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Author Topic: Bench trimming  (Read 1545 times)

Offline Ron Merrill

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Bench trimming
« on: October 09, 2006, 09:33:01 AM »
Hope this is the place to ask this question. What is involved in bench trimming? What are we trying to accomplish or prevent. ??? y1 <= #^ Ron.

Offline Bill Little

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Re: Bench trimming
« Reply #1 on: October 09, 2006, 09:36:53 AM »
Bench trimming is the process we all go through before that first flight.  Checking the CG, wing tip weight, lead out position set ups.  Keeps some "surprises" to a minimum we hope!

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Offline Ron Merrill

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Re: Bench trimming
« Reply #2 on: October 09, 2006, 10:54:41 AM »
  • Does that in include the vertical CG? ???  Are there articles out there that cover this process?  #^ Ron.

Offline L0U CRANE

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Re: Bench trimming
« Reply #3 on: October 10, 2006, 01:21:38 PM »
Ron,

Vertical CG has been mentioned occasionally, as In I think, Tom Dixon's magazine bipe article (38Special?).

Vertical CG shouldn't need much attention for a 'pretty standard' layout stunter. However, extreme low- or high-monoplane wing mounting, or a biplane, or extreme dihedral might make it more important.

The consequences of wrong vertical CG can only be a banked flying posture, with the  model banked  out in upight flight in the direction the CG is "off". that would be banked "in" if CG is "low", or "out" if CG is high.

Could be worth a check - easily done when it is too late... Hang the model from the leadouts and take a look from the front...
\BEST\LOU

Offline Dick Fowler

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Re: Bench trimming
« Reply #4 on: October 10, 2006, 05:34:30 PM »
Lou's suggestion of hanging the model by the leadouts is a good one. It can tell you something about CG location in general. I usually do that to all my planes when complete to make sure the nose hangs just a bit down. Kind of gets it in the ballpark for the first flight.

 A plumb-bob from the center of the leadout spacing to the fuselage will give you the CG location on the fuselage too.
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Offline Ted Fancher

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Re: Bench trimming
« Reply #5 on: October 10, 2006, 07:10:24 PM »
Hi Ron,

Simply put, bench trimming (a term originated by Big Jim Greenaway a couple of decades ago) is nothing more than the process of evaluating a new airplane from operational, safety and aerodynamic  points of view prior to its first flight.

The basics are insuring the wings are straight, flaps and elevators are neutral at the same time and that roughly equal amounts of up and down control are availabe without binds.  Flaps and elevators should also be symetrically (side to side) neutral at the same time.

The outboard wing should be slightly heavier on an airplane with built in assymetry (longer inboard wing) and noticeably heavier on one with equal span wings.  The precise amount necessary can only be determined through test flights.  For our purposes here be sure the above criteria are met and any errors should be in the "slightly heavier than anticipated" rather than less.

The location of the CG is an important aspect of bench trimming, especially if you fear the ship might be tail heavy.  Tail heavy airplanes can be difficult (impossible if extreme) to fly safely and even harder to land.  Generally speaking the designers recommendation can be a safe place to start although some older designs provided questionable (far aft) suggested CGs.

Any remotely normal stunter will fly safely with the CG located at 15% of the average chord (halfway out each wing) aft of the leading edge.  Balance the ship on your finger tips at that half chord location and measure the distance aft of the CG and then divide that number by the length of the chord (distance from the leading edge to the trailing edge "including flaps")  If the chord is 10" for instance, 15% aft of the leading edge will be at 1.5 inches.  If it's forward of that I'd consider adding weight to the tail until it gets to 15% for first flights.

If the CG is aft of that you need to make a more critical evaluation to see if it is safe to fly.  Figure the area of the tail (multiply the span by the average chord of the stab/elevator) and then divide that number by the area of the wing (normally given on the plans somewhere or else figure it the same way you did the tail, span X average chord -- don't get hyper critical about area inside the fuse or tip shape, etc.  this isn't that critical)

The CG can safely be located at a point aft of the leading edge (at the average chord) equal to the percentage of the tail area to the wing.  Thus, if the tail is 150 square inches and the wing is 600 square inches the tail is 25% of the wing area.  The CG on such an airplane can safely be located at a point 25% aft of the leading edge at the average chord.  For instance, if the average chord is 12" the CG could safely be located at 25% of that number or three inches aft of the leading edge.

If it is behind that number you will need to add weight to the nose until it balance at least that far forward of where it started.  Don't let anyone talk you into "just give it a try".  Tail heavy airplanes can be extremely difficult to get up and down safely, especially if the control system is very sensitive.

