aug 19, 2019 8:19 pm

I had the prop redone on the Cherokee - "0" timed. The previous 2 inspections were done signed off by a small AME but it looked pretty rough so I sent to the prop shop. It was worth every penny, and wish I'd done it 3 years previous. I gained 5mph cruise and about 150ft/min climb. I would have saved the cost of the overhaul in fuel over the 3 years I owned the airplane. (Sold it 3 months after) But that is a perfect example of how performance may vary from one aircraft to another with the same configuration. Maybe if I'd had it re-rigged I could have gained another few MPH :)
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