Geoff Bradley Posted February 15, 2015 Share Posted February 15, 2015 I've recently bought a JP Prangster for a Winter Hack and it noses up very severely. Everything is set up as per the handbook with the C of G at 70 mm which is as recommended for a beginner. Has anyone had experience of a similar problem ? I have noticed that the wing chord is not as stated in the handbook.....it states 52 inch span and 520 sq inch area which should give a chord of 10 inch but it actually measures 9.25 inches ! This would give a C of G of 59 mm at 25% of chord ! I considered increasing the down thrust but it already appears to be at around 5deg. Any held will be gratefully accepted. Quote Link to comment Share on other sites More sharing options...
Cuban8 Posted February 15, 2015 Share Posted February 15, 2015 I think I'd start off by checking that the wing incidence was OK and that there was sufficient down thrust on the motor. A lot of these funfly type models do need plenty of down thrust, especially when over powered. I remember having to add a lot of packing to the top of the engine mount of my old Weston Cougar to get it to behave. I assume that the model is nosing up when full throttle is applied - what's the glide like on tick over? Is it a new model or a second hand example that might have been pranged? If all else fails, perhaps a bit of throttle-elevator mixing will help tame it. Edited By Cuban8 on 15/02/2015 16:37:53 Quote Link to comment Share on other sites More sharing options...
Geoff Bradley Posted February 15, 2015 Author Share Posted February 15, 2015 It's a second hand job but is in good condition and the wing incidence is around 2deg. It was a nightmare to control in the glide and the down thrust is around 5 deg that's why I wondered if the instructions about C of G may be incorrect ! Quote Link to comment Share on other sites More sharing options...
Cuban8 Posted February 15, 2015 Share Posted February 15, 2015 Nightmare in the glide..................nose up and stalling or nose down & speeding up? Quote Link to comment Share on other sites More sharing options...
Geoff Bradley Posted February 15, 2015 Author Share Posted February 15, 2015 Sorry..can't answer thet one ..it was windy and all I wanted to do was get it down in one piece. I was hoping for some advice before I risked another flight ! Quote Link to comment Share on other sites More sharing options...
Plummet Posted February 15, 2015 Share Posted February 15, 2015 Check the mounting of the tail surfaces. These rely on dowels to locate them and the foam and correx can allow play. The wing mounting dowels can also loosen. We added ply 'doublers' to the sides to strengthen things. As far as the chord is concerned ... I recall that they did produce two models of the prangster. I think that the difference was the dihedral and the depth of the ailerons. Plummet Quote Link to comment Share on other sites More sharing options...
Dave Bran Posted February 15, 2015 Share Posted February 15, 2015 AFAICR there was a Prangster and a Prangster 3D variant Quote Link to comment Share on other sites More sharing options...
kc Posted February 15, 2015 Share Posted February 15, 2015 The J Perkins catalogue shows both a Prangster and a Prangster 3D but I cannot see any difference in the small photos. But it does say it was created by " Dr Newman" which sounds like Avicraft to me, so maybe a phone call to them would provide the correct CG. But there wont be any harm in moving the CG to 25 percent for a trial flight. Quote Link to comment Share on other sites More sharing options...
Chuck Plains Posted February 15, 2015 Share Posted February 15, 2015 Posted by Geoff Bradley on 15/02/2015 16:05:16: I've recently bought a JP Prangster for a Winter Hack and it noses up very severely. Everything is set up as per the handbook with the C of G at 70 mm which is as recommended for a beginner. Has anyone had experience of a similar problem ? I have noticed that the wing chord is not as stated in the handbook.....it states 52 inch span and 520 sq inch area which should give a chord of 10 inch but it actually measures 9.25 inches ! This would give a C of G of 59 mm at 25% of chord ! I considered increasing the down thrust but it already appears to be at around 5deg. Any held will be gratefully accepted. From what I've learned over the last, nearly, 2 years, with zero wing incidence angle, on most non scale planes, you ought to have a reasonable flight. So that's a starting point that you could set in with a bit of packing. As for the ol' CG, I always use the adamone.rchomepage.com CG calculator, even on ARTF jobs, just to get an idea of what's going on in that respect. Quote Link to comment Share on other sites More sharing options...
Geoff Bradley Posted February 16, 2015 Author Share Posted February 16, 2015 Thanks for the advice......I was hoping someone had the same problem ! I've checked the angle of incidence and it seems to be zero so I'll take some measurements tomorrow and use the calculator to work out the C of G. If that gives the same result as the handbook states I'll give it a try at 25 % and see how it goes. Thanks again for all your help......fingers crossed .... Quote Link to comment Share on other sites More sharing options...
J V R Posted February 16, 2015 Share Posted February 16, 2015 Hi GB On my Prangster I glued a 10mm foam block to the fuz for the wing to butt up against so that the wing would always seat in the same position and a couple of strips about 10mm square on the under side of the wing to set the wing central on the fuz, I've not flown mine for about a year now but I think I put a bit of a mix EL to TH. I think my C of G is set around 75mm. Some of the later Prangster's have a smaller wing cord compared to the first ones. One thing I will say about them is that they take a lot of abuse, mine has landed/arrived at every possible angle, it's had 3 wing spars, 2 engine mounts / ply box, the last spar I replaced is a solid length without the joiner in it as that was causing a weak point just after the alloy joiner. Fantastic hack model....... Edited By J V R on 16/02/2015 00:50:08 Quote Link to comment Share on other sites More sharing options...
Paul Jefferies Posted February 16, 2015 Share Posted February 16, 2015 If incidences and CG are as they should be, could it be that the wing is lifting? Is the wing held on with good strong rubber bands? Paul Quote Link to comment Share on other sites More sharing options...
Cuban8 Posted February 16, 2015 Share Posted February 16, 2015 Start from first principles. Check that the wing and tailplane are correctly rigged with respect to each other. I'd put a spirit level on the TP, and on the bench pack the underside of the fuz to level it. If you've got an incidence meter pack the mainplane to give no more than 2 degrees positive with respect to the 0 on the TP. Without a meter, judge the datum (depending on the wing section) then eyeball the wing so that the LE is no more than 1/8 inch above the TE – If it looks excessive then it probably is. If anything err on the side of too little. Set the CG to between 25-30% of chord to start. Having a non tapered wing, I've never found this type of model to be overly fussy with CG. Depends on flying style. Check elevator and ailerons are all set to a true neutral. Down thrust - check that the firewall has not been fitted badly and is at 90 degrees to the TP. View the prop sideways on at 12/6 o'clock position. For an 11 or 12 inch prop pack engine mount so top of prop is ½ inch forward of bottom tip. Slightly less for smaller props. Don't worry about side thrust just now. Set control throws to minimum for test flights, ideally we want the model to fly like a trainer at first and then steadily wind things up to hooligan mode as the control issues are sorted. Hope this is of some help. Quote Link to comment Share on other sites More sharing options...
Geoff Bradley Posted February 16, 2015 Author Share Posted February 16, 2015 Thanks again for all your advice. JVR.....I've also put a foam block to position the wing so I'm happy with that....once she's flying 'safely' I'll try the Thr - ele mix. Paul...I'm happy that the wing is seating...6 strong bands. Cuban8....I'm sure the incidence is ok..I projected a line parallel to the tail along the side of the fus and compared it to a chord line on the end wing section and they appear parallel. There is abt 5deg of downthrust. Chuck Planes....I've measured the plane and entered the info to the C of G calculator...Using a Static Margin of 15% it says C of G should be 63mm So..I'll set the C of G at 60 mm and give it another try. Off on my holls this week for a couple of weeks so I'll post the result when I return. Quote Link to comment Share on other sites More sharing options...
Chuck Plains Posted February 16, 2015 Share Posted February 16, 2015 Good idea Geoff. Going on the conservative side of a 15% static margin should be nice and safe, and you can then progress from there. Quote Link to comment Share on other sites More sharing options...
Geoff Bradley Posted March 23, 2015 Author Share Posted March 23, 2015 Well....I set the plane up with a C of G at 60mm......it still wanted to nose up ! So I tried 55 mm and it still had a tendency to nose up ! A fellow flyer suggested packing the wings at the rear to give a negative incidence angle...so a 10mm packing was inserted and it flew well. I removed a lot of the weight up front and flew again and it flew a treat. I'm going to play around with the packing and try to get it to fly without all the extra up font weight. It's not set up as the instruction manual but it flies a treat so the packing will stay put ! Quote Link to comment Share on other sites More sharing options...
Geoff Bradley Posted March 23, 2015 Author Share Posted March 23, 2015 Well....I set the plane up with a C of G at 60mm......it still wanted to nose up ! So I tried 55 mm and it still had a tendency to nose up ! A fellow flyer suggested packing the wings at the rear to give a negative incidence angle...so a 10mm packing was inserted and it flew well. I removed a lot of the weight up front and flew again and it flew a treat. I'm going to play around with the packing and try to get it to fly without all the extra up font weight. It's not set up as the instruction manual but it flies a treat so the packing will stay put ! Edited By Geoff Bradley on 23/03/2015 20:31:32 Quote Link to comment Share on other sites More sharing options...
aduncan Posted March 23, 2015 Share Posted March 23, 2015 My mate has a Prangster which he has flown for ages. Had the same symptoms as yours. We found the problem to be that the wing bands pulled the wing dowels together, bending the fuselage into a "banana" shape. He made a couple of ply plates to fit over the wing dowels and take the strain of the wing bands. Problem solved. Quote Link to comment Share on other sites More sharing options...
J V R Posted March 23, 2015 Share Posted March 23, 2015 GB, don't forget that the ailerons need to be set in line with the center of the wing not set in line with the top surface of the wing, a lot of people have made this mistake ( me as well ) it makes the ailerons act as flaps if they are set incorrectly. Quote Link to comment Share on other sites More sharing options...
Geoff Bradley Posted March 24, 2015 Author Share Posted March 24, 2015 Aduncan.......I've noticed that the new replacement cowl has an extension to go aver the wing band pegs.....I'll make a couple and see if it makes a difference. JVR.....I'm sure that I'm set up as you suggest but I'll double check. Thanks for your advice chaps. Quote Link to comment Share on other sites More sharing options...
Tim Morton Posted July 11, 2018 Share Posted July 11, 2018 Hope this thread isn't dead... I've had a Prangster for years and every now and then it comes out - it's a good plane. I've replaced all sorts of things over the years - engine mounts, control runs, undercarriage, and have just changed out the radio from 35 to 2.4 gives you an idea of it's vintage... But one issue which has been gradually getting worse is the correx tail has started to warp. It's got quite bad and is affecting flight now. Any ideas of how to fix this? Replace is an option but I didn't really want to do that, besides it will only happen again. Any advice ? thanks Quote Link to comment Share on other sites More sharing options...
Plummet Posted July 11, 2018 Share Posted July 11, 2018 I found it useful to brace the tail and tailplane to keep them at 90 degrees. I used plastic angle pieces cut from an old piece of square drainpipe. Can you stiffen the correx by poking carbon rod or bamboo squewers(sp?) through the correx tubes? Plummet Quote Link to comment Share on other sites More sharing options...
Tim Morton Posted July 11, 2018 Share Posted July 11, 2018 Good idea Plummet - but the warping is more a twisting than along the length if that makes sense? Even the elevator has taken on a nice curve... I've not had any problems with keeping the surfaces at 90 degs. Could you explain that a bit please (or some photos if you have) I heated it this morning (with a heat gun on low) and sandwiched it between two flat heavy boards. Hopefully that will straighten it a bit. I was thinking / hoping possibly some bullet tape may help Quote Link to comment Share on other sites More sharing options...
J D 8 Posted July 11, 2018 Share Posted July 11, 2018 With a pointed wire pierce holes along the length of the tailplane through the futes, then fit a carbon rod in place. Quote Link to comment Share on other sites More sharing options...
Tim Morton Posted July 11, 2018 Share Posted July 11, 2018 Quote Link to comment Share on other sites More sharing options...
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