E-Flite's J3 Cub 25

[Handyman]

Hi all, I have just completed one of Horizon Hobbies excellent Cubs, the 25 electric jobby.  I have assembled it with great care and resisted the urge to alter or otherwise improve the model with the exception of the u/carriage fairings, which were supposed to have been fitted using yellow tie-wraps. Not my idea of a good job.

I fitted the E-Flite 25  motor that is recommended and with a 3s 3000Mah Lipo, the C of G was spot on. Being a devout coward, I elected our club instructor to make the first test flight.  The take off was ok, climb-out was a bit steep, and I could hear my pilot bleeping my trim buttons like crazy.  He handed over control to me, remarking that he thought it needed some more weight in the nose to correct the steep climb on take-off.  I can fly quite well but I noticed a tendency for the aircraft to want to climb or dive when trying to fly straight and level, and the same occurred when altering the power.

I noticed that on my Spektrum DX-6i that the forward trim indicator was half way between neutral and the fully forward point.  I attempted to bring this trim back to the  neutral point, but this made the flying  worse. The first landing seemed to be uncontrollable with the aircraft landing quite heavy.
We decided to cut the control movements down to half of what they had been for the first flight, but not much better, with the plane still showing the same problems, and the same heavy landing.

I decided to call it a day while it was still in one piece and set it up on my workbench, less wings, into a flying position, with the tailplane set up at zero degrees using a small spirit level. When I moved the spirit level to the wing seat, I fully expected it to read Zero, or possibly 1 or 2 degrees of positive incidence. What I got instead was  3 degrees of negative incidence, on both sides of the wing seat area.

Can some-one correct me if I am wrong on this, but I was always led to believe that a flat-bottomed wing is usually rigged at a positive angle compared to the tailplane, unless it is a fully symmetrical wing when it is usually a Zero-Zero set-up.

I think that my motor may need a bit more downthrust, but I am also convinced that the incorrect longitudinal angle of the wing compared to the tailplane was the direct cause of the strange flying characteristics of my Cub.

I am open to suggestions on this.
 

[Martin Harris - Moderator]

The chord line of a flat bottomed wing is positive with regard to the bottom surface so a zero incidence of the bottom surface would give positive incidence but without knowing the wing section it's impossible to say how much - but it's likely to be around 2 degrees at a guess so with some latitude for the guesswork and your measurements it might still be set correctly.

 

If the model pitches up with power and dives when reducing it if trimmed for level flight at half throttle then I'd go for some more downthrust.

[Handyman]

Hi Martin, thanks for the reply. I do agree with what you have said, this confirms my first thoughts on this, but when I set the fuselage up on my bench into a flying attitude, I then checked the angle of the tailplane fore and aft, and it was at zero degrees on the bubble. Without disturbing the plane, I put the spirit level onto the wing seating area and as I said in my post, I had over half a bubble of negative incidence where I would have expected zero or possibly 1 or 2 degrees positive angle.

 

I am now re-fettling the wing seat to zero to give my flat bottomed Clark Y wing the necessary angle of incidence that it was lacking in the first place.

 

I welcome any more suggestions. I will keep you all informed. I will try and test fly before the weekend,hopefully,weather permitting.

 

I

[Handyman]

 

 

I

Edited By Handyman on 19/07/2010 18:47:52

[Handyman]

Ooops finger trouble. I think I sneezed and got two postings. Sorry

[Simon Chaddock]

Handyman

Remember you flew with a lot of down trim.

I presume this meant the elevator was 'down' a bit as well so if you think of the effective shape of the complete tailplane in this condition it is effectively under-cambered. In combination with Clark Y section wing this tends not to be a good combination.

 

A truly 'flat' tailplane and elevator set parallel to the bottom of a Clark Y will still have a perfectly adequate 3 degrees longitudinal dihedral so setting the underside of the wing to have a positive incidence to the tail plane will increase this even further and make the plane even more difficult to control, unless you move the CofG forward to compensate.

 

I do suspect your test pilot's original suggestion was probably correct.

 

I hope this helps and do post how you get on.

[Handyman]

Thanks for your input Simon. I had got the C of G spot on according to E-Flites instruction manual. It recommended it to be 2. 5/8" (66mm) back from the leading edge and described the C of G range as 2.1/2"-2. 3/4" (63mm-70mm) behind the leading edge.

 

I think I had better do one thing at a time. I have altered the wing seating now, so I will test fly in this configuration. Next I think I will give the motor more downthrust, and if that does not improve matters, I will adjust the C of G to be a little further forward and see how that goes.

