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Old 04-27-2006, 08:27 PM   #1
Franny
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Default Great Planes Sportster Bipe 40 conversion

Hi All,

I am currently building a GP Sportster Bipe and am looking for some suggestions for lightening it up. My general thinking is to lighten as much as possible behind the CG but the plans call to glass the center section of the bottom wing. The top wing is flat and swept, but the bottom is straight and has a few deg of dihedral. They also say that if you don't glass it, you will have a structural failure. The funny thing is that they really donít have any dihedral braces. I would think that a couple dihedral braces would be lighter than glass. The wing is symmetrical with an upper and lower spar (1/4"X1/4") that gets webbed. Ailerons on the bottom only...

The fuselage is going to be another place to lighten, but in my experience, these things always come out tail heavy and I'd rather have structural weight in the nose than lead.

I don't need to put this thing in orbit and all I would do is mild aerobatics. I do like a bit of strength for nose overs and rough grass. I'm thinking of using an AXI 2826-10 and a 3S 3300mA LiPoly that I already have. Final weight should be 5-5.5 pounds Any input would be helpful!

Thanks,

Franny
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Old 04-28-2006, 03:19 PM   #2
Matt Kirsch
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You can do other things to save weight. Don't skimp on the glass reinforcement at the wing center, especially if the wing doesn't have any internal dihedral bracing.

If you want to be REALLY ambitious, you can use the following rules to lighten the plane even further for electric:

1. Replace all aircraft ply parts with lite ply of equal thickness.
2. Replace all lite ply with balsa of equal thickness.
3. Replace all sheet balsa with stick structures.

With modern brushless motors and relatively lightweight LiPoly batteries, all that effort is generally not required.
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Old 04-28-2006, 04:02 PM   #3
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Thanks Matt,

Most of that heavy stuff is forward of the CG - ply and such... I did drill some large holes on the tail fins as they are solid 1/4" balsa. I know those pieces don't addd up to too much weight, but they are so far from the CG in the wrong direction. I'm not sure the fuselage would be strong enough as a stick structure. There is some sheeting on the wing, but mostly the LE, so it is "good" weight.

It's interesting that you mention that with the new technologies, such drastic measures arn't necessary... It really is getting to that point, huh... You can take an airplane you have flown forever with a slimer and convert it to electric and presto! equal or better performance. How cool is that?

I'll take your points into consideration when I build the fuse. I've just been scratching my head trying to make a real difference in weigh without compromising the strength. I'd rather trade a little performance for a bit of structural insurance.

Thanks for your help!

Franny
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Old 05-01-2006, 03:16 AM   #4
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Hi Franny
With all the new technologies, we're getting as bone idle as the best of the good ol' fashioned slimer buyers !

The point is usually forgotten in the rush for instant gratification, that a lighter AUW is the cheapest and most effective power upgrade around

Way back when I got into high performance electrics - 20 x SCR 2000 into a MaxCim 13Y - I built a GP CAP 232. First mod was to lose the 5/16 square 'hardwood' wing spars for two laminations of 1/4 x 1/8" spruce, with the innner ones shorter than full span. Dropped the spar weight from just over 4oz to 2oz before I even started! Ended up with a wing that weighed about 14oz with Monokote covering and two Hitec HS200 mini servos.

The fus - well, I built the kit's, minus a lot of wood, but it still flew pretty loggy. So I took the cowl and rudder, propped them up on the board and put a new fus between them. You can replace Liteply sides with 1/4 x 1/8 spruce longerons and ditto balsa uprights/diagonals in the needed places and just use them with the same shape formers.

I've never hung my 600-off watt packages on thicker than 1/8" Birch ply firewalls either.

As I have been known to fly enthusiastically on odd ocassions - okay, more like have been known to fly an aerobatic model straight and level, but not much - it might be surprising to some just how light you can build a model and it still stay together. My present aerobatic own-design weighs about 5-1/4lb with 16 x 3000 NiMh - with the covering off, the fuselage looks more like a big Comet 10-cent than a 700W electric.

GP do great aerodynamics, but no-one there can spell "weight". Good luck with your "Bipe" project, it's a good looking model and one that'll stand out some in our cookie-cutter world.

The above is a bit brief, if you want any more details, please ask.

