Black Horse Piper Cub Fault

[Bill Brown 3]

Hi, one for the much more experienced here. I recently bought a Black Horse Piper Cub which was in brand new condition having had very few flights, the engine was new when initionaly put together, it is a 72" span using an A.S.P. 61 FS motor and turning a 13x6 prop. When it had it`s first flight (piloted by a guy with a fair amount of experience) not myself as i am a lot lower on the learning curve, we experienced some odd behaviour, it taxied along fine, got about a metre off the groundand dropped the port wing suddenly, it was well caught though and once trimmed out flew very well including a steady landing. However on the second take off it did the same, dropped the same wing, yawed round to port and hit the deck causing some damage. Damage now fixed i am looking for a reason, i have downloaded the manual and can see that i must check the aileron throws are set to specs. also the C.O.G. described as 100mm from the l/e on top of the wing balanced upside down. Now here is the odd bit, having had to strip off the engine to do the repairs i have noted that the firewall is set biased to starbord, nothing unusual, however the mount has had three washers put under the two nearest to port fixing points and as the engines rocker cover sits at about 10 to 12 i would have thought that when this was done it would not only add to the normal bias but also tilt it down slightly.In addition It must have been thought to be tail heavy because the engine mounting position has been moved foreward by 10mm from it`s initial placement along with the 6v battery pack being moved from its allocation to just behind the engine and 50 grams of lead added to the mounting bars ???????? Your thoughts on this one would be most welcome and do i re-fit the washers when i bolt things back?

[John Tee]

Hi Bill. I don't think the washers would be a problem , just giving a bit more right thrust on the motor. Trying to visualise the rocker cover and the engine mount I don't think the downthrust would be altered. It sounds to me as if the plane is taking off too slowly and stalling and this could be made worse by using the ailerons at too low a speed. use rudder until the speed has built up. Someone else may have more knowledge of this particular model. I have a BH Cub but it is a smaller electric one as yet unflown.

John

[Chris Bott - Moderator]

Bill, dropping a port wing on takeoff sounds to me like a classic symptom of pulling it off the ground too soon.

The engine sounds a bit big too, so that will add torque in the same direction.

If it flies fine onece it has been sucessfully got away, then I'd say it flies fine once the required airspeed has been reached.

The big engine, probably explains the previous owner putting in extra down and right thrust, assuming that's the effect that the washers have?

I'd suggest trying a longer takeoff run, to allow the airspeed to build up before allowing the Cub to gently lift it's self off.

[Colin Leighfield]

I've got one of these, although I've not flown it for a couple of years. However I found it to be good overall and do intend to resurrect it at some time.

My first comment is that I would have thought it to be well over-powered with a 61FS. If I remember the suggested engine size was .46 two stroke. I flew mine with a Thunder Tiger 54 four stroke and it was more than it needed, I will probably re-engine it with a 40FS. Apart from that, with the engine mounted directly to the fire-wall and no alteration to thrust angles, it flew ok on low throttle settings. I'm sure I set cg and control throws to plan.

I did learn though that a scale Cub isn't in the trainer category. It will drop a wing at the stall, although recovers easily. Also, the narrow undercarriage means ground handling needs some care. Perhaps the problem you have is connected to control throws, perhaps the elevator is particularly sensitive. I can well imagine that excessive up elevator at low speeds would cause a tip stall and from my experience, it would be to the left. Sorry if this isn't specific Bill, but it might give you some clues.

[fly boy3]

I agree with Bill, seen this down the patch many times, luckily with no damage. Wonder if the explanation for this is engine torque ? Cheers

[Bill Brown 3]

Thanks for your views on this one chaps, some good food for thought and things to check, your right about the engine size Colin, the manual states a 52 size f.s. although being aware of this maybe will be o.k. if take offs are gentle without perhaps giving it full throttle, please feel free to correct me if i am wrong, i also feel that the 13x6 prop could be too big as A.S.P.s largest recommendation is 13x4 or options of 12x5,6,7, and 11x7,8,9,.

[Colin Leighfield]

Bill, the prop may well be an issue because it is probably maximising torque reaction and not giving maximum thrust at low rpm, not good if happening at the same time! I'd probably go for 12x5 or 6. Do bear in mind though that once you're up there you've got 25% more power than you want and just screwing in more down-thrust isn't a good solution.

Assuming that cog is spot on, I'd be very cautious about elevator movement because there is a tip-stalling tendency and too sharp up elevator (even if it doesn't look a lot) at take-off speeds could definitely exacerbate this. Overall it's a great flyer and very scale, but it does have a couple of vices at the take-off stage for the reasons described. A bit of sensitivity is the issue in control throws and pilot handling.