 

Thanks to you all for the help and advice. I will post results at each stage.

[James40]

I'd move the CofG first, it's the easiest to do by moving the battery. It sounds to me as if the CofG is too far to the rear if it has a tendency to be pitchy.

[Handyman]

Hi all. Handyman back once again with the saga of my E-Flite J3 Cub, and its strange maiden flight characteristics.

 

I was e-mailed setup instructions by E-Flite of America and they confirmed that the cub should have an angle of  2-3 degrees of positive on the main-plane with the tail set at zero degrees.

 

My cub must have been a rogue one or the jigs used to build the fuselage must have been incorrect, because I measured a negative angle of 3 degrees on my Cub. I went ahead and built up the wing seat to rectify  this situation because I had checked the C of G several times, and it was smack bang in the middle of the recommended range.

 

The end result was that the wing now had the correct incidence, but was sticking up above the top of the windshield, where it had been flush before. However the second test flight on Sunday last was a revelation. I used my friendly BMFA Instructor to do the honours, and the take-off was perfect, control authority was exactly as it should be, and Ian remarked that it was flying just the way that a Cub should.

 

The same evening I e-mailed E-Flite and told them the good news and attached 12 photo,s showing the checks that i had made using a spirit level before the remedial work and afterwards the views showing the raised up section at the wing leading edge.

 

I have received an e-mail back to the effect that after looking at the photo,s sent they are convinced that the Cub was not assembled properly during manufacturer. They want to have the Cub back, and want to have my details so that they can arrange shipping for this cub and then authorise a replacement back to me.  What do I do in this situation. I have a Cub that now flies properly, but they want it back, after all my work.  I will then have to assemble this other Cub, but no consideration for the extra work that I had to carry out on the faulty one.  But here is my burning question. If they ship this replacement directly to me, then might I not fall foul of "Her Majesties Customs Service" and have to pay duty.

 

What do I do.  All clean suggestions welcomed.

[Pete B]

Ask them if they'll let Horizon Hobby UK deal with any exchange - that'll avoid any possible duty issues. HH have brilliant customer service so I'm sure you'd be looked after.

 

Or, point out that you've put a lot of effort into getting it in the air and you'd prefer to keep the model, but if they wish, you'd be very happy to accept another product or a voucher by way of compensation for your trouble.............

 

Pete

[Handyman]

Hi Pete, why didn,t I think of that. It must be my age. I must admit that i have been well pleased with my other purchases from HH. I have one of their Beavers and a Piper pawnee crop sprayer on the back burner.

 

I must admit that I would rather keep this cub and perhaps give the downthrust a little tweek. It looked absolutely brilliant on it,s approach to land and Ian commented on it,s ground handling, right back to my flight box.

 

Does anyone else have a view on what has gone on with this J3 Cub.

 

Tony

[Pete B]

I'm a bit of an HH products fan, Tony. Spekky radio, Beaver, Cub 25, Sea Fury and P-38 from the E-Flite range - and drooling at the unaffordable Twin Otter.....

 

If I have one gripe it is their persistence in using UNC (?) bolts rather than accepting the world is now metric! It's a pain having to buy odd Allen keys to fix u/c's, motor mounts etc

 

I was flying my Cub 25 last evening and it a pretty little thing in the air, but it does seem to need some flying. The C of G does seem to be rather critical in terms of  its stability - whether that is a model issue or just a feature of the Cub per se, I'm not sure.

 

Here's mine, taken last year, but still in same condition, with pilot sitting comfortably and a bit of scale-ish bungeeing on the u/c - definitely worthwhile as it does tend to splay its feet - or it might just be my landings..........

 

Pete

[Handyman]

Hi again Pete. My cub was flying hands off for quite long periods and the C of G was exactly where e-flite recommended it to be.  Have you had a good look at your cub with regards to the angle of the horizontal stabiliser and the mean thrust line of the fuselage. On my cub, the zero plane of the stabiliser seems to be following the angle of the upper longerons.

 

I have been told that some cubs were rigged to fly in a nose down attitude in order to give a better view over the nose.  The handling of my cub on Sunday last compared to the previous week was nothing short of a miracle, and all down to getting the angle of the main wing to be at 2-3 degrees positive.

 

I have started to add a bit more detail to my Cub now that I know that it will fly well, and my landings were floaters. Perhaps you ought tto check your cub out as I have done mine. Who knows, yours miught be setup wrong.

 

Bset wishes...Tony