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Measuring stick position in light aircraft

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Measuring stick position in light aircraft

Old 4th May 2020, 18:55
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Measuring stick position in light aircraft

I've thought about this for a while but what prompted me to make this post is that I will be flying a Pawnee for the first time, hopefully in about a month.

I will be investigating the handling, particularly stall characteristics. Is there a simple way to record the stick position, particularly in pitch?
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Old 4th May 2020, 21:35
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Use a fabric tape measure or marked 'calibrated' length of string.
Anchor the measure on the front instrument panel, extend it, by-passing the pilots torso - under arm, and secure on the seat / rear bulkhead (or fix it to self and sit back to hold the measure tight).
Note stick position as required, static datum (1.3 vs ?), thence record values relative to the datum.

Alternative, use an extendable tape measure with the boss secured to the stick top (double sided tape), but free to extent.
Attach the zero end to the front instrument panel and record measurements relative to datum as the tape extends vs stick position.

Voice recorder to note measurements vs speed.
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Old 5th May 2020, 03:49
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India,

You might find that stick FORCE per attitude change, rather than stick position is generally what the handling pilot perceives, unless the stick travel is very unusual - reversed or very large. Think back to your first flights in the 182, I am sure the heaviness in pitch - especially at the flare - is more prominent in your memory than how far the yoke came back.
A more suitable implement may be a “Fish weighing scale” used to measure the pull forces required to generate “g” loads, or pitch and roll rates. Correlating these with the varying airspeeds found during the climb, cruise, descent, & approach would be interesting. Measuring the pull force required to maintain an off-trim speed can give insight into the pitch stability characteristics.

However, one thing I do, especially when first flying a new-to-me aerobatic aircraft, is to measure/estimate the position of the stick at the stall. I also look for the amount of aft (for upright, forward for inverted) stick travel available past this point. Stick position, as well as load, does give a powerful clue to the angle of attack & nearness of the stall boundaries.

As well, these numbers can be used to quantify the control harmonisation. The classic handling formula is 2-4-6 units of force to generate a rate of attitude change in roll, pitch, and yaw - the venerable Chippie is one aircraft that meets this. Although it is not the whole story, as I understand the Spitfire does not. Measuring the rudder load would take more flexibility and a smaller physique than I personally possess, an estimation would suffice here, I think.

Have fun, & congratulations on planning a more scientific checkout than most....

Jamesel

Last edited by Jamesel; 5th May 2020 at 04:03. Reason: Stick position
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Old 5th May 2020, 09:32
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Having used, and given up on, various devices specifically made for flight testing, I've flown with something like this for years for handling testing - purchasable for usually around a tenner from fishing shops and travel luggage shops (and doubtless at the moment online suppliers), a handheld force gauge with a tape measure built into the handle. I define a convenient reference point, usually the front lower edge of an instrument panel for pitch displacement - but as Jamesel says quite rightly, for most handling evaluation work, it's force, not displacement that's critical. A velcro strap is a useful addition for attacking it to the relevant hand control. Remember that you can do all the data reduction you need out of the cockpit, so it's fine if your neutral stick position is 223mm, or whatever - just record the numbers.



Guessing you aren't trained originally as a test pilot, a few pointers...

- Get all of your recording needs pre-defined on your kneeboard, and work from that. The term used for this is "test cards", and it's best to get everything - instructions to you, and place to record data, together on the same cards.
- Don't let any normal airmanship, especially lookout, lapse whilst doing your tests. It's much much too easy to forget to lookout whilst fixated on your experiment in the cockpit.
- Have clear, including time and fuel state based KIO/RTB criteria defined - you do not know how long these tests will take until you fly them, and need to be prepared to land, rest and refuel, and return to the task safely.
- Record all of the "common data" to the test - ensure you have data to record W&CG, engine state, trimmer position(s) for everything you do.

W.R.T. Stalling particularly, a few further thoughts...

- Don't compromise all and any safety practices you'd normally include in stalling. (Safe height, HASELL checks, pre-brief spin recovery, etc.)
- Deceleration rate to the stall can have marked effects on stall characteristics: I usually test , 1, 3 kn/s and 5 or max achievable without climbing for aeroplanes in that class
- There's a lot to observe in a stall test - buffet, pitch attitude, airspeed, audible warnings, tendency to wing rock, control response as the aeroplane slows (3 axes!), height loss, response to recovery actions. In my experience, expecting to capture everything in one go is unreasonable. Expect to fly multiple instances of each test, recording 1 or 2 variables each time. This will get you much better data, and at the same give you a further insight into consistency of the stalling characteristics.
- Consider calibrating the ASI against a GPS down to, say, 1.1Vs and assume some consistent extrapolation down to stall of the low speed end IAS/CAS curve. There are multiple ways of doing this - here's a good basic guidehttps://bura.brunel.ac.uk/bitstream/...0A%20FINAL.pdf . That said, there should in a certified aeroplane be a published curve, although depending upon how rigorous your assessment needs to be, a cross check on that may have value nonetheless.
- I have seen some light aeroplanes where the stall characteristics are also heavily affected by pitch trimmer position. To be fair, most are not, but consider it.

