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  1. #1

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    jet legend F 15 diet



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    Last summer i completed my JL F 15.

    It was originaly made to fly with tailerons. I done an extensive modification of instaling ailerons and flaps, as all hiden control rods and servos.

    I equiped it with JetCat P 120, two A123 bateries, Hitec servos and jettronic air valves and sequencer.

    It also used Futaba gyro on nose wheel.

    Here is the video of a maiden.

    http://www.youtube.com/watch?v=jwUHT...layer_embedded

    Since this was my first jet, not counting some df flying, i was very pleased with the performance. Althought it flew without any trims and seems very neutral, it was evident to me that this was very serious airplane.

    The take off roll seemed longer than what i saw on videos of sutch planes.

    Also, it had to bee flown fast to feel controlable.

    Also, as you can see, the landing was a bit fast.

    It did slowed down nicely on touch down, but the final was very fast.

    I used a ½ flaps on landing, because, to be onest, my knees vere shaking and i was afraid to deploy it totaly.

    I drove quite far to fly on abendoned airport with tarmac runway.

    First trys of taxi on local grass field make me wonder should sutch plane be flown from grass runways.

    The retