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Engine choice for Graupner Extra 300s?

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Howard Tomlin26/11/2007 17:59:00
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69 forum posts

Recently I built (ok its an artf) a Graupner 300s, the Patty Wagstaff one i think it says for 61 2st on the box.

For the first engine choice I installed the saito 90T ( the original version) c/w on board glow system (to keep both cylinders lit, I hate unreliable engines) and a bigger tank.

 This combination looks really good with both the cylinders poking out of the cowl, however it really wasnt powerful enough for the model and I pretty much had to fly it on full throttle just to get loops, rolls, knife edges etc.,

 So unfortunately the saito has had to go back on the shelf in the work shop.

 Its not all bad news though as I happen to have a Laser 90 and an irvine 61 2st also waiting on the shelf, the irvine has a lot of hours on oit whereas the laser doesnt, I also prefer 4 st to 2 st for noise reasons.

 Question is ( yes i finally got to it) do you guys think the laser 90 will be able to power it- bearing in mind it wont have the 2x c cell on board glow system anymore and it should have less mechanical losses as its a single pot.

 Does anyone have a similar model and what have you used in it if so.

 note:

I have a 3d model which is completely loony so this plane is just for standard "real" aerobatics ie vertical rolls lomcevaks etc..

Ultymate26/11/2007 18:13:00
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All I can say is the Lasers are loved by scale modellers but are not renowned as powerhouses very rarely do you come across one being used for aerobatics. If moneys no object stick a Y.S. 110 in it and light the blue touch paper.
Tim Mackey26/11/2007 19:35:00
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Yes but if money is an object - which it is for most of us, then I would try the laser. true they are not the most powerful 4 strokes, but if it is designed for a 60 2 stroke, I would think the laser 90 should do. Of course, a lot depends on the AUW and drag of the model.
Howard Tomlin26/11/2007 20:46:00
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69 forum posts

if anything it would be a saito if i had to install anything bigger than the laser, i've heard some nightmare stories about YS engines failing to start etc.

 The all up weight of the model c/w laser 90 should be 3220g approx.

 As for the drag, i have no idea, is there a formula for that?

Howard Tomlin26/11/2007 21:14:00
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69 forum posts

So i put all the data into the thrust hp program and it reckons that i should have a 1.4 -1.6 thrust to weight ratio,

 Does that mean that it should be accelerating in a vertical climb????

Ultymate26/11/2007 22:58:00
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If your calculations and formula are correct then yes it should accelerate vertically, as for the YS motors yes like a lot of sophistiacated bits of kit they take some skill in handling but they are supreme in the fourstroke model motor market place bar none JMHO.
Steve Hargreaves - Moderator28/11/2007 10:38:00
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Hi Howard, I have this very model & I have an ASP90 two stroke in it turning a 14x6 prop & on this combination it is absolutely ballistic...takes off on half throttle & will pull straight into vertical (on full power...) until you can't see it anymore!!!!!! That said it isn't that fast in a straight line as the wing is quite thick which slows it down a bit.

Personally I like having excess power because then you're not flogging the motor all the time & can do e-n-o-r-m-o-u-s loops with the thing....its all down to throttle management.

I have often thought that a 90 4st would be the perfect powerplant for it though....the only problem you might have is ground clearance for the prop.....a 15" is about as big as you can go.

I'd be interested to know what you think of its flying characteristics.....the review in RCM&E some years back (which used a Super Tigre 90 incidentally) claimed it was a pussycat......I wouldn't agree with that....it has some nasty habits which can make it a bit of a handful. For instance pull too much up elevator too quickly & it flicks out (always to the right)....not fun if you get a bit low at the bottom of a loop or exiting a spin. Also it can tip stall on landing so you need to keep the speed up a bit on approach...don't bring the nose up too far or it will drop a wing. I thought at first it was just me but another guy at the club bought one & he was scared of it!!!!

Two points.....fly it smoothly with a bit of speed & its fine......watch the C of G...DO NOT go further back than stated in the instructions (I think its 90mm from the LE but check!!). Go a bit further forward if in any doubt.

Despite all the above I do like mine...it looks great in the air. Just make sure you respect it & don't fly it like a funfly...........

Howard Tomlin02/12/2007 11:26:00
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69 forum posts

Thanks to all of you guys for your help, i will persevere with the Laser 90 ( maybe wit h some 15%nitro ) and see how i get on with it.

 When i flew it with the saito 90 twin it was a bit tail heavy, this was mainly due to the twin layout being very compact +  rear mounting arrangement ( i had to remove an inch from the rear of the cowl to suit this shorter nose set up). 

During its maiden i carried out the standard : Stall at height (dropped right wing but only gently), low speed circuit at height (all good), spin and recover (L&R recovery ok), outside loop, inside loop (both straight with little rudder correction), roll right and left (all ok - and quick as well), four point roll (some rudder coupling noticed ) and knife edge (again rudder coupling but not too bad)

On landing I brought it in on a fairly steady approach and subsequently flared it into a perfect three pointer engine running about haf way down the strip, I was quited chuffed with that considering it was a tail heavy maiden flight.

When i got it back to the pits i checked the fuel remaining and noticed that i was virtually out of fuel, this was probably because the motor was nearly at full chat for the whole flight - hence the requirement for an engine swap before i fly it next.

