11-Dec-18 - 04:29 PM

Thermal Circle Text - Thermal Circle 09

Written by Hayden Daley - 8th September, 2007.
Welcome to this edition of the soaring circle. In this edition, the topic will be something we all don’t like dealing with, crashing and repairing models. Although some people seem to never crash, eventually even the best model pilots have to deal with some sort of incident or accident occuring with their models. Some of these accidents are minor allowing the model to be repaired; and in other instances the model is too badly damaged to be repaired and written off.. The hobby of flying radio control models can be dangerous and we as aeromodellers are responsible for the actions that occur when we fly our models. I believe it is the responsibility of every model aircraft pilot to do as much as possible to keep our hobby safe; and to do as much as possible to prevent models crashing. This includes pre-flight checks of the aircraft, radio range checks, proper battery management in the model and to be as careful as possible in regard to the well being of your fellow club members. I also believe it is necessary to try and get to the bottom of what happened after an accident, to prevent the same sort of crash occurring again in the future.

After an accident that had a heavy impact, I prefer to use the receiver that was involved in the crash in combat models only. The reason is that after a heavy crash, receivers can become intermittent; they work fine for a while, then go off the air. This does not happen to every receiver that is involved in a crash, however it is possible that receiver damage that is difficult to detect has occured. I personally always dispose of crystals that have had a heavy impact, they are the most delicate part of the receiver and are easily damaged. If relegating receivers to foam combat models is not an option; then I feel it is important to get the receiver checked. Your local hobby store can assist with having radio gear accessed or fixed. In a heavy impact it is also quite possible that you will strip gears in servos. The price of a gear set is not high but it is a fiddly operation to change servo gears, this is best left to the radio repairer. Any sign of damage to battery packs should result in them being put in the rubbish bin.

If you have an accident with a traditional all wooden model and you wish to repair it, you must collect all pieces from the crash site. Wooden models are the most repairable of all types of gliders and repairing wooden models is easier if you have all the pieces. With models constructed from Balsa wood, cyanoacrylite is the best adhesive for this task and using dress making pins will help to hold things in place during repair. Once the model’s pieces are returned to their rightful place you may be missing some pieces. With the front of fuselage’s for example, after gluing the pieces back the best option with dealing with pieces missing is to line the inside of the crashed area with a suitable width of balsa wood and then fill in the gaps with a hobby filler. After filling, sanding and reinforcing you can then re-cover the repaired area. However, if you have damage that is behind the trailing edge; and you have pieces missing then you may have to “splice” new material into the damaged area. By removing the wood and re-gluing in a fresh piece, these repairs are better hidden and more structurally sound; and they will also allow for sanding and filling of the area before recovering.

With traditional type built up balsa wooden wings, repairs are more difficult than a fuselage. To return structural integrity to wings it is often necessary to brace broken wing spars to form a “d-box” repair. This is done by gluing the spars back together and using either balsa wood or plywood, cut to a rectangular shape and glued to each side of the broken area, between the ribs. Aliphatic glue and epoxy are suitable adhesives for this task. The damaged wing spar will once again regain its integrity with this type of repair. Also, you may need to cut new ribs where damaged ones once were. The style of repair that was described for wooden fuselage repair is not really an option with wooden wings. They have a higher load placed during launch and in flight; need to be kept light, and need to be structurally as strong as they were before the accident. Making templates from cardboard is a good way of getting accurate replacement ribs for wings and templates can be helpful with other repairs. However the main goal with traditional models is to keep the structure as light as possible after repair; whilst restoring the structural integrity to the airframe..

If you damage the tailplane on a wooden model, you are faced with task of keeping the repair light. All repair material that is added to the tail has to be countered with nose weight and this can result in a heavy model. Selecting light balsa wood for the repair is important, and using light weight adhesives such as cyanoacrylite will also aid in keeping the repair light. Also, when re-covering tailplanes or wings, stripes and other shapes are a good way of hiding repairs.

Models with pressed foam core wings, foam wings “skinned” with either balsa or lite ply often present as the easiest to repair of all types of model sailplane wings. Firstly you have to access whether the damage has structurally affected the model. If the damage is minor the best fix is to cut the damaged area from the wing and replace the damaged area with balsa and then sand to shape. You can then fill, sand and re cover the damaged area. However if you have broken this type of wing, the repair becomes more difficult. In this situation the first job is to glue the broken wing back together with epoxy. The next task is to “brace” the broken/damaged area. This is done by using a suitable sized ply wood and cutting “slots” in the damaged area for these rectangular braces. By gluing in the braces with 30 minute to 7 day cure epoxy the strength is returned to the wing with this type of reinforcement. Also, the longer drying epoxies are stronger, so are better used for this task. 1.5 to 3 millimeter thick plywood is a good size to use as bracing material and between ten and thirty centimetres is a good size for a brace. Also important is when using several braces to return structural integrity do not make them the same length, this will create a hard point and may fail at the repair site; it is better to use braces of different size to spread the load. For aesthetics filling the joins with filler and sanding will give a better hidden repair.You must also take into consideration that the closer the break is to the fuselage, the more bracing will be required. For interests sake, I have seen a Southern Sailplanes Eclipse that had both wings broken in several places return to the field with bracing and covered in three-quarter ounce fibreglass cloth. The weight was up, so it was relegated to slope duty; but it demonstrated the repairability of these types of wings.

