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Old Feb 27, 2011, 01:29 PM
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Letchworth, Great Britain (UK)
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Question
Angle of incidence for Yamamoto trainer?

I've just about finished restoring a Yamamoto trainer that was donated to our club by a retired member. It's an all-wood traditional model with foam-cored semi-symetrical wing of 56" wingspan.

Having repaired and re-covered the fuselage and wings, I've only just put the two together and realised that the wing doesn't sit properly in the wing-seat cutout in the fuselage The wing-seat cutout is more curved than the underside of the wing, so I need to either build up the middle part of the cutout or sand down the front.

So, my question is, what angle of incidence should I be looking for in a trainer aircraft, please?

If I extend a line forward from the horizontal stabliliser, it just about intersects the rear wing dowel, as the line I've drawn on the picture. I guess this will be my reference line, and the "centreline" of the wing will be a line from the tip of the leading edge to the trailing edge.

P.S. That "windshield" sticking up at the front is over-sized, and will be cut down.
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Old Feb 27, 2011, 02:35 PM
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I would set it to be level versus the tail, zero incidence.
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Old Feb 27, 2011, 03:45 PM
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Thanks, ChillPhatCat, that's what I suspected.
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Old Feb 27, 2011, 03:57 PM
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Depends on the wing profile I would say. For a flatbottomed profile I would agree that zero angle is fine but for a semisymmetrical you might want to have a degree or two of incidences angle. Trainers are usually set up so that they climb with speed, in order to teach newbies how to fly properly, and for that you need a bit of incidence. For more experience fliers this is not necesary as it can feel like the plane is ballooning with speed.
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Old Feb 27, 2011, 05:34 PM
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Well, if you increase speed of a plane without doing anything else, it should climb regardless of the incidence, when a plane is flying level, that indicates that the lift being produced is equal to the weight. If you increase power, you increase lift.
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Old Feb 28, 2011, 01:59 AM
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OK guys, I'll see what angle it naturally sits at now. If it's zero, or slightly positive (i.e. leading edge slightly up), I'll simply simply fill the gaps where the wing doesn't touch the seat; if it's at all negative I'll have to reprofile the whole seating.
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Old Feb 28, 2011, 02:22 PM
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Well, I've done some checks this afternoon and the least-work option (a little bit of trimming of the wing-seat, and no filling) will have the wing sitting with the centre of the leading edge about 6 or 7mm higher than the trailing edge, when compared with the line extended from the horizontal stabiliser -- that calculates out at about 1-1/2 degrees of incidence.

I've always worked to the plans before, so I normally just do what the plan says without actually measuring the angle. I suppose 1-1/2 degrees means that I'll have to maybe apply a bit of up trim for level flight?
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Old Feb 28, 2011, 02:34 PM
Fueled by Arabica Beans
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Sounds good, if you think there was originally some incidence in it then by all means put it in.

The incidence will make the nose sit a degree and a half lower in flight since there is in a sense 1-1/2 degrees of angle of attack "built into" the plane... most planes are at a couple degrees positive angle of attack in level flight, but you never notice since it's such a small angle. It may need a touch more down elevator to fly level at each throttle setting than other planes. Only the first flight will tell you exactly what it will do, but it should behave the same as any other plane. No real surprises.
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Old Mar 01, 2011, 02:37 AM
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That's the problem, ChillPhatCat, I don't know what the angle was originally

The previous owner had put approx. 3/16" strips of ply on top of the wing seatings for their whole length, which raised the wings above the deck at the trailing edge, so I removed the strips (they came off easily, due to previous poor workmanship, and old age) to get back down to what I though was the original wing seating. Now, having done that, the wing sits correctly with its trailing edge just about flush with the rear deck, but I need to take about 1/8" off the top of the former at the leading edge of the wing, and adjust the curve of the wing seating near the front, to make the wing sit properly over its whole length. Doing that will give me the 1-1/2 degrees I mentioned previously.

The wing was such a bad fit that I wondered if it was the right one, but the dimensions tallied with what I was able to find about the model, and the decals were consistent on fuselage and wing, so I think it was just bad building/repairs by the original owner.

Anyway, things should be finished this week, for a maiden flight at the weekend. I'll post how it flies after then. I've managed to lose almost a pound of weight in the restoration, mainly due to old heavy covering which also had a thick layer of paint on it, and six ounces of lead in the fuel tank bay due to the heavy covering on the tail, so it should fly better than it did
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Old Mar 14, 2011, 06:43 AM
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Letchworth, Great Britain (UK)
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Good news ... bad news

Rugby matches on our field prevented flying last weekend, but I was able to maiden the model yesterday. Good news is that it took off okay, though perhaps a bit longer run than I'm used to, with its old OS40 engine. And it landed on its wheels.

Bad news is that the landing was about 1/4 a mile away from me, out of sight behind a wooded slope, following complete loss of radio control as I was just about at cruising height, joining the circuit. But the fact that it landed on its wheels, with only one aileron displaced when the wing was nudged as it hit a bush, after last being observed vertically nose-down, suggests to me that it's basically stable.

Next week we'll try again, and I'll remember to extend my 35MHz antenna this time Maybe all the onlookers who swarm around for a maiden flight will also remember to remind me to extend my antenna
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Old Mar 14, 2011, 01:57 PM
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lol Yikes! At least no serious damage and if it can fly on it's own, it's probably good to go.
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Old Mar 18, 2011, 12:53 AM
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Next week we'll try again, and I'll remember to extend my 35MHz antenna this time

Yeah I've done this too!
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Old Mar 18, 2011, 04:04 AM
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I did it once before too, before I'd converted to 2.4GHz, so my reaction then was automatic when I saw the model not responding -- pulled up the antenna and regained control.

But now I've been mainly on 2.4GHz for a couple of years, and the thought process is not quite so automatic. I only realised the antenna was still down when I started to walk towards the crash site.
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Old Mar 21, 2011, 03:49 AM
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Not so good news ...

I remembered the antenna yesterday, but the flight still didn't go well. Despite having the c of g at about 1/3 chord, slightly forward of the main undercarriage, a location that all the experienced flyers at the field thought was okay, it was clearly tail-heavy when it took off. In addition, or maybe because of that, it was very twitchy on the ailerons. I ended up putting two clicks short of full down trim on it (eyeballing it, the elevator looks like it's at about 5 degrees down) but it still didn't fly well at all.

After a few minutes flying circuits I heard a ripping noise, so decided to land. On its final turn for the runway, at about 50ft height, the model put its left wing down and went into a vertical spin. I managed to convert it into a flat spin by giving full rudder and up elevator, and it pancaked into a soft field, bending the front undercarriage wire leg, and breaking the nylon screws holding the main gear. No other damage that I can see.

The ripping noise turned out to be the covering tearing loose on the upper surface of both wings -- I've never had that happen to me before.

So, what to do next? I'll bring the c of g forward by adding more ballast to the nose, and reduce the rates on the ailerons for starters. But I'm beginning to think that this model may never be suitable as a club trainer.
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