Messerschmitt Bf109D Kit 401 build for FAC

[woundedbear]

Hello! Everyone, There is something I need to ask. The rubber motor for a free flight model airplane acts like a tourniquet. When winding up the model for a flight it does the same thing that a tourniquet does. How do you prevent the dreaded implosion? As a boy, (9 or maybe 10 years old at the time) the fourth Guillows kit I built was the 900 series P-51 D Mustang. I was quite pleased with myself after I had completed this model. I had painted the plastic prop hub cover bright Tester's red, "and because it was all I had at the time", I gave it a neutral gray paint job from a spray can. It did some surprisingly good test glides. Then I was winding the rubber motor for the first short powered test flight when I heard the crunching sound to my horror, the little airplane's propeller was pulled all the way back to the former behind the rear of the canopy, I was distraught with grief How could such a thing happen I had read the instructions so well and had done such a good job. This experience traumatized me for days on end. It was more than a two years after this before I tried to build another Guillows model. I did build the 900 series Cessna Birddog and because it had a plastic nose piece that spread the stress over a larger area it could take the pull of the (tourniquet) rubber motor. It lasted for a week or so of many successful but short flights. Now I see you guys building these incredibly lightweight and beautifully built model airplanes, and I know that you all use contest rubber for the motors, and achieve many long duration flights. How do you all manage to prevent the heartbreaking implosions that I experienced as a boy I do want to be able to build rubber powered free flight model airplanes as well as the RC conversions. But how can such light and delicately built models withstand the powerful pull of a tightly wound rubber motors I have seen videos of guys stretching and winding 4, 6, and 8 loop rubber motors and am awestruck by how much stress these lightly build model airplanes can take . I can see how a model airplane powered by an electric motor can work being built so lightly built. "On a side note here", I think I now understand why there are not too many companies making timers for electric powered model airplanes anymore. It occurred to me that if a twisted person who wanted to build a bomb How such a timer it would make things easy for them to do this. I don't understand why anyone would want to do such a thing but we now live in a world that doesn't make a lot of sense to me. I know this doesn't have anything to do with building model airplanes, but for the longest time, I was perplexed by the lack if entrust in electric powered free flight. All of the digital wrist watches I have ever owned have had this adjustable countdown feature, and such a device could be made for limiting the powered flight times of electric powered free flight model airplanes, but such a device could be used by some "lunatic fringe" person to cause great harm to folks. But rubber powered free flight is something that could not be used by men with evil minds to cause harm. So electric powered free flight will never be the truly cool part of this hobby it could have been

[Mitch]

Hello WB and everyone.

I found that there are at least 3 things can go terribly wrong when winding a motor. First off I primarily build the 400 and 1000 series kits. In these kits you get the grey rubber. If you use that (and it works) it has a lot more torque than tan rubber that I use for competition.

From the story you told WB it sounds like the plastic front end failed with catastrophic consequence. If you are using the plastic cowling as a structural piece and planning on a rubber powered model you MUST reinforce that plastic with built up wood structure. The second thing that can go wrong is if you use a wooden rear peg, that can slip out of position, with a different, but equally catastrophic result.

So now that I have a solid front end and do not rely on the plastic, and I do not use a wood dowel. I use an aluminum tube with fuel line hose on each end to lock that in place. The only thing that can, and does go wrong is motor failure. While stretch winding a motor to maximum torque one is always getting close to the breaking point of the rubber. Moreover as you use that motor a few times you are getting dangerously closer to the failure. You will not avoid the failure but save your model if you use a winding tube that is inserted in the model during winding, and taken out before flight. This will not prevent the motor from exploding if and when it fails, but will contain the energy inside the tube rather than 're-kiting' your model.

If you look at the picture of my FW you will see some damage visible to the skin just forward of the rear peg. I blew a motor without using a blast tube.

Just thought of a 4th thing can go wrong.

