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Old Jun 26, 2014, 07:53 PM
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Coro / Corflute Pusher Trainer Plans

Hi all.

I think a similar question has already been asked a while back but hoping there's some new ideas/plans out there now.

I'm looking for an all coro plan (don't mind aluminum rail fuse) for a reasonably light, trainer style pusher. I already have a delta, so looking for a more traditional rudder, elevator and even aileron design.

I don't currently have any carbon rod suitable for making a twin boom tail style plane either.

Any help appreciated.

Thanks, Paul
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Old Jun 27, 2014, 02:53 AM
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Ah, I've been struggling to get this one right since I returned to RC a few years back, and still don't have a design that I consider perfect for a beginner. By that I mean it flies slow, is docile, predictable and largely self correcting. Then I want it to be very crash resistant, and in particular not break motors and props on impact (hence the pusher configuration). Lastly, I wanted it to be cheap and use low powered, cheap components. Not to mention simple to build and repair.

Here are my lessons - some may be of use to you.

The first problem is that the effect of the prop wash on the rudder seems to be more pronounced on a pusher than a tractor...so the pusher tends to yaw (and then roll) significantly as soon as you throttle up and therefore needs to be re-trimmed regularly. Not a problem for those who can fly, but not an ideal trainer characteristic, esp when the yaw is more than can be corrected with the available trim.

Oddly I have not seen this problem reported in any of the foamy pusher threads. But I did eventually solve it by using an inverted V-tail - I worked out that this gives the propwash spiral the least amount of surface area to push on. A vertical stabiliser with as much area below the boom should do the trick as well, but I don't thing that is a good configuration for a trainer as the bottom half is likely to break off on landing.

The second problem is one of weight, which is a proxy for flying speed. My twin boom glider flew well, but needed a bit more speed than I felt was right for a beginner. My plan is to build up the next version of the wing from 3mm depron.

I think I solved the other challenges in the pusher glider - the nose is a pool noodle, so acts as a shock absorber. Construction is all 2mm correx with wood dowels for the spar and boom, so its cheap. Booms are held on with elastic bands and cable ties, so don't break in a crash. Lastly it flew happily on a cheap ~150w E-max CF2228 motor.

Here are pictures of my two attempts so far:

1) Pusher version of the 1970's Airtronics Q-tee. Fuse was correx and the boom was 20mm aluminium channel. Flew well, except for the yaw problem. Also, the wing was an old built up balsa one; it became too heavy with a correx wing. I eventually totalled it when I exceeded the structural limits of a light weight single skin correx wing....

2) Twin boom pusher. Flies well, but still too heavy and fast. I have in-progress plans in Sketchup if you want to take this further.

Have fun
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Old Jun 27, 2014, 06:30 AM
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Originally Posted by Extreme Sports View Post
Ah, I've been struggling to get this one right since I returned to RC a few years back, and still don't have a design that I consider perfect for a beginner. By that I mean it flies slow, is docile, predictable and largely self correcting. Then I want it to be very crash resistant, and in particular not break motors and props on impact (hence the pusher configuration). Lastly, I wanted it to be cheap and use low powered, cheap components. Not to mention simple to build and repair.

Have fun
Yes, seems to be a fine balance.

I'm not too hung up on getting something to be super slow/docile or self correcting. Predictable, durable, cheap, simple etc would be nice though!

Years back I had an Electrafun XP trainer which looks a lot like the Q-tee actually and this is where the inspiration sprung from.

A web search revealed a couple of interesting(?) things, one of which I'd considered in the past…

Crashtest Hobbies - Albatross.
http://www.crashtesthobby.com/albatross-46.html
A puller but with the motor mounted on stick protruding from fuse just forward of wing. This plane design apparently flies at 14-40mph and can turn itself right side up, level the wings, and put its nose on the horizon without pilot input.

Also, a new (old) find, in corex: The Snow Master
http://www.benjamminsvideos.rchomepage.com
Same principal as the Albatross. I actually quite like this design and would be keen to have a crack at building but with dihedral. Plans are included on his site. It can even do water take off and landings if you're game!

These two plane designs got me thinking…. to simplify the build process, could I not just mount the motor aft (or forward) of the wing via an aluminium "L" mount that allows prop clearance of the fuse and wing - rather than constructing a full pusher style fuse as per the Electrafun or Q-tee ?