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Old 05-01-2006, 08:38 PM   #5
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Thanks Derek,

Yeah, this kit is a little funny. I'm curious as to why they discontinued it... It isn't particularly easy to build and it is a bit odd structurally IMHO... The cabane mounts in the wing are not attached to the main spar and are actually reinforced to the sheeting. They are attached to two ribs, but... The bottom wing has no dihedral braces. The only real structural bit comes from the glass sheeting once again attached only to the sheeting. Seems like things could have been designed a bit differently to ease construction and be lots stronger and a bit lighter. The wings are actually coming out pretty light. It is the Fuse I'm worried about. It sounds a bit like your CAP232. Lots of thick balsa and some hefty ply. The only saving grace is that 90% of the heavy stuff is forward of the CG. What is your feeling on that? It is hard to get a balance before all the pieces are built so I'm inclined to leave the weight forward of the nose. I'd hate to remove a bunch of stuff just to then add lead to the nose. I can get the entire battery forward of the CG as well.

Oh, another issue... Cooling. Any suggestions on motor/ESC/batt cooling? Without a huge cylinder and muffler sticking out, I could close things up a bit and get a much cleaner look. The forward most part of the nose is the same dia as the spinner. Always a mystery to me as to how to get airflow for the motor and such...

Thanks bunches,

Franny
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Old 05-01-2006, 09:35 PM   #6
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Not sure I'd want to fly a bipe with the fuselage hanging off the wing centre sheet! If you're actually building, you could add a couple of spruce spars from the centre to a bay or so outside of the strut pick - I'd go for one top and bottom, with interspar webbing. On your lower wing, there's been zillions of wood sheeted foam wings joined by gluing the panels together and bandaging around the joint. The engineeringly minded would suggest a second layer that tapers somewhat, to spread the loads and avoid building a "stress riser" in, but most English foam winged models survive on a single wrap of 2 or 3" inch glass bandage despite its shortcomings.

My feelings on structures run pretty simple - "make it light, don't make it heavy and take out all the weight you can" is one, the other I lifted from the CL aerobatic lot - "It's easier to take a gram off each of 100 pieces of a model than to take 100gm off the finished model".

Kits, however - and especially those from large mass-production manufacturers - are designed firstly for ease and cheapness of production, nextly to look good and sell well, then comes flying, followed a long ways back by such as elegantly engineered structures and light weight. There's many small kit makers around who do far better, but often they're doing it as much for the love of it as to make a profit.

Huge slabs of wood - any chance of gluing them up, tacking them in place, shaping the outsides and then removing them to give them some attention from Mr Dremel and his little grinders? An alternate would be blue or pink foam covered in brown paper and white glue! Fairings are only 'tertiary structure' - giving the model its shape, while not doing a thing for the structural strength.

Cooling - big holes! On a shape like this, I'd go for a hole under the motor - take a look at full size light aircraft cowlings for inspiration. Split the airflow entering this with light sheet balsa deflectors, to push half of it around the motor and the rest through a matching hole in the slimer firewall. I usually put a big, ESC sized hole in the fus floor aft of the firewall, sit the ESC just inside this on a couple of strip wood supports, so it gets cooling air top and bottom.

Down the back, engineer a hole in a fuselage bottom bay just aft of the wing as an air exit. Put a deflector - more 1/16" balsa sheet - inside, from the rear edge of this hole all the way up to the upper longerons, sloping forwards at 30 - 45 degrees - this will offer an exit for hotter air that's done the rounds inside the fuselage and dissuade outside airflow from ending up making the back end of the fuselage act like a parachute...

It's not only about getting air into the fuselage, it's as much about getting it to cool the hot stuff down as efficiently as possible. Not sure if it applies to LiPo, but it got proved by someone technical years back that it's almost impossible to cool a battery all that much during flight - too much mass, too much heat, not enough cooling airflow. What you have to look after is your motor and ESC in flight, and arrange to run the battery at what it can live with.

With my CAP 232, I used an old computer cooling fan mounted in a piece of Liteply (the stuff has some uses ) that covered the fuselage opening left by the hatch and blew cooling air onto the 20 cell pack in place. That was easier than taking the pack out, whereas my Four Star 40 had so much hatch, I'd just pop the pack out and put in my cooling tube.

