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Author Topic: Trimming a model  (Read 306 times)
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bbdave
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« on: April 22, 2019, 05:57:33 PM »

I built a rubber Piper cub about 4 years ago and have never flown it I have no idea how to trim it do I just bend the surfaces to induce turns climb etc.? I have flown rc gliders which is obviously easy to trim but now fed up with failed rc gear I fancy going back to simpler modeling.

Dave
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Jack Plane
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« Reply #1 on: April 23, 2019, 02:32:26 AM »

Don Ross - Rubber Powered Model Airplanes


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Pete Fardell
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« Reply #2 on: April 23, 2019, 03:16:58 AM »

Hi Dave,
Good luck with your Cub. As well as bending surfaces there’s also thrust line adjustments, incidence adjustments, nose weight/CG adjustments and motor selection/power adjustments. That Don Ross book is handy. Also, this is a trouble shooting page in Bill Hannan’s classic book, ‘Peanut Power’.

Best bet really though is to try and get to an event where some old hands are flying rubber so they can help. One of them will probably take one look at your trimming flights and say, “Ah, that needs a bit more ... [whatever it needs]“ and you’ll be up and away! Alternatively, post a vid or two on here and see what people think. Usually works for me!
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Re: Trimming a model
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billdennis747
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« Reply #3 on: April 23, 2019, 03:37:36 AM »

Best bet really though is to try and get to an event where some old hands are flying rubber so they can help.
You don´t say whether your interests are indoor or outdoor, although you are allowed to do both. If possible, go to the Indoor Nationals at Walsall next Sunday and/or the outdoor Nationals at Barkston Heath at the end of May. If you have any latent enthusiasm it will be sure to be activated.

PS Pete, I assume the previous page covers setting up of balance and alignment. Get those right and the Cub can´t fail.
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Bryanair
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« Reply #4 on: April 23, 2019, 04:42:23 AM »

Hi Dave      If you tell us where you live perhaps we can put you in touch with someone near you.   Bryan
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Pete Fardell
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« Reply #5 on: April 23, 2019, 04:47:58 AM »

PS Pete, I assume the previous page covers setting up of balance and alignment. Get those right and the Cub can´t fail.
The whole preceding chapter is on set up and flying, as applied to peanuts. The Don Ross book Jon recommended is possibly more relevant.

You don´t say whether your interests are indoor or outdoor...
I was sort of assuming it was more of an outdoor model due to your previous experience of RC gliders, but yes, more details please Dave! Kit build? Size?

EDIT: I think I've found it: https://www.hippocketaeronautics.com/hpa_forum/index.php?topic=19285.0
Looks good!
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Jack Plane
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« Reply #6 on: April 23, 2019, 05:14:11 AM »

1. Check static balance.  Is it as per plan (with rubber in place)?  If not then add nose-weight until it is.
2. Check ALL surfaces are warp-free and and are attached exactly as per plan (i.e. wing and tailplane incidence). If not, then correct now.
3. Check prop is balanced.  If not trim and scrape the heavy side.  Then smooth both blades with fine abrasive paper.
4. Attach trim tab to rudder.  Assume will be flying in a left-turning circuit, but only put a very gentle bend in for now.
5. Attach down-tab to left wing about 3/4 of way towards tip, and bend in very gentle down.
6. Make up shims (1/32 and 1/64 ply) ready for adjusting thrust later.  You'll almost certainly need some down and some right.
7. Prepare some extra nose-weight (will almost certainly need a bit more than static balance).

Then start glide-tests on a very calm day over long/soft grass.  Aim is to get a gentle descending turn to the left without stalling/diving or spiralling in - balance the amount of down-tab on the wing with the amount of left-tab on the rudder.  Might need to add more nose-weight now (in small increments) until glide is consistently regular without hints of mushing etc.

Power trims are a whole other kettle of flying-fish...!  Shocked

DO come along to the Indoor FF Scale Nats this Sunday in Walsall.

Jon
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Jack Plane
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« Reply #7 on: April 23, 2019, 09:12:02 AM »

The next step is to start trimming for power.

