Had a sad incident at the field this weekend. Someone who I would classify as a beginner moving on the intermediate level got a 46 size Chipmunk with retracts and all. At others advice, he removed the retracts completely, and made it into a trike wheeled configuration instead of the original tail-dragger. When he removed the retracts, he covered up the holes with monokote and more tape over it, to retain the shape of the wing.
Took off, and it flew beautifully. Then he moved on to some agressive flying at higher throttle, and one of the wings broke apart right at the place where the retracts were removed. No need to say, the model was totalled.
A costly lesson :( Should have placed some reinforcement instead of the moulded plastic retract cage.
Why would others give him such advice to make the plane trike wheeled?
Because a lot of people had them fail, after couple of impacts on hard landings.
Damn! thats bad. He must have cut through some balsa to remove the retracts.
Nope, the retracts were housed in a moulded plastic piece, which was removed.
http://www.rcgroups.com/forums/attachment.php?attachmentid=3735936
http://www.rcgroups.com/forums/attachment.php?attachmentid=3735935
if it was Phoenix models, they use some kinda plastic glue gun which is automated i think, glue is at regular interval and quite flimsy, it is a good idea to inject /brush wood glue. reinforcement is an option difficult to execute in an ARF, unboxed one looks so good externally that you are reluctant to operate upon it. I use some thing called an endoscope tied toothbrush to reach places where normal brush can't reach to brush it with glue
My take - Removal of retracts probably had nothing to do with structural failure. The probable cause lay elsewhere
1. No 1 culprit - Damage to strcture while removing the retract housing, and converting it into a trike. Being a newbie even more possible. I say this as for con version to nose wheel , the main U/C has to be moved to the rear and still be in line with the old housing (and offcourse add a nose wheel). Now I am assuming that the flyer did not add the main struts to fuse but just moved them to the rear. Which led to further weakaning of the structure at this line and fianlly it failed.
2. No 2 culprit - As said by Gusty insufficent glue/improper gluing of structural components which is a common deficiency in most ARF's orginating from far east (and most of them do )
3. Damage to the wing (which went unnoticed) either in transport from the factory/during assembly or later which led to the failure
Balsa is not the culprit here. Strength of the airframe is mostly derived from the way it is engineered rather than brute material strength.
Can't see your point Captain. The wing folded right at the place where the retract was, and was held with just monokote + tape.
that's poor workmanship by the OEM
Which point anwar? If the wing was held with just covering and tape, what else would one expect? It would have failed sooner or later.
Also that possibly can not be true (unless the manufacturer had missed certain components). For the retracts to be fixed, the entire housing had to be screwed on something substantial which in turn needs to be supported by the ribs, which again pass on the load to the spars. If rest of the wing was there, there was a spar which must have passed over/under the retract housing. So possibly the wing being held with covering and tape can not be correct, the spar has to be there else the the entire structure outboard of the retract location could just not hold up.
Given the geometry of the retracts (at the pictures at RCG) the legs fold back, this geometry is much friendlier to a model wing structure as the gaps between the ribs can be effectively utilized to locate the retract housing/bearers and other load bearing components (and also the space for wheel wells) without compromising the structural integrity of the wing.
Also pertinent information which is missing is what engine was the flyer using? It is common to see people shoehorn the largest engine which can be fitted on the model and then if there is a structural failure blame the desgin/manufacturer. Could that be a possible cause?
Captain - I was surprise by the below quote of yours.
Quote from: flyingboxcar on May 20, 2011, 06:46:29 PM
My take - Removal of retracts probably had nothing to do with structural failure.
It was obvious that not reinforcing the hole from the retracts was THE cause of the whole incident. Anything else may have at the most played a contributory role (if any). What I noticed was the retracts housing was a solid piece of moulded plastic, and would have served the purpose of ribs/spars more than adequately. When that level of support is taken away and replaced with next to nothing, what happened was inevitable !
He was using a .46 engine, so that was not the issue. And the plane was NOT the one in the link I posted above, it is another make with the retracts a bit more away from the fuse on the wings. But the orientation of the retracts were similar, oriented like a rib (unlike some other models where the retracts are oriented parallel to the spars).
He was flying like the plane was a bullet, and crashed right after someone asked him to slow down (like 2 seconds after the person commented). We all had a good laugh about his "evil eye", and people were offering him money (as a joke) begging him not to open his mouth about anyone else's flying or their airplanes ;D
Anwar
If the landing gear was converted to a trike, the "mains" would have to be moved back. How was this done? Could be a clue here.
Quote from: augustinev on May 01, 2011, 05:04:30 PM
if it was Phoenix models, they use some kinda plastic glue gun which is automated i think, glue is at regular interval and quite flimsy, it is a good idea to inject /brush wood glue. reinforcement is an option difficult to execute in an ARF, unboxed one looks so good externally that you are reluctant to operate upon it. I use some thing called an endoscope tied toothbrush to reach places where normal brush can't reach to brush it with glue
I have a PHOENIX At 6 with retracts. They are mounted on what looked like a substantial ply (10-12 mm) plate. The gear came off on a slightly "hot" - but not hard - landing on the second flight. Examination revealed that the "plate" was made of several layers of ply with NO glue between! And the screws fro the gear mounting barely entered the second layer. Have fibreglassed the plate and used 2 additional screws (6 total) of 15 mm length so they go through all the layers. Holding up well. But the retracts themselves are "cheapo". Would have changed to fixed but the airplane flies much, much better with the gear up.
@ Anwar
My reasoning was that the plastic housing was just cosmetic and at max to support the retract itself, and also pass off the landing imparted loads on to the other structural members of the wing . The housing itself should not have had any role to play in the structural integrity of the wing. Atleast that is the way a wing should be designed.
My statement (to your surprise) was based on your statement which said that the entire wing was held with the covering material and some tape, which possibly can not be correct as this would not have lasted even a minute in the air, not to mention high speed flying where the wings would have given in to the aerodynamic loads.
As said earlier, and as also echoed by Sushil the main gear would have been moved to rear and in line with the originals (which means more cutouts in the wing) and top this off with the now vaccant location left as is which creates a weak point across the wing (along the chord line), what else would one expect?
Modifying a model is all fine, but without adequate attention to appropriate engineering??? Well! the conclusions are foregone...
And if I may? (not to deride the flyer or anyone else) The flyer was a comparitive newbie, with apprently not much experience in either building (note not assembly but building in traditional sense) or repairing, hence this oversight could have happened. No point blaming a balsa build or the manufacturer for poor design
Never meant to blame balsa builds or any manufacturer :) And I meant to say the "the hole" was held (covered) with monkote and tape, not the entire wing.
He had the two hind wheels for the trike stretching from the fuse to just before the opening for the retracts (if I remember correctly).
The point is that reinforcing to cover the hole left by retracts seems to be a must if anyone is trying such modifications in future.
Does drag due to the non-retractable landing gear also lead to an increase in stress ... which might have eventually lead to structural failure (especially at the higher speeds) that the a/c was being subjected to during the aggressive flight?
While an extended undercarriage, will sure induce additional drag, in a model airplane this is not the extent to stress the airframe to the point of failure.
Any airframe has a VNE, this would substantially reduce if the structure is weakend for any reason, and structural failure (due to flight loads) could result in case the VNE is exceeded