 The shape of the wing no doubt makes it more susceptable to tip stalling and i have also heard about cap's etc flicking out during tight loops (probably because they have similar shaped wings), however i was fairly gentle with the loops etc. as the speed really wasnt there from the saito twin so no problems.

I will certainly bear in mind your comments during the next flight and probably try to write up some more details if anyone is interested.

 Perhaps its a good idea for a section for reviews from forum users on models they own detailing engines used etc. this could become a great database for others to choose models and engine setups

Braddock, VC02/12/2007 11:46:00
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1634 forum posts
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You may well be surprised by the power of the laser, remember to prop it to suit the torque characteristics that'll mean largest diameter and finest pitch that'll fit the airframe.

I had a WM midget mustang about 4kg and 62" ws IIRC and a laser 80 on 10% nitro was definitely up for it, only about 2 vertical rolls going upward but unlimited downward dependent on starting height. I used 14 x 6 and 14 x 8 props on that a/c.

Ultymate02/12/2007 12:31:00
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"Unlimited Downwards" I love that, unfortunately most of them are
Frank Skilbeck07/12/2007 19:46:00
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I had one of these and initially had an ASP91FS in it and it was quite well powered but I thought the elevator lack a bit of authority so I dialed in a bit more and that is when is flicked out of the bottom of a loop, but fortunately with not too much damage. I then tried an Irvine 72 and that was very good, but the plane still had a tendency to flick when you least expected it.

Your Laser 91 should be OK, but watch for the flick, my last one was too low

Steve Hargreaves - Moderator08/12/2007 15:34:00
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Frank, I'm glad you've found this too...I thought I was getting too old & slow for these aerobatic semi-scale jobs & would have to content myself with flying Cubs for the rest of my life!!!! This tendancy to flick out under elevator is really scary...it once did it at about 50 feet at the end of a spin.....my thumbs were just a blur but I just managed to save it more by luck than skill I think.

Keep a bit of speed on (don't go mad) & fly smoothly & you should be OK...problem is it usually gets you just when you don't need it (ie bottom of a manoeuver) & aren't ready for it.....

Howard, can I suggest you get some height & go looking for "the flick"......fly a horizontal at a reasonable speed & then feed in a fair amount of "up" elevator....don't snatch a bucketload...you'd be asking for problems with any aircraft doing that......just apply about 2/3rds up elevator smoothly but quickly & see what happens. If it flicks....it will probably be inverted by then so release everything & let the model establish its direction & attitude...roll to upright & gently bring the nose up to recover (this is when you get the problems...it flicks, you panic, try & recover too fast, pull in loads of up cos its getting near the scary green bit, it flicks again......repeat until you manage to pull out or you run out of altitude, airspeed & ideas all at the same time!!!!!

PS you might like to check the lateral balance too......mine was about an ounce out (right wing was too heavy). Correcting this improved matters but the problem is still there.....

Eric Bray08/12/2007 16:08:00
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Shades of Alex Henshaw writing in 'Sigh for a Merlin', he'd had one with a skew gear failure, and was lined up on a strip of back gardens, when the Spit flicked on him.

This also happened to the last real Mossie, at Barton, after an engine failure. It flick-span, recovered, then flicked again too low to recover. 

Howard Tomlin08/12/2007 17:39:00
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69 forum posts

just to pull this back on thread ! 

I'm more intestered on whether the laser will be a suitable power plant.

but thanks the same for your opinions on flick recovery....

Brian Cooper13/12/2007 20:59:00
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452 forum posts
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I have one of these models with an ASP .91 F/S, running a 14x6 APC prop.   There is plenty of power available and the model will happily go vertical.

B.C.

Howard Tomlin31/03/2008 21:38:00
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69 forum posts

Success At Last!

It made its first flight since the engine swap today with the Laser 90 and a 14x 8 prop.

SO.

It pulled like a train on 10% nitro,  - great noise too 

 It literally tore up the sky, well happy.

Thanks for your help guys.

Howard Tomlin10/05/2009 19:05:17
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69 forum posts
Update:
 
I've been having a lot of trouble getting the laser to run properly due to the carb being positioned below the tank and also airflow.
 
Unfortunately today the laser shed its inlet tappet during a trial flight, which led to the engine losing all power.
 
I judged that I had plenty of height so I ruddered it around ( This is where it all starts to unravel- I should have carried on in a straight line) and whilst turning it started to drop a wing,
 
I let it recover some airspeed in a dive and attempted to return to level flight at which time it tip stalled and fell - literally- out of the sky. Tthis time too close to recover, smash!, in it went.
 
Damage was terminal, wing sheared in half on starboard side 1/2 way along, fuselage broken in half and bulkhead torn out.
 
 
I got it home and removed the radio and engine, and traced the fault to the inlet tappet
 
 
I was surprised to note the general lack of glue!, this was shown by the way that the bulkhead and other parts came apart, all along the glue lines.
 
 
Perhaps this is the way of the ARTF, light and cheap due to lack of glue????
 
Ah well onwards and upwards!
 
David Ashby - Moderator11/05/2009 08:44:33
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Glue is at a premium in many of 'em Howard. Sorry to hear about that. 

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