When I first flew mine I soon found out it wasn't the pussy cat I expected when taking off and I probably went off the runway sideways, but you get there and in truth, I'm not so sure that the full-size Cub isn't a bit tricky in experienced hands either. I used to fly a full-size Auster and that could tie you up in all kinds of knots if you weren't tuned in to it.

Good luck Bill.

[Martin Harris - Moderator]

I suspect that the ground angle has a lot to do with this behaviour. A scale Cub has a surprisingly high tail in the flying attitude (hold it with the tail parallel to the ground to appreciate this) and you should be getting it running at this attitude before a (very) small rotation to get it off the ground. If you simply open the throttle and leave the tail down it will unstick at a high angle of attack - and at model sizes, the critical angle is less than at full size.

As you've taken on board, a steady and controlled take off run should pay dividends - I deliberately limit the throttle opening on my 1/4 scale version in order to get a slightly more scalish run, only using full throttle (after take-off) if I need a rapid climb out or to extend vertical manouevres.

Flying a scale Cub well is a rewarding experience and one that takes a fair bit more skill than wanging a Wot4 or other typical sports model around the sky, needing some co-ordinated left thumb work (mode 2 of course).

I'm sure you'll soon get to grips with it!

P.S. Does the manual really advocate setting the C of G inverted - good luck if you can manage it but I would definitely do it the right way up with any Cub - it's low wingers that need to be inverted! You haven't downloaded a low wing Cub (Pawnee) manual by mistake?!?!

Edited By Martin Harris on 30/08/2013 15:17:15

[Martin Harris - Moderator]

P.P.S. After finding the manual and confirming their advice to balance a Cub inverted, I'd take anything they say about the C of G with a large pinch of salt! 100 mm seems a long way back if it's a scale wing  (I'd have expected somewhere around 70 - 75 mm).

Measure the chord, divide it by 4 and use that as the C of G measurement from the LE for your initial flights. You may be able to go back a bit towards 30% or so when you get to know the model better...

Edited By Martin Harris on 30/08/2013 16:15:04

[Biggles' Elder Brother - Moderator]

I think there is a lot in what Martin says here. I have the World Models Cub - just a shade bigger than at 78" span. The Cub is a lovely model to fly but, as Martin says, it really does have a few "features" that make it interesting.

I think the problem you might be having is that, as with any taildragger, you normally hold in a little up-elevator in the early phase of take-off. The problem is the Cub has a very high lift wing section and as Martin says a low sitting tail - result up it goes from the three point position much sooner than you might have expected. In fact its airborne sooner than it expected as well! So not enough airspeed and too high an angle of attack. It stalls and drops a wing - in your case the left one!

Try a take-off run with little or no up-elevator, if you do use a bit just at the start of the roll take it out PDQ. Even use a tiny push of down if needed to let the tail come up so she runs level on the mains. Build a bit of speed, then squeeze just a touch of up-elevator to rotate and lift off. I think you'll find the Cub will behave itself a bit better then!

BEB

[Pete B]

I'd second BEB's view on the elevator. From something I read on another forum a while back, I stopped using up-elevator on my 76" CG Cub at the start of the take-off run, unlike just about every other taildragger I fly. This immediately solved the slow-speed directional problem I often experienced and it behaves very much better for it.

I also agree with others that insufficient airspeed, coupled with an over-powerful engine, seems to be the likely cause of the OP's Cub's behaviour.

Pete

[Robert Hynes]

I'd like to second BEB and PB's comments and add that elevator trim can also help.

This can facilitate the best take off run with mains firmly on the deck so that the aircraft rolls with tailplane off the ground

If you get this level of trim correct the aeroplane will need slight up elevator to lift off and and this will need to be maintained for the climb out. If done optimally it takes all the pressure out of the takeoff run.

The advantage is you get ground speed above the stall before lift off, making a wing drop unlikely, (see below), after lift off, after which the aircraft continues to accelerate to normal air speed. If you are not comfortable with using different elevator trim settings during flights then do practice first using something like the Phoenix flight simulator.

If you overdo the down trim and take your thumb off the elevator as soon as as you lift off you will likely pile in, so take care, again you can duplicate this with the sim. Just enough down trim to make it easy to give you that long accelerating take off run.

The only other situation I have seen with an unexpected roll at lift off was caused by a wing warp. The reason here is that in flight the warp could be effectively tuned out using aileron offset. Ailerons are much less effective at liftoff however and so the aircraft rolled on takeoff with the warp. Cure of course was to fix the warp.

[Bill Brown 3]

Thanks again all, your tips and views are very much appreciated.

[Colin Leighfield]

Good luck Bill, it's a very sound model actually, once you've sorted the minor quirks, you'll be fine.