I'm hoping you recall my scaled down "Hell on Rails" posts from the "Beyond Yardsticks" thread…. which is the plane I have in mind to modify/experiment with when I suggest this type of pusher mount.

I wonder if the v-tail on this plane would suffer from the prop wash yaw you mentioned? The v-tail would be lower than the props direct line of thrust with that type of motor mount.

Anyway, just a thought.
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Old Jun 27, 2014, 08:56 AM
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My hunch is that pusher prop models suffer the yaw problem because the prop is so much closer to the tail and so the prop wash has proportionally more effect. Obviously pusher jets with the prop behind the tail don't suffer the problem at all. I suspect that a high mounted motor would lessen the yaw effect, but high mounted motors introduce a new set of problems related to thrust line...once again all solvable, but adds additional variables that need to get set up right.

Bear in mind that the main reason why the albatross will fly so slowly and gently is its very low wing loading, rather than its motor configuration. Substitute all correx for the foam and this property is quickly lost. My calcs suggest that the wing is the main issue - a correx fuse is only a few grams heavier than a foam one, but MUCH stronger. However, even a light correx wing is about 3x the weight of a foam one, so you've lost even before you start. Hence my conclusion that a foam wing and a correx/pool noodle fuse is the way forward.

Correx makes great intermediate planes, and probably good .40 and .60 size trainers, but I still have to see a good park fly trainer made from correx.

Be very happy if you can prove me wrong though
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Old Jun 30, 2014, 09:11 PM
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http://spadworld.net/forum/viewtopic...hlight=avipark
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Old Jul 01, 2014, 08:03 PM
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I like it. Had found this before and forgotten about it.

I think I disregarded due to its AUW given the running gear at my disposal
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Old Jul 01, 2014, 08:10 PM
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"However, even a light correx wing is about 3x the weight of a foam one, so you've lost even before you start. Hence my conclusion that a foam wing and a correx/pool noodle fuse is the way forward."

ES. WHAT do you use to reinforce the pool noodle fuse? Surely it's not rigid enough on its own? Although I've never experimented with these. Probably the usual suspects like Grp or cf rod? Or do you just use noodle as nose crash protection?

I'm thinking a small gauge PVC pipe thru the middle of noodle would fit snugly but probably still too flexible on all but the smaller length model? I think that you can get some quite sturdy/heavy gauge PVC pipe tho but unsure of the weight vs strength / rigidity. A new SPADable idea?
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Old Jul 02, 2014, 01:56 AM
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Noodle is just for crash protection. Fuse is correx and the pool noodle sticks out about 5 - 10cm, which is enough to not need any reinforcement (which would negate the crash protection objective anyway). There is a trick to shaping the nose....I'll post in the techniques thread if you are interested.

I have considered making a full fuse pool noodle fuse for a Stearman-like bipe, but the noodles we get here are a bit small. Such a construction would need reinforcement.
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Old Jul 14, 2014, 04:30 PM
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There is a trick to shaping the nose....I'll post in the techniques thread if you are interested
Yes please ES, if/when you have time.

Think I'm going to peruse this kind of build after the Hell on Rails. I was thinking of rounding the correx nose a bit like the front of a Mig sloper prior to attaching a pool noodle or similar but am interested to see how you created the nose of yours.

Regards, Paul
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Old Jul 16, 2014, 11:35 AM
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Details of shaping a pool noodle added to the Techniques thread....
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Old Jul 16, 2014, 11:43 AM
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Originally Posted by pzanni View Post
I like it. Had found this before and forgotten about it.

I think I disregarded due to its AUW given the running gear at my disposal
Likewise.....nice plane, but its heavier than I would consider appropriate for a beginner in a park with kids and dogs running around. Remember BEX lives in a semi-desert region of the country with a very low population density, so not much chance of hitting anybody or anything if something goes wrong
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Old Sep 10, 2014, 11:32 PM
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Tuffer Tuffy

Finally got my hands on some NEW 2.5mm corflute here in oz, as well as some packaging grade EPP I assume.

Settled on a Tuffy Trainer scaled to 800mm WS as base for plans and this will be a composite build of those two materials.

Question is: what do you see as an all round ideal method for building the corflute wing?

I've read and re-read all the tips in the BEYOND YARDSTICKS thread and still unsure if I should go span or chord flute direction

THE PLAN THUS FAR WILL BE AS FOLLOWS. Input welcomed.