Leadout position goes hand in hand with the CG.  Once you've located the CG per the above, use the adjustable leadout guide to locate the mid-point of the leadouts about an inch behind the CG.  Ideally you'd use the LineII program from Pete Soule to determine a more precise first flight location but an inch aft will cover most conditions with full size stunters.  It'll be within a fraction of an inch of correct and won't pose an issue for first flight safety.

On the subject of leadout location, many older designs have a fundamental flaw in that they have the leadouts exit the wing the same distance apart as the size of the bellcrank.  Doing so is a big mistake form the point of view of yaw trim.  If your airplane is one of those that show the leadouts three inches apart, modify the wing and leadout hole locations in the ribs to as to space them no more than an inch apart.  Mine are typically only 1/4 inch apart. 

Having them far apart causes the airplane to yaw when control inputs are made and the load on the leadouts changes due to the airloads on the control surfaces.  Giving up control (the rearward leadout historically) will cause the airplane to yaw out increasing drag and inducing roll away from you and giving down control will do the opposite.  With the leadouts very close together this trim anomally goes away.

Make sure all the wheels roll freely (equally so on the main gear to avoid turning in at you) and that the airplane tracks straight ahead if you give it a push on a smooth level surface.  A little turn outboard is acceptable.  Fine tuning might eventually have you cock the main gear to provide a very slight left turn for those "pro style" long smooth take off rolls.  Not for your first flights though!

A nominal pull test (don't try to break the thing) and a check of all serviceable nuts and bolts should be included ... don't forget the prop nut and spinner.  While you're up front be sure to check cowl security and peak inside to make sure all the plumbing is hooked up correctly and that you have a good filter installed on the feed line.  New tanks can be notorious for shedding flecks and flakes of residue from the soldering process.  Better to catch them in a broad filter that won't obstruct the fuel flow and burn up your brand new engine.  Check the filter after each of the first half dozen flights or so to find out of blockage is incipient.

Make sure the engine, prop, and line length/diameter combination are servicable for first flights.  Don't try to come up with that magic 5.5 second lap the first flight out (besides which, read my comments elsewhere on seeking slow lap times for their own sake ... don't!).  Pick a combination that wil almost insure lap times of around five seconds or so a lap.  Better a little faster than slower.  Generally I'd use a bit more pitch or lower diameter than I expect to ultimately use.  The higher speed will pay dividends if trim issues surfact on first flight by providing greater line tension and more positive control.  If you've any questions about what combination is appropriate ask a expert local area flyer for advice or come back to one of the on-line forums.  Give specifics of your aircraft so guys can respond promptly.

Ideally, the power train/line length set-up will be something you're familiar with from other similar acft.  If you can match a known set-up that is the best way to go.  Otherwise err on the faster predictable lap time.

Double check your handle and lines to insure that any adjustable features are appropriately set (again, ask a local expert if you have specific questions ... too many variables for a simple one size fits all response).  Make sure all the adjustable features area secured properly and check the condition of both the control lines and any handle cables installed.  Both are sources of more accidents than you care to know about.

Use some system to insure you always install the lines and handle correctly to avoid taking off with the controls backward.  I personally paint all the leadouts on my airplane and the the control line ends with red and green paint.  Green is up and red is down.  Others always leave one connector on the airplane and one on the lines so that they can only be hooked up one way ... just make sure that you always use the same combination so that lines may be interchanged if necessary.  I.e. always leave the connector on the "up" leadout.

Finally, be sure that there is no restriction to the prop,  No rubbing spinners against the fuse, nothing inside the cowl rubbing on the thrust washer, etc.  You don't want anything dragging the engine which could cause it to overheat.

This should have been covered during design/construction phases but before the first flight double check that adequate cooling airflow is available both into and out of the cowling.  More exit than entrancce is a long standing and valid determination.  The cool air expands when heated by the engine and requires more area to expel the additional volume.  Be careful with piped cowls.  It often looks like a lot of exit area is back there until you put the pipe and header in the holes.  If so, get out the knifes and sandpaper and saw away some of those restricting bulkheads.

That's all I can think of off the top of my head.  Anybody want to fill in the holes?

Ted Fancher

Offline Ron Merrill

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Re: Bench trimming
« Reply #6 on: October 13, 2006, 12:36:02 PM »
Thanks a million, for the input. My bench trimming didn,t go near far enough.  AP^  I will do better with my LA Heat. Ron.


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