Hope that helps

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Old 05-01-2006, 11:53 PM   #7
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Thanks Dereck. That is helpful. Great ideas on cooling. Interesting about the batt cooling. I have a LiPo that actually has a bit of space between the layers for airflow. They are built much more like a capacitor than a battery. I'll put a bit of thought into the entrance and exits. On my glider, the cockpit is hinged and opens up with the throttle so when the motor is on, it works like a scoop and when it it off it is clean. Maybe something like that would work. Those little micro servos are really light and work great for stuff like that.

Thanks again for all your input!

Franny
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Old 05-02-2006, 04:11 AM   #8
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The example I mentioned was a long ways back and concerned pylon racers, before they became something in FAI speak. Still, am inclined to believe it, as I have been known to get my round cell packs a tad hot after typical flights.

Still, toss 'em in the Kool Charge Tube for a while, slap them back on charge when they're handle-able and go fly another in the meantime...

I now actually have some lipo's but after everything I've read, am inclined to treat them very carefully indeed. A steel box with lockable lid seems to be the first purchase The idea of cooling space between cells sounds very sensible, mine are devoid in that region. Still, they're not going in an all-out aerobat, so should be okay.

I've seen a magazine article on a P51 fitted with a working flap system housed in the underside air scoop aft of the wing, but that seems a little complex for a sports model. I'd go with big, carefully placed holes as much lighter and nothing to go wrong on a great flying Sunday! Am very much in favour of simplicity - these electric models are complicated enough to start with.

Keep us posted on how you get on

D
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Old 05-11-2006, 04:31 PM   #9
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Hi Dereck,

I opted for an actual dihedral brace instead of just glassing the center section of the bottom wing. I also webbed the spar out from the center line a few rib bays (if that makes any sense). It is quite strong now and I'm betting a bit lighter than the 3oz glass would be. I'm wondering if I should still glass it, but the glass is attached to 1/32" balsa sheeting only and I can't think that it would add more stength than the weight it would add... tough call. Well, I will start the fuse this weekend and will lighten as I go. I was able to fine a couple snipits on the web that implied that this model comes out a bit tail heavy (suprise, suprise!). I'll keep that in mind. Thanks again on the cooling suggestions. I'll build something in.

The wings did turn out quite flat and seem quite strong... So far so good!

Thanks,

Franny
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Old 05-11-2006, 07:24 PM   #10
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If the tail feathers are solid sheet, that has to go!

My favourite structure has, loosely, a 1/4" thick structure - not too aerodynamic, but it works on most sports designs. I'll use a 1/2" wide TE, which I'll often taper from just outside of the centre. LE - 3/8" wide, splice jointed at the centre, as the LE usually sweeps backwards. Tip, to suit - either 1/2" wide sheet shaped to outline, but if the tip is well rounded - like a Piper Cub say - I'll laminate the outline from 3 or 4 laminations of soft 1/16" balsa.

Ribs, never more than 1/8 x 1/4, they only keep the edges apart. I'll splash out in the centre with a single piece of wood that will allow 1/4 - 3/8" either side of the fuselage sides, to fasten the covering too.

Elevators, similar, with some 1/64" ply reinforcement to take any joiner piece - wire or wood - and the control horn. On a larger model, I'll make the elevators completely seperate, with a split pushrod system to the seperate halves with their individual horns.

Why any model aircraft should end up tail heavy is beyond me. It's barely rocket science to make the nose longer. For an electric, all mine have the battery right over the CG so even my ancient nimh fuelled designs will be perfectly happy with a lipo pack. That makes it easy to hit and trim the CG, plus battery removal for charging is no sweat either.

Your lower wing sounds like it should do okay. When you apply a GF centre wrap, the main essentials you're joining are the spars, LE and TE - joining the centre sheeting is gravy - thin gravy in your case

Replace the Liteply sides with 1/4 x 1/8 strip - spruce longerons, balsa for the rest, some 1/64 ply doublers to join the high stress parts together - that's motor mount former, wing LE former, UC mount, rear wing hold-down plates or whatever.