- 800mm WS, 160mm chord
- 30mm dihedral at wing tip
- flutes chord wise
- crush LE AND score another 3 to 5 times say 10 mm apart back from LE
- Foam spar 16mm high at 30%
- 3mm MDF joiner
- 1 x 6mm carbon Ribbon spar laminated to joiner and foam
- 2.5mm corflute ailerons reinforced with 2.5mm fibre rod

Fuse, boom and v-tail bare currently at 70gms. I can easily trim down fuse further to save a bit of weight and make more aero at same time.

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Old Sep 11, 2014, 06:45 AM
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Correx and EPP should make a nice tough combination. Your plans sound fine, but consider the following:
1) I don't think you need to crush the LE AND do the scoring. I've not tried the scoring, but my guess is that it will mess up the profile rather than improve it. Simply crushing the LE (with heat) and maybe adding a bit of pre-bend ahead of the spar seems to the trick for me.
2) I'd put the spar at 25% chord. Remember that the correx will try to bend evenly, so the thickest part of the wing will be behind the spar, not at it. So if you put the spar at 25%, you might be lucky and end up with the thickest part closer to 30%.
3) Not sure if you need to add the fibre rod to the ailerons....I use 2mm correx for ailerons on my 800mm span Mugi (fast delta) and they have never given me any problems. I expect the Mugi goes a lot faster than the Tuffy, so you should be more than OK with just 2.5mm correx.
4) If you want to keep the cost down, two 'anti-crease' strips rather than the CF spar should suffice at this size...but no harm in using the CF if you have it. Just glue it firmly to the TOP skin, as that is the one that will tend to buckle under normal flight conditions (i.e. not doing negative G's).

At this size a wing with spanwise flutes would also work pretty well and would probably only need the foam spar. Mind you, a single skin curved plate would work too, but you've already successfully done one of those

Hope this helps.
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Old Sep 11, 2014, 11:18 PM
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.

4) If you want to keep the cost down, two 'anti-crease' strips rather than the CF spar should suffice at this size...but no harm in using the CF if you have it. Just glue it firmly to the TOP skin, as that is the one that will tend to buckle under normal flight conditions (i.e. not doing negative G's).

Thanks again ES.

When you say two anti crease strips do you simply mean two foam spars, one at 25% and one perhaps closer to the leading edge?

Also, I was leaning toward chord wise flutes for ease of build in terms of getting a reasonable airfoil but do really prefer spanwise so I can do ailerons and wing all in one piece (as you say, should be strong enough) but I often end up with saggy airfoil forward of spar?!

Probably due to two things, slitting too many flutes on inside of top skin between LE and spar AND not enough tension prior to glueing. I'm sure creation of a jig to hold LE whilst glueing will solve the tension issue but when running spanwise flutes, maybe I should be slitting less flutes, further apart or not at all?

Cheers, Paul
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Old Sep 12, 2014, 03:25 AM
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By anti-crease strip I mean a thin strip of 2mm correx with the flutes running spanwise that is glued to the inner side of the wing skin just behind or ahead of the spar. For the Tuffer Tuffy, I'd guess that 15mm wide is more than enough, especially as you are using 2.5mm. I'd glue just one just behind the spar on the top wing skin only, since you won't be pulling huge negative G's on this plane. I add these to all my planes now...add minimal weight and give a lot of strength plus stop the unsightly creasing. Here is the original post on the subject: http://www.rcgroups.com/forums/showp...&postcount=165.

I have only made a few wings with spanwise flutes and can't say that I have noticed a sagging problem. I do pull the LE quite tight as I 'roll' the skin over the ribs. You must slit some flutes else the correx will 'kink', but try cutting fewer flutes on some scrap to see how few you need to slit. IIRC, I slit every second one at the LE, then increase the spacing as I move towards the spar, with none slit behind the spar. Your wing will be quite thin, which will further reduce the need to slit too many flutes. Also, you can add a few half ribs in front of the spar as well.

If you want to go spanwise, try forming some LEs at this scale with scrap....I have a sneaky feeling that it might be hard to get a decent LE this small with 2.5mm correx.

Also, having the aileron 'sandwiched' between the wing skins on a chord-wise build won't have any adverse effects this scale and speed, so don't let that bother you.
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