While I was attaching my prop to the FW with a wound motor, the shaft wire slipped over the propeller stop ramp. I did not have the nose cone on at this time. So I was holding on to a rubber powered dremel tool as the wire sliced though 3 of my fingers. No pictures for that as children may faint. I was attended at the aide station and continued to fly, as I was in competition at WestFAC 5.

So respect the power of the Rubber Motors

Mitch, Keep Building, Keep Flying, Keep Safe

[woundedbear]

Thanks for the input Mitch, as always your advice is welcome. I agree with you 100% on the power that can be stored in a rubber motor it can destroy weeks of work in a nano-second. But still how do you guys build so light and not have the tourniquet effect implode your airplanes?

[Mitch]

Lets see... all the parts of the plane work together. My planes are fairly light and they are still delicate and need to be handled with care. I was just showing my Hellcat to some friends and crushed the side a bit...oh well another two hours to repair that.

But to the point of the question, the power of the Rubber Motor is down the length of the fuselage, and in that direction the model is very strong. The frames of the fuselage are lightened, but they are only holding the stringers, and the keels. They are where your strength is, also as you wind and add power the force will want to compress and make those longrones explode outward. The doped skin of the airplane is working to hold that all together.

So it is the sum of the parts, and there are parts that are critical and those that are non critical. Remove and lighten the non critical parts as best you can.

Hope this answers your question.

Mitch

[Chris A.]

Mitch's comment is similar to Lotus founder Colin Chapman's design mantra, "simplify and add lightness". Chapman was a fanatic about weight saving and efficiency. In the Lotus 25 GP he gained stiffness and strength but dropped weight by using monocoque construction instead of a frame and making the engine a stressed part of the system.

Of course the entire 109D fuselage system has to work in harmony. It may get to the point where the original motor overstresses the fuselage at a vulnerable area and you have structural failure. Back to the drawing board where instead of beefing up failed structure, carbon fiber thread is used to strengthen the fractured part in a vulnerable area such as fuselage stringers at the motor mount or alongside the cockpit. If the Bf 109D comes out significantly lighter, it may be possible to shorten the motor length for the same endurance and save more weight. And it may be further possible to take a little weight off the nose if the CG is moved forward because of the motor being moved one bay forward, which may allow a little more sanding and weight reduction at the tail. And perhaps the prop can be sanded to lose a little weight. And so on until you reach a failure point. In the real world the urban legend was Lockheed's Kelly Johnson offered engineers $.25 if their design work could save a pound on the SR71. The quarters were rarely paid and were highly prized.

[woundedbear]

Thanks, Mitch I was thinking maybe I could lighten the formers and the keels and make my own 1/16th x 1/16th stringers from (A) grain medium weight balsa wood sheet. In fact replacing the center and side keels the way shown in the posted photograph is a good I saw a guy holding a rubber powered free flight model airplane for some Dude who was stretch winding his plane Not for all of the tea in China would I do such a thing Now I have some friends that I would lay down my life for but holding something that could hurt me soooo bad not in a million years If that rear rubber mount were to let go, "like we use to say about a racing engine that blew out the crank through the oil pan", that kind of let en' go That rear motor mount would be like a bolt shot out of a crossbow OUCH That could be a fatal injury, I'd rather be beheaded with a hand saw That reminds me of a funny old story. There once was a guy who was deep sea fishing, hooked a big Blue Marlin, you know the ones with the long sharp bills. Anyways, It jumped into the boat and it's bill stuck this guy through the chest, right through the heart He was having a good fishing trip one second the next he was dead Can you imagine standing if front of the pearly gates and St. Peter ask "cause of death " You were what "Wait a minute" , he picks up the phone, "Jesus this you got to see this " There's some guy here...Well, you know the story. God, please don't let me die doing something stupid . When I die all I want to be is surprised, when my time comes. No whoops stuff please Whoops now there's a word for ya' Good name for a kid, "this is my boy Vinny this is our daughter Debby and you know our first son Whoops". "Yep, got married cauz'a of a' that you'ng ".