Kits are massively overbuilt - for ease of production and to mitigate the effects of sub-standard patches in the Liteply sheeting. If you build the model, you are the QC dept on a production run of one! With a little practice, you can pull some real tricks - the wing in my 54" span pattern ship weighs just over 12 ounces RTF, Monokote covered and with two HS200 mini servos. A flying buddy has a BARF wing that comes in at around TWO POUNDS!

Keep banging your head on the wall

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Old 05-11-2006, 08:15 PM   #11
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Thanks again Dereck,

I think the Fuse is where the real savings will be... The wings are actually quite light at this point (I probably should Actually weigh it at some point ) I really like your dual pushrod idea for the elevator... the plans call for (and is included) this thick steel wire to which a long steel control horn is brazed. That piece alone must weigh three or four times what both stabs weigh! And yes,... they are solid 1/4" balsa. I did drill several 1" dia holes to lighten things up, but I've never been a huge fan of flat plate airfoils as the have yucky stall characteristics. I might end up building them up as you suggest. There doesn't appear to be a lot of ply in the fuse; just for formers and mounts. I wonder sometimes what percentage glue is of the final weight. I've been using mostly CA, but I did set the dihedral with epoxy for the strength and working time. Still, I've already run through 2+ Oz of CA. All that sheeting sucks up adhesive...

I'll try to get some pictures up soon...

Thanks again. I'll be re-reading all you info. Great stuff!!

Franny
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Old 05-11-2006, 09:14 PM   #12
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The traditional joiner is a fine example of a low cost/ease of production item! I use Sullivan Goldenrods for driving split elevators - that way you have 'universal' adjustment on one end, plus independent trim ability at the elevators. Big plus - you just hang the elevators on after you've covered the whole model.

I've always liked the 'old' idea of a stripwood flat plate with a pair of 'spars' at around 30% MAC, to develop a diamond shaped, sort of symmetrical airfoil section. Simple and cheap! Plus if it was good enough for the Phil Kraft era, it can't have been all that bad. I'm thinking of retro-fitting one onto my E-Rotica pattern bird.

If I could get off my ancient duff, the best stab would be an airfoil sectioned, balsa skinned white foam jobber. Done right, it wouldn't be all that heavy and could possibly be made a little smaller than a flat plate would be.

Not sure about holes - you have to weigh the holes after removal from the mother sheet, and figure out if the percentage weight drop is worth the effort. Still, Chris Golds likes them (but he's a retired RAF officer ) so there's mileage there.

The fuselage is just a big lump that holds all the flying bits in order, keeps the rain off the electronics and looks good (or not, in the case of many modern kits...) Holes lightly trimmed in minimal wood to keep the covering in shape work fine for much of a fuselage.

Glue - I regard CA as fine for smaller parts, good for tacking things in place and as a sub for pins! For real gluing, I prefer a good woodworking adhesive, though Pica's white glue from some hobby suppliers is a very handy glue. I pour a little into one of those little plastic cups you buy for mixing epoxy in and put on the joint with something like a toothpick or coffee stirrer. PVA type glues are way stronger than most aeromodelling woods as long as they aren't asked to fill gaps on high stress areas, plus being evaporative means a lot of their weight vanishes as they dry. CA, you add the weight of whatever adhesive you use and epoxy is something I apply very sparingly indeed.

I'd almost bet there's been a lot of models trashed by the weight of that epoxy fillet around the tailplane and the lead that should have been added to the nose to counter it - but wasn't

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Old 07-18-2006, 07:14 PM   #13
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Just a quick update...

I have the wings built. I decided to install dihedral braces in the lower wing instead of glassing the center section. It seems really strong. The Fuse is about 1/2 done. Lots of balsa, but a bunch will be sanded away. At this point it is about 8oz but the majority of it is forward of the CG. I will be thinking about cooling while I close up the nose a bit. No need for the huge hole for the engine and muffler! I posted over in the building section about all the mold in this kit. It is really stuffing me up. The box and wood showed a little water damage and I can see black dots on some of the wood. It is a bummer to have to build using a dust mask. Something to look carefully for in the older out of production kits that have been sitting in someone's wet basement for years...

Any rate, I'll post some pictures soon. I plan to add outboard flat struts (like a Staggerwing) to aid in strength and looks. Biplanes should have struts, just MHO

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Old 08-20-2006, 09:30 PM   #14
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Smile Pictures! Finally finished it...