G

Last edited by Genghis the Engineer; 5th May 2020 at 20:05.
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Old 5th May 2020, 11:36
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For control position, I use the small pocket 36" measuring tapes, and carry a few spares. Masking tape to the stick and appropriate cockpit feature. I do this more in helicopters than airplanes. Some helicopter testing has stick position as a primary element, not so much for planes.

For stick force, I use a digital luggage scale with a "peak" feature. If I miss recording the peak control force, I get get it right after the maneuver.

I tested the rather unsettling pitch control force reversal on the Siai Marchetti 1019, after experiencing some non compliant handling during testing. The negative pitch forces I had encountered were quantified by my testing. Those negative forces, in this case, did correspond to abnormal stick position - during a test climb with C of G well within the range, I reached the forward pitch control stop, and the nose was still rising. This seemed to be a design characteristic of the type, and was never resolved, but I didn't certify it either! In this chart, "Tq" means engine torque
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Old 6th May 2020, 08:12
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Thanks chaps for your detailed and informative responses. I shall review in detail and decide how to approach my Pawnee evaluation flights. I'm also thinking of doing some evaluation of my club's DG-1000 glider. Depending on which seat I sit in and whether the 12 Kg of tail ballast is installed, I can evaluate CG positions from near the forward limit to about 60% of the way to the aft limit. The advantage of evaluating a two-seater is that I can have a safety pilot and therefore spend more time "eyes in".

Genghis, you guessed correctly - I'm not a test pilot, although I do have a copy of "Handling the Big Jets" and I have stayed in a Holiday Inn Express!

As well, these numbers can be used to quantify the control harmonisation. The classic handling formula is 2-4-6 units of force to generate a rate of attitude change in roll, pitch, and yaw - the venerable Chippie is one aircraft that meets this. Although it is not the whole story, as I understand the Spitfire does not.
Jamesel,
I've flown both types. A couple of hundred of hours in Chipmunks a long time ago and 30 minutes in a Spitfire a few years ago. Compared to the delightful handling of the Chippie, I was surprised by the Spitfire's handling. It was very sensitive in pitch but the ailerons were very heavy, even at only 250 kts. I think here is a reason for that two-handed spade grip!

I did wonder whether the pitch sensitivity might have been due to an aft CG - I was a bit heavier in those days!

PS I've just looked up the Siai Marchetti 1019. A turbo-prop Bird Dog with up to 400 HP!

Last edited by India Four Two; 6th May 2020 at 08:23.
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Old 6th May 2020, 12:08
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Genghis, you guessed correctly - I'm not a test pilot, although I do have a copy of "Handling the Big Jets" and I have stayed in a Holiday Inn Express!
As do I, but also I have a copy of this, which is free to download, and a really good intro to the basics of flight testing.


https://www.faa.gov/regulations_poli...mentID/1027429

G
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Old 8th May 2020, 04:17
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Thanks for the very useful link, Genghis.
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Old 10th May 2020, 03:39
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Compared to the delightful handling of the Chippie, I was surprised by the Spitfire's handling
Yet the gossip by supposedly knowledgeable folk say the handling is similar. Only ever flown the Chippy, delightful as you say, have read elsewhere the Spitfire is heavy on the ailerons, will take your experience on board whenever I next hear the conversation saying the two handle the same.
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Old 10th May 2020, 08:03
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Originally Posted by megan View Post
Yet the gossip by supposedly knowledgeable folk say the handling is similar. Only ever flown the Chippy, delightful as you say, have read elsewhere the Spitfire is heavy on the ailerons, will take your experience on board whenever I next hear the conversation saying the two handle the same.
Consistent with this article https://www.pilotweb.aero/features/f...munk-1-4669362 I tried to find a more quantitative comparison but failed.
I was aware of this roll rate and aileron stick force data for the Spitfire.
Attached Images
File Type: png
WW2FighterRollRate.png (86.9 KB, 10 views)

Last edited by djpil; 10th May 2020 at 08:04. Reason: typo
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Old 11th May 2020, 01:54
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... have read elsewhere the Spitfire is heavy on the ailerons
megan,
I have to say though that the "view out of the window" and the extra 1500 HP or so, more than makes up for any handling deficiencies!


Plus of course the bragging rights.

A well-known instructor flew in it after me and he told me afterwards in inimitable Kiwi fashion: "You sit in that back seat and it feels just like another rubbishy old Harvard, but then you look out at that wing and you know you are flying something special!"

djpil,
Thanks for posting that interesting graph. An amazing improvement in roll-rate when the Spitfire's wings were clipped.
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