The amount of thrust and therefore torque will depend on the thickness of the rubber you're using.  The duration will depend on how long the loop is.  There is a formula for how many turns you can put on before the rubber breaks, etc.  But let's assume for now that you're only going to use the (old probably UV-degraded rubber) that came in the kit which is of unknown length, that you haven't pre-stretched or lubricated it, that you don't have a geared winder (usually 1:10), and that you aren't going to stretch-wind it.  So, very conservatively guessing that you can safely put 250 turns on the motor, you'll have to do this each time by winding the prop round 250 times with your finger.  Don't change this number at all during the following trim tests.  (In fact, never change more than one variable at a time when trimming a model.)

1.  Wind 250 turns on your model (which has already been trimmed for the glide) and gently launch it into the prevailing breeze.
2.  If there is no down-thrust built in to the nose-block/button, then the model will most likely climb rapidly into a stall.  Add shims to create down-thrust until this doesn't happen anymore.
3.  If there is no side-thrust built in, then the model will most likely quickly spiral in to the left due to the torque.  Add shims to create right-thrust until this doesn't happen anymore.

---

In summary, these are your basic controls:

* Nose-weight (sometimes tail-weight) to bring CG in practice to very slightly forward of the theoretical neutral position.
* Rudder trim tab to yaw model into a left-hand circuit.
* Inner-wing down-tab to counteract tendency of the left-rudder tab to roll the model in.  (Sometimes tip weight can be used on the outer wing instead or as well.)
* Down-thrust to tame tendency of prop (on initial power-burst) to climb the model into a stall.
* Right-thrust to neutralise (left-rolling) torque effect of prop.

Every model will be different - but these basic principles and the order of sorting them out are normally always exactly the same.

Start with the glide and sort that out, then deal with the power side of things.

---

If you make up a new motor of different thickness (i.e. power) to the previous one, then you'll normally have to adjust down- and right-thrust to compensate.  A thicker motor for more grunt and/or an increased loop-length for increased duration will move the CG rearwards, especially as the the rear peg is usually very far back towards the tail - in which case add more nose-weight to bring the CG back to where it was on the original motor.

Always check flying surfaces are true before each session.  A very dry day could tighten the tissue, inducing warps.  The damp of early morning or the evening will make the tissue sag as it acquires moisture.

Finally, get yourself a 1:10 winder and make yourself a simple stooge to hold the model while you wind.

Hope this brief overview is of help.

Jon
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Indoorflyer
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« Reply #8 on: April 23, 2019, 09:20:55 AM »

1. Check static balance.  Is it as per plan (with rubber in place)?  If not then add nose-weight until it is.
2. Check ALL surfaces are warp-free and and are attached exactly as per plan (i.e. wing and tailplane incidence). If not, then correct now.
3. Check prop is balanced.  If not trim and scrape the heavy side.  Then smooth both blades with fine abrasive paper.
4. Attach trim tab to rudder.  Assume will be flying in a left-turning circuit, but only put a very gentle bend in for now.
5. Attach down-tab to left wing about 3/4 of way towards tip, and bend in very gentle down.
6. Make up shims (1/32 and 1/64 ply) ready for adjusting thrust later.  You'll almost certainly need some down and some right.
7. Prepare some extra nose-weight (will almost certainly need a bit more than static balance).

Then start glide-tests on a very calm day over long/soft grass.  Aim is to get a gentle descending turn to the left without stalling/diving or spiralling in - balance the amount of down-tab on the wing with the amount of left-tab on the rudder.  Might need to add more nose-weight now (in small increments) until glide is consistently regular without hints of mushing etc.

Power trims are a whole other kettle of flying-fish...!  Shocked

DO come along to the Indoor FF Scale Nats this Sunday in Walsall.

Jon

Pretty good summary, I'd do steps 2-5 first, then static balance the thing.
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bbdave
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« Reply #9 on: April 23, 2019, 01:59:15 PM »

Thanks for the replies I am in Buckfastleigh Devon the model is a gillows piper cub I bought a prop. rubber winder and lube from Sam's when I built it. I will be flying outdoors probably on the moors but the local cricket club is just a short walk up the road.
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billdennis747
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« Reply #10 on: April 23, 2019, 03:21:03 PM »

I will be flying outdoors probably on the moors
I remember the hazards of flying on the moors - the sudden fogs, disorientating landscape and Hound of the Baskerville bogs!
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bbdave
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« Reply #11 on: April 23, 2019, 04:46:16 PM »

I will be flying outdoors probably on the moors
I remember the hazards of flying on the moors - the sudden fogs, disorientating landscape and Hound of the Baskerville bogs!

I always have a map and compass with me when I'm on the moor.
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