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Comparative Propeller Test Flying printer friendly version
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Bob Grimstead
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Posted Thursday, July 3, 2008 @ 11:00 AM  

Hi again Folks,

I have done a bit of propeller test flying in my Fourniers over the past few years. I thought I might share some of it with you all.

First, let’s talk about how to do it.

Like everything else in flying, you only get good results if you are careful and meticulous.

I’m sorry if any of you find the following insulting. I’ll try to go step by step through the process, just in case anybody is new to it.

First, it is vital to compare like with like. You just cannot compare Fred’s standard aeroplane at maximum weight on a hot day with Arthur’s modified, big-engined one in mid winter with a nearly empty tank.

If you want to compare propellers, it is really only possible to do so on the same aeroplane – there are just too many variables. Last weekend we flew a formation of four Fourniers and compared our instant airspeed readings. They varied by over ten mph! My British Fournier’s electronic tachometer reads more than 400 rpm different from my Australian one’s original mechanical one with the same prop in identical conditions.

Comparing speeds at anything other than full throttle is fraught with problems, too.

Add in altimeter errors, varying weights and centres of gravity and you will immediately see that you just cannot compare apples with oranges.

Don't even start to think about quoting VSI readings. The VSI (variometer) is much the most inaccurate instrument devised by man. Climb rates can only be assessed accurately by using altimeter and stopwatch.

When I fly to compare propellers, I try to make both flights on the same day, in quick succession, so ambient pressure (altimeter setting) is constant, and the temperature is very similar. I either fly at maximum weight (with radio, battery and full fuel plus me, normally dressed, but no baggage) or at aerobatic weight (ditto, but with only 17 litres of fuel aboard). This means I have 15 litres, the maximum aerobatic fuel, by the time I have climbed to 1,500 feet.

If your spinner won’t fit over both propellers you’re comparing, leave it off for both flights. I don’t know the change in drag effect, but there will be one.

I set 1013 HPa or 29.92 inches Hg on the altimeter, to eliminate that variable, and airspeed checks are flown at either 1,500 feet or 500 feet, the upper and lower limits of my aerobatic display.

For your timed climb, it is important to fly in a straight line at a steady airspeed ACROSS the prevailing wind. Otherwise the ‘kiting’ effect of positive windshear will skew your readings. Those of you who’ve flown glider winch launches on a breezy day will be well aware of this effect.

You should also stay well away from thermals, ridge or wave lift and cloud streets. No fair cheating now!

Indeed, the more still the air, the more accurate will be your readings, so try to pick a calm day and fly perhaps either early in the morning or late in the afternoon.

Test pilots fly in ISA conditions: pressure of 1013/29.92 and 15°C temperature. For those Americans among us still stuck in the stone age, that’s quite a cool day, but I no longer understand Fahrengrade numbers. About 60-65 perhaps?

It is important to hold your airspeed accurately, to within +/- 2 mph/knots.

Climb checks are generally timed from 500 feet, because that gets me out of ground effect and the most turbulent lower levels. It also enables me to establish steady at the correct climbing speed (I use 60 knots or 70 mph) and get trimmed and settled. Simply hit the stopwatch as the altimeter’s big hand swings past 500 feet, then check the height gain after 60 seconds and you have a useful comparative climb rate.

For a ‘proper’ test, climb at full throttle for five minutes. Note your height at every minute (or every 30 seconds if you want to be really accurate) and then draw a graph of your points against time. You should of course get a gently sloping curve, as your climb rate drops off with height. If it is accurate enough, you could presumably extrapolate to get your service ceiling (that altitude, with standard altimeter setting, when your climb rate drops off to 100 fpm).

The most important speed check is at full throttle, although partial power speeds might be of personal interest. Again, it is important to get your aeroplane exactly trimmed out, and to let the speed settle. You should fly exactly level (+/- 10 feet) for a MINIMUM of two full minutes before reading your airspeed. If you cannot do that, then you can get an approximate reading by averaging your maximum and minimum speeds as you float up and down around your datum altitude, but this will be no good for comparison with other props or Fourniers.

Now for some numbers….

These are taken verbatim from my Fournier test flying notes, and I apologise in advance for any discrepancies.

Yours, Bob

Donald
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Posted Friday, July 4, 2008 @ 07:40 AM  

Hi Bob,

Puzzled by this part: "For your timed climb, it is important to fly in a straight line at a steady airspeed ACROSS the prevailing wind. Otherwise the ‘kiting’ effect of positive windshear will skew your readings. Those of you who’ve flown glider winch launches on a breezy day will be well aware of this effect."
Yeah, but the difference there is that in a glider launch you are anchored to the ground by the cable so wind speed does have a bearing, but in a climb by yourself your airspeed is just that, your airspeed. It's what your pitot head 'sees' and your ASI reads, it's what your wing and your prop 'sees' too. That you are also drifting across the ground below due to wind is surely not relevant.
I do however agree that when doing flight trials of anything it makes the job easier and more reliable if conditions are calm.

Donald

Bob Grimstead
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Posted Friday, July 4, 2008 @ 04:40 PM  

Yes Donald,

You are not alone. Many people have trouble with this concept. Your compatriot and my esteemed colleague, James Allan is one of them.

Please accept that your aeroplane, however light, has a mass and therefore inertia.

One day when you're feeling brave, take off into wind and time your climb to 1,000 feet. Then land again and take off downwind. Time your climb the same, and see the difference.

Then you'll understand why test pilots always make their climbs acorss the prevailing wind.

Yours, Bob

Donald
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Posted Saturday, July 5, 2008 @ 03:33 AM  

Bob,

I'm going to take your word on this for I have no intention of taking off downwind for a flight test. I'll do it if my life depends upon it and there's no alternative.
I have only once made a full downwind landing when I uncritically accepted the runway direction from air traffic. I have never felt so close to wiping the gear off my plane!

Donald

Bob Grimstead
Captain

Gender: Male
Location: Perth, Western Australia or West Sussex, England
Registered: Dec 2006
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Posted Saturday, July 5, 2008 @ 04:34 AM  

Aha,

Downwind take-offs and landings are different.

You will only rarely catch me landing in a tailwind. There is no airflow over your controls, so they don't work too well (depending on wind strength of course).

Downwind take-offs are less demanding, because you have slipstream over your elevator and more imoprtantly over your rudder.

I often have to take off downwind, because my airstrip is just 360 metres long, and slopes significantly to the north-east. That climb-out is comparatively clear, while the other way has phone wires and power lines (trees, too, but you can see them)

Predictably, it's not the actual take-off, but the climbing away that's the problem in a Fournier, due to this wind shear, or the change in wind gradient with height.

That's why we make timed climbs across the prevailing wind.

Yours, Bob

Donald
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Posted Sunday, July 6, 2008 @ 05:29 AM  

Bob,

I can understand your point in relation to wind shear but I have never done my climb tests from ground level. Just getting tidied up after take off would blow any chance of me achieving a steady climb rate. All my tests therefore have been begun below 1000ft and I start the clock as I pass that height. That said, next time I do it I'm going to try it into wind, down wind and across the wind to see if I can measure a difference, or anyway a difference beyond what my sloppy handling allows.

Donald

[Edit by Donald on Sunday, July 6, 2008 @ 05:30 AM]

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