Performance Degradation Over Time

[slgrossman]

Did you ever fly a brand new ship for awhile then come back to it a few years later to find that it's become a real dog? What are some of the factors and what is their relative weight in decreasing the performance (or perceived performance) of an aircraft over time?

I can think of a few likely contributors:

First off the airframe gains weight, both through repairs and the addition of supplementary equipment. The main rotors, having been replaced individually rather than as a matched set don't track and balance as well. Erosion and other wear causes turbine engines to lose efficiency. Panels and doors no longer fit perfectly, disrupting the airflow.

What other factors might there be and how much of a decrease are they responsible for? Does surface dirt substantially affect the performance of modern rotors? How about erosion? Do worn bearings in the flight controls have much effect?

-Stan-

[Old Man Rotor]

The pilots gain a little [or a lot] in their Operating Weight..........

[Lu Zuckerman]

Theoretically, There should be no endplay in a flight control system. This is especially true for those systems that have hydraulic servos. On most hydraulic servos the pilot valve must be moved about .012” in order to start fluid flow. If a push-pull rod end bearing has endplay it is a sign of poor maintenance and if one bearing has excessive wear it can be assumed that many if not all bearings have endplay. This accumulative endplay could cause the control system to hit the stops prior to making a full input on the pilot valve. This could effect the flight control input and the flight characteristics of the helicopter. Conversely when the control is moved in the opposite direction the controls will be displaced a significant amount before the pilot valve is displaced. Considering the pilot valve must move .012”, a significant amount could be an accumulated ¼ to 3/8ths of an inch or possibly more. And, that is a lot considering some helicopters are equipped with auto pilots that are controlled by variable differential transformers that require about .001” or less to generate a signal. If you have a SCAS system there might be a mismatch between the mechanical input of the pilot valve and the L/RVDT input to the autopilot.

If there is a mismatch between say the left lateral input and the right lateral input there could be force fighting between the servos.

:cool:

[Nick Lappos]

Lost performance is almost all due to two factors - engine power degradation and rotor blade erosion.

The engine can be checked by using a routine hit/power assurance check, and comparing the engine to delivered norms. Power assurance margin is directly usable as a tracking tool.

rotor blade erosion is harder to track, since there is no easy way to find the lift vbs drag characteristics of a specific set of blades. One way is to conduct a climb power check, using a well calibrated torque gage and a known gross weight to compare the climb performance to that called out in the flight manual.

Wear on controls, increased door gaps and such will not cause any appreciable loss of power performance although control slop can cause vibrations.

Aircraft (and pilot!) weight gain is easy to find, just use a scale. Most aircraft gain about 1/2% empty weight per year due to equipment increases, spilled oils, dirt and paint.

[slgrossman]

Nick,

Is there any measurable effect from a coating of dirt or dust on the rotors? How about blade repairs that don't perfectly match the contour? Worn and ragged scarf joint tape? Tip cap seams that aren't perfectly smooth? Is any of this stuff significant?

I've seen significant differences in level flight cruise speed from one ship to another. I've also noticed that one ship will seem to leap off the deck, while another will struggle off at the same weight and ambient conditions. I suppose a lot of that can be explained by instrument calibration errors, but I was wondering if there weren't some other subtle factors at work.

-Stan-

[Barannfin]

I also wonder how much lift/efficiency is lost when the blades have a layer of dust/frost on them. At our flight school we keep the blades clean, so there hasn't really been an opportunity to fly a ship with dirty and then clean blades to try it. I know the layer of dust interferes with the laminar (sp?) flow on the blades. Just not sure as to the effect on power reqd.

edit: Umm actually I just found some numbers on the web, says that ice or frost accumulation with the consistency of med. grit sandpaper on the leading and upper edge of the wing can reduce lift by as much as 30 percent and increase drag by 40 percent.

:eek: seems pretty high.

[ShyTorque]

Don't forget the rose tinted specs effect of flying a new aircraft....it flies better when it looks and smells new.

My car always always goes and handles better when I have cleaned it. I always write better in a brand new notebook....etc

But we do make sure our rotor blades are washed along with the rest of the hardware.

[Nick Lappos]

slgrossman,
You are right on, all those blade condition factors I imprecisely lumped into the word erosion. The drag of the blades will directly rob power and performance, to a degree greater than the typical bug-smash's effects on a light airplane. The rotor tips of even a light helo are moving at about 675 ft/sec, or about 360 knots, where the typical light airplane moves its wings at 110 knots.

I can believe a 1"reduction in MP going from dirty to clean.

barranfin,
Frost is a real problem, dust perhaps less so but still not good. Never fly with such pervasive stuff on your aircraft, especially lifting/control surfaces. It is quite alright to see pure frost on preflight inspection, run up and pull no pitch, then shut down to see if the frost has sublimated. Never do this with ice or thick frost, your neighbors on the flight line will get rich from the insurance settlement from the chunks you hurl into their aricraft. Your tail rotor might also take a pounding.