With molded competition models that are severely damaged for example, in a heavy crash from a high altidude; it is generally easier to buy new parts. Hard impacts will cause too much damage to the models wings to repair. Keeping the repair light weight and strong, which is the main issue when repairing; will be a difficult task. Also after a hard crash damaging the leading edge, it is also difficult to keep the accuracy of the wings profile when completing a repair. However with sailplanes which have for example, have sustained light damage through an outlanding and hitting something, these minor damages to wings are normally repairable. The crucial assessment when considering repair in a molded competition model’s wings is whether the spar has been damaged, and to what extent. F3B and F3j competition models are built “on the edge”, they are built as light as possible for their intended purpose. The spars are made separately and built to flex under load, for example during launch or turning at high speed. If you damage the spar on the inward half of the wings it is probably better to relegate the repaired model to slope duty. With damaged wings that have had repairs to the spar, they create the problem of not flexing as they are designed to; the repair will create a hard point on the spars where material has been added during the repair. Under a shock load; such as launching in gusty conditions, these hard points will break first, causing a structural failure, because the repaired spar cannot flex as intended too during design.

When repairing hollow molded wings it is neccesary to “fill in” the void where the damage has occurred. Balsa wood or blue foam as used for insulation in houses, is suitable for this task and when shaped fills these voids well. You then need to chamfer, create an angle in the damaged area, by sanding. This sanding is to allow for the composites used in the repair to build up in layers and intergrate with the original structure. The next task is to lay up the wing using a suitable composite and to blend the repair. This is done by using carbon fibre and a suitable resin. A suitable resin for this task and popular amongst Victorian modellers is called R180 and it is available by the litre from the Air Strike winch company. It is an excellent quality, strong epoxy with a seven day cure. The task of repair entails laying up either uni-directional or woven carbon fibre cloth; or a combination of both types over the the damaged area; to a suitable strength and allowing good time to dry. Preparation is important when laying up repairs, you will need to cut all material to size, have appropriate mixing tubs and to be able to accurately mix the epoxy, weight measuring scales are useful for this task. You also must make sure all resins used are mixed thoroughly before use. When the repair is dry it is then time to use two pack car body filler to fill in the weave and then sand to shape. Also, stripes look good on these types of repair; as matching molded model finishes can be difficult.

With molded fuselages repairs become infinitely more difficult to complete neatly if it cant be reached from the inside. When repairing broken fuselages it is also neccesary to make sure they are straight. Mould lines where the fuselage has been joined provide excellent reference points to keep things straight; as do long length rulers. The first task though, is to glue the fuselage back together. Thirty minute epoxy is useful for this task. When dry you will need to “remove” in a semi-circle shape enough material from the fuselage for a suitable strength repair. Then lay either carbon fibre cloth or depending on suitability; or the strength required standard fibre glass cloth could also be used. You may need to do several lay-ups to complete the repair. When ready to finish, use two pack body filler available from your local car products dealer to fill the weave; which will allow you to sand and prepare the repaired surface for paint.

With damage to V-tails on moulded models it is best to replace the part. These types of tailplanes are also built “on the edge” to be as light and strong as possible. They use similar construction as the wings and also have the same problem of hard points, affecting the designed flex of the tailplane. For peace of mind, I advise the replacement of the V-tail, as they are under extreme loads on launch, and are cheaper to replace than a whole model.

If you have “dings” or hanger rash on you molded model this can be fixed. Polymers by their very make-up have a “memory”. The composites used will return to their original shape as long as the hangar rash has not gone through the wing skin. The way to complete the repair in this instance is to boil water, then using a 1 part cold water to 4 parts hot, place a tea-towel over the ding, poor the mixed water and wait between 5 and 8 seconds and remove the tea towel. This will remove 95 percent of all hanger rash on composite wings and other parts.

In this edition I have tried to cover as much as possible on repairing models and to provide useful information on how to deal with these disasters. I have thoroughly enjoyed writing the soaring circle and can be contacted by email at soaring@newlitho.com.au. If you have any topics you would like to see in the soaring circle then please email. Otherwise happy, safe and successful soaring. Hayden Daley

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