Hi All,

Well, I finally finished the Bipe and it is ready for it's maiden! Woo hoo! I don't know how long it took (4+months) but I believe it turned out well. It isn't the easiest or, more correctly, the best designed kit. Alignment was a big issue and since the kit had water damage, there was a bit of warped wood but all in all it looks like it will do just fine. One cool thing is that it came out at about 4.5 pounds which a bit less than the plans called for (5.5-6 lbs). I did have to stuff everything into the nose and the balancing point is a wee bit towards the rear of the CG range but shouldn't be a problem. Also, I have an aluminum spinner on order that should help a tiny bit. The power is a little thin at around 370 Watts at full throttle. That'll teach me to put off the Watt meter purchase . Anyway, I don't need vertical performance and I've yet to do a pull test but it feels like it pulls well. Oh, about the color :o... 'K, so there was this hobby store going out of business only I didn't get there until the later days and all the "normal" monokote colors were gone but they had a bit of the pearl colors so this is pearl wine and pearl white. A little funky, but I really like it and it has a softer, cuter look. The wheel pants are a neat touch but I imagine they will feel the wrath of my patented "everything was just perfect until the last second" landings. Poor things... Well they look nice now. The canopy was a "Rit" dye experiment. Mauve and pink - and a bit of bleach when Franny left it in to long...:o It *almost* matches... The original kit of course had a huge cutout for the engine (.40 size) so I needed to cover that up a bit. I didn't find many airplanes with an air scoop on the top of the noseÖ I'm guessing that is because it would just make it that more difficult to see over but since I donít have to actually sit in the thing I could put it anywhere I wanted, so... I thought about the sides but that would have been a bit to "Pitts" like. The nose cap is removable to get in and futz with the motor as needed. There are two big air holes just aft of the firewall in the battery cover for cooling. There is also a large hole in the firewall. In theory, the incoming air will be directed over and through the motor and then down past the speed control and the battery and then out. We'll see. Without further ado, here are a few photos. Let me know what you think! Thanks for reading,

Franny


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Old 08-20-2006, 09:34 PM   #15
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Default ...And a few more

Ok, I have more nuttyness...

f


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Old 08-20-2006, 11:38 PM   #16
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Nice job! The colour scheme is so good I'd not be surprised if the Chinese copied it

Did you put a big hole somewhere in the back to let the air out? I figured out how to do this with a minimal bill from Mr Gravity. Otherwise all that hot air just hangs around in the tightest spot it can find - isn't that why the Capitol has a domes, to capture hot air?

Your neat big bipe fair makes me wish I'd kept that kit/plans to the Aeromaster I had back in my slimer days...

Good luck with the test flights
(maybe you should leave those spats off initially - they are really too pretty to scrape and scratch)

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Old 08-21-2006, 01:25 AM   #17
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Thanks Dereck,

Good call on the spats No great complement than imitation... I'll have to keep checking the Hobby Lobby flyers...

I'll keep an eye on the heat too. I actually have to check out a new flying site that is much bigger than the little park where I fly the small ones. I'm such a slacker... I have three maidens backed up now. Probably should go out and fly a bit, huh? Although; I have a Balsa USA Taube on the bench now. Hmmm... Fly or build... Such tough choices!

Thanks again,

Franny
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Old 08-21-2006, 08:20 AM   #18
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Franny, your improvised colour scheme looks great, and the cowl-top air scoop adds a certain aggressive look to the plane, as well. I agree with Derek re taking the spats off till you know how the model handles on the ground, especially if you fly off grass. The whole thing has a very purposefull look to it, and I can see where you added lightness where it counts, so it should be a very sweet flier. Can you tell I have a soft spot for bipes? Good luck on the maiden, and don't keep us in suspense for too long.
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Old 08-21-2006, 06:29 PM   #19
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Thanks Brad. Great advice. I really like Biplanes too... I have a PT17 kit from AerodromeRC that I'm really looking forward to and I just finished up the triplane at the beginning of the year. The Sportster Bipe is something I have wanted to build for a really long time. I did add the outboard struts to give it a more finished look. It is strange without them. They aren't really functional as they are just held in their slots with magnets. I will get all these in the air soon. I will be checking out that new field this weekend. Then I just have to get my nerve up!