[woundedbear]

Yeah! Chirs A, I think I remember that race car . Changed the way builders built the cars for good. Can you recall a little chevy Smoke Yunick took to Indy one year? Had a custom made crank shaft turned from a single piece of Crome-Moly, brought the displacement down to 200 cubic inches, had two turbochargers . Was having a good race then something broke going into the last 10 0r 7 laps That Smoke he was an American original, one of my childhood heroes I was a strange little boy Use to get caught drawing airplanes and race cars at school. Got whooped good and hard a time or two for that Yeap! I was a weird you'ng didn't know a thing about baseball, football, or basketball. But I could tell you anything about a P-51 Mustang, or how o rebuild a simple carburetor, Ya' know go-carts and stuff. Of course, now I wish I had been a better student I am sooo deficient in math metrics and now I can see that There guys here on this forum who are good at the math metrics needed to do the modifications I want to do Live, learn, die, and forget it all, "old Scottish proverb", here's another one, "experence is dear school en' and some will learn no other". Yeap that's me alright

[Chris A.]

One of the considerations on lightening a fuselage is the leverage effect of taking weight out of the tail. By taking weight out of the entire structure to be able to use a shorter rubber motor, just moving the motor mount forward one bay not only reduces weight, but by taking it out of an area further away from the CG, it effectively leverages the weight savings effect. However for strength, the motor mount should be glued against the bulkhead and closely fitted and glued to the top and bottom stringer and keel surfaces. By not carving out the bulkhead, it stiffens a part that takes a lot of torque load when both winding and running the rubber motor and spreads the torque load throughout the structure instead of concentrating it at the mount. My rear mount consists of a hard balsa bulkhead and mount with an aluminum motor tube, not a match stick. Areas that are open and very large are a weak spot in the structure. The Bf 109D model by having the stub wing mount glued into the bottom of the fuselage closes off a large opening and strengthens the bottom of the fuselage. This is preferable with a light structure to having a one piece wing held on by magnets or rubber bands. A carefully designed and built stub section with mounts like this 109D adds very little overall weight. This is an example of good detail design engineering.

[Mitch]

Gootta go...going to Alaska...

WB remember when we stretch out the rubber and wind there is very LITTLE torque on the motor. It is only towards the end of the wind that the torque shoots up and then you MUST be carefully. At that time the motor is in the model. So if it blows at that time... well... the energy is absorbed by what is holding it.

Mitch... SYL

[glassblade]

Finally back to completing this model.

Covering is complete. Nothing interesting for the flying surfaces. I used Japanese tissue rather than the kit tissue. Jap tissue may be a little lighter than domestic tissue, but the main thing for me is that it has a smooth, shiny side that does not absorb dope and paint so readily . One coat of light dope and I am ready to airbrush the paint and have a nice smooth surface. Most domestic tissues, and that supplied with the kit, tend to take a few coats of dope to fill the grain. I have seen very nice, light finishes using this tissue, however, so this is mainly a function of my own preferences, skill, and just the inertia of habit.

This photo is of the covered fuselage. I painted some of the interior of the cockpit a light grey after covering. The covered wing center section is alongside.

Guillows vacu formed parts are unique in that they are done in a misty white colored plastic, very similar to the tissue. This means I can paint with a very light coat of paint, as the model parts are the same color and translucency.

[glassblade]

More fuselage photos.

These photos show the covered fuselage with the wing center section glued on, as well as the canopy and wing fillets.

I used velum to make the machine gun troughs, as the supplied plastic parts are too heavy and also not scale. Also used velum for the wing fillets. Like the cowl, they are also a misty white, which results in a perfect translucent surface for the paint. The 109 has very small wing fillets, which for me is a relief, as I always have a hard time doing them so they look right, and the big ones are the hardest.