More insideous and more unsafe than the loss of performance is the loss of max lift (at high angle of attack) that you might get from dirty airfoils. This could lead to premature retreating blade stall and loss of control. I had a flight in an old S-58 where the blade erosion (which was ageneral sandpaper condition on the abrasion strips) lead to blade stall on gusty afternoons, and a 10 knot cruise speed reduction. Swapping blades made everything better. On a tail rotor, this could lead to loss of anti-torque margins that might embarass you at the wrong time.

[Old Man Rotor]

Ggggeeeeeeeeee.

Don't you guys ever have a day off.....maybe a warm or cold beer or two..............

[GLSNightPilot]

Flying in rain will cause lots of roughness on the leading edge of blades. I've checked them after flying in rain, & they are really rough, even if they were very smooth prior to the flight. I've seen large improvements in performance from just having maintenance smooth the blade leading edges. This is supposed to be done during some inspections, but it often helps to have it done more often, especially when the aircraft has been flown in the weather on a regular basis.

[902Jon]

On their s76 fleet, Bristow helis changed the screws on the blade tips to a round head from a flush fitting. Were there any flight manual considerations in terms of performance by this change? The aircraft seemed in my experience to seriously underperform from book values during still-wind / warm days.

[Shawn Coyle]

Remember that the torque or manifold pressure you see in the cockpit is only a measure of the drag of the rotor blades. Less smoothness will generall mean more drag, and when you run out of power to turn the blades at higher angles of pitch, you run out of lift.
So, keep smooth, clean blades, and if you fly in heavy rain, take the time to get leading edge blade tape, and keep it in good condition.
I'm surprised no-one makes a kit of some cleaning and waxing solutions to keep blades clean. I understand some manufacturers specifically prohibit waxing the blades - can anyone elaborate?

[Helinut]

One summer, a few years ago, a private R22 owner at where I used to work started to moan about his engine being down on power. This went on for a while; engineering checked the engine and no problem found.

Finally, I was asked to do a flight test with the chief engineer. When we looked at the main rotor blades they were absolutely filthy with an accumulation of dead insects, pollen all set in gooey hard stuff.

When we flew it, it did seem a bit slow, and a height climb test showed it way down on power. R22s often struggle to pass this test; sometimes an engine just seems to be low on power from out of the box.

We then got the blades cleaned and hey presto, an extra 5 knots cruise speed!

[Aladdinsane]

Helinut,

for those not in the know, can you explain the R22 height climb test and where one would find it documented?

cheers

[Lu Zuckerman]

Many years ago we waxed our Bell 47 wood blades every 25-hour check or more often if required. Having several Sikorsky helicopters we figured that we should also wax the metal blades on the those helicopters. This lasted a short time when we found that the solvents in the wax were attacking the Epon adhesive that held the pockets on the spar. This might be why some manufacturers warn against waxing.

Regarding erosion of the leading edge of rotor blades the B-105 when first introduced had blades that were lifed for 5000 hours and that cost was added to the cost of the helicopter. Many operators in the Gulf of Mexico operated in rainy conditions and had to blend out the erosion on the leading edge of the blades. This occurred so many times that the operators were afraid that they had blended out too much material and structurally weakened the spar so, they removed the blades around 1200 hours. The operators asked Boelkow to change the blade material and they were told that all operating parameters were based on the installed blades. The operators in this case were not getting what was guaranteed in the purchase agreement.

Regarding leading edge tape it works best when there is no rain. When exposed to operating in rain the tape will de bond slightly and the CENTRIFUGAL FORCE will cause the tape to move outward and when it reaches the tip the high relative wind will cause the tape to shred off. Eventually the tape will be gone.

:cool:

[Barannfin]

uh oh.... :rolleyes:

[GLSNightPilot]

I've had tape come loose, & it gets your attention. The noise makes it sound like the blade is coming apart. Tape usually works well until you fly in rain for a few hours, then it's worse than nothing.

[reynoldsno1]

I worked with a large heli operating company in SE England many years ago, and, on average, all the hulls gained 5% in weight a year and was the most significant factor as far as payload/range was concerned.

[Nick Lappos]

reynoldsno1,
I would certainly believe 1/2% per year (about 50 lbs on a big machine, all due to dirt and such accumulation. The old Bristow bunch told me a few years back that they put the aircraft into a thorough vacuum cleaning after lifting the floor boards to regain some performance, since 50 lbs of dirt spread across the aircraft is quite understandable.

[Tail Bloater]

Restating the obvious:
Clean and 'polished' main and tail rotor blades will benefit lift. Clean compressor blades will benefit power.
In UK compressors should be washed after every flight as the fresh air is contaminated with salt.
Salt is sticky stuff and corrosive and is sure to degregate aerofoil performance.
Polish the blades first, clean the windows inside and out next and worry about the shine on the paintwork last.