Thanks again,

Franny
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Old 08-23-2006, 03:32 AM   #20
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Your Sportster Bipe looks great, Franny!

I built one of these in the '80's. I remember the plane having a lot of big balsa blocks in the fuse that had to be hollowed and shaped. Tedious.

I had a Supertigre .40 on mine, IIRC. It flew quite well, and I flew it every chance I got until one day at the field, when I was flying around in lazy circles, I discovered I didn't 'have it'.

She came down slowly at about 1/4 throttle, in a series of big lazy circles. Everybody even had time to land their own planes and watch, once I announced 'no control' on the flight line. She landed in a level upright attitude in a muddy area of the field. It would likely have landed itself undamaged if the mud hadn't allowed the wheels to dig in and cartwheel it hard.

Post mortem discovered a receiver switch that had somehow vibrated itself into the 'off' position in the air. :o


FWIW, I've built the AerodromeRC PT-17 kit--one of the very first prototype kits (it's in my avatar). It's a shame that Kurt decided to take this kit off the market--it's a good 'un.
Expect a very light finished airframe. Mine only weighs 4 lb. RTF with battery! If you have any questions about the build, I did a build thread on it over on RCGroups late in the fall of 2004.

I still have it--in flying shape, but I am afraid to fly it much, since it is a defunct kit now. That's why I did a Great Planes PT-17 Stearman last year--also electric. Easy to get replacement parts for it if needed.


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Old 08-23-2006, 04:00 PM   #21
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Thanks Rick,

Yeah, I'm really looking forward to flying it. Sad to hear about yours... I watched a .90 size Cosmic Wind go in like that after it vibrated it's spinner and prop off and the engine just started whining at super high RPM. I guess that knocked out the radio and down it went in big circles.

I noticed that Kurt took down the PT17. I wonder why? I must have gotten one of the last ones. I'll look for your build thread on that. Glad to hear it is a good kit. Such a majestic airplane - a real beauty.

I'm still trying to decide if I want to build the Taube or the Stearman next. I might try the Taube... We'll see.

Thanks,

Franny
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Old 08-23-2006, 09:54 PM   #22
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I read somewhere (maybe on Kurt's website) that he decided to pull the kit because of the whole US aircraft manufacturer royalty demand controversy.

Apparently Boeing was demanding (or he thought they might) royalties on each Stearman kit he sold.

Can you imagine? Paying royalties to a mega-conglomerate so you can sell a model of a 70 some-odd year old antique design.

I'm sorry he decided to give in, but he's a small enough operation that one lawsuit would've been the end of him, I'm sure. I understand his decision.

Rick
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Old 08-27-2006, 02:26 AM   #23
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Originally Posted by Electrick View Post
I read somewhere (maybe on Kurt's website) that he decided to pull the kit because of the whole US aircraft manufacturer royalty demand controversy.

Apparently Boeing was demanding (or he thought they might) royalties on each Stearman kit he sold.

Can you imagine? Paying royalties to a mega-conglomerate so you can sell a model of a 70 some-odd year old antique design.

I'm sorry he decided to give in, but he's a small enough operation that one lawsuit would've been the end of him, I'm sure. I understand his decision.

Rick
This matter has been mentioned around the house quite a few times, but never seems to get folk sufficiently aroused.

There's more on the front page of the AMA's website, with a sample letter to the Grande PooBahs on the Hill at

http://www.modelaircraft.org/supportletter.asp

(That AMA. The one the hobbybuyers tend not to join because they're too tight and it's nothing to do with them anyway )

This moneygrubbing new idea won't only cover scale models of present day types and makers - it will include all those companies long since swallowed up. Imagine an aeromodelling world with no Lockheed Lightnings, P51s '47s etc? Maybe it could spread as far as the Piper Cub?

But be reasonable - big airplane company lawyers deserve those new BMWs, like any other shyster

D
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Old 12-01-2006, 03:13 AM   #24
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Smile More pictures!!

Hi All,

I thought you might want to see some pictures of the cute little biplane in the air! Many thanks to The Sky Sharkster for the great action photography.

Thanks,

Franny


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