I glue on the cowl and canopy with RC-56, which is great stuff. You should use nothing else for canopies. After gluing on the canopy, I add strips of tissue around the edges to help it merge in with the fuselage structure and covering. This will be worth the trouble for the appearance after painting.

[scigs30]

Woundedbear,
That is too bad the experience you had as a child, I remember having problems getting Guillows kits to fly as a kid as well. First thing, if you want a good flying plane, don't build a Guillows kit. There much better kits and plans out there that are set up for freeflight. Now that does not mean a Guillows kit wont fly out of the box, especially with the laser cut wood. I have many posts here showing that you can build a Guillows kit out of the box and enjoy 10-20 sec flights with all the supplied parts. The only thing you really have to change is don't add landing gear or modify the landing gear on the 500 series. The landing gear will break off each time unless you remove it or beef it up. I have never had a problem with the thrust button snapping back even when I use the supplied rubber. You can also modify a Guillows kit for a lighter build and modify the nose block, just depends on what you are trying to achieve. I built the same Mustang that you did and she flies great and no issues with the nose plug. Of course it would make life much easier if you add a nose block instead of the simple thrust button, the button alone will work.
http://www.balsamodels.com/phpBB/viewto ... f=2&t=1822

[glassblade]

Final parts to add to the BF 109D before painting today. The spinner, oil cooler and tail wheel. I vacuformed these parts - this spinner is more accurate than the kit supplied one, which is too rounded. Oil cooler goes under the left wing. Starting with the E model, the oil cooler moved underneath the nose. The tail wheel is turned from pink foam with a light balsa core. It is very light, which will be a problem with this model- it has a long tail moment, and will certainly turn out tail heavy.

Right now, total weight, without prop and paint, is 33.5 grams. For rubber scale models, I usually aim for a weight in grams that is 1.5 times the wingspan. For this 25" span model, that would be 37.5 grams. I'm going to go over that a ways, I can see.

[woundedbear]

Hello glassblade! I have been following your build with great interest, and have learned a lot from the photographs you have posted I am currently building a 400 series P-51-D Mustang (kit # 402) as a rubber powered free flight model, and I am applying the techniques you have described in your build. I am very familiar with the Messerschmitt Bf 109 E through the K models, the earlier models of the Bf. 109s and the (D through F) models are a bit of a mystery to me Old Willie Messerschmitt's design went through so many changes and improvements it's difficult to keep up with them all. In your last posting you said you vacuum form your own parts Can you vacuum form the parts for the old, out of production 800 series P-39 Airacobra kit ? One of the other guys here on the forum sent me a copy of the build sheet of that kit and I want to build it at some point, but whenever one of the out of production Guillow's kits like the P-39 comes up of sale eBay the prices are way too high for builders like myself. I do wish Guillow's would bring back the P-39 as a laser cut 1000 series kit with the option of building it as a P-63 Kingcobra, but once Guillow's discontinues a kit it never comes back. This has been discussed her on the forum many times. I think the P-39 was one of the neatest designs to come out during WWII and if the airplane had, had a supercharged engine it would have no doubt produced many aces during that war I wish you the best of luck with your Bf. 109 during the FACs contest and hope you do well! and a little

[Chris A.]

Keeping the weight down on single engine WWII fighter models is tough because most of the inline engine aircraft had fairly high wing loadings, the Bf 109D is no exception. Their leading edge slats went a long way to ensure low speed control, but wing loading continued to be a problem throughout the production life. However there are two versions where wing loading was addressed, the 109T1 and T2 versions for the carrier version and especially the 109 H high altitude version with stub wing extensions. The 109 T versions are very close to the 109E series except for the extended wingspan and the carrier arrester gear. Either of the T version could be made from an E kit. The H version is more like the 109F. What is interesting about the real 109 planes is just how small they really are. And of the 6,000 lbs fully loaded and armed, over 2,000 lbs of that weight was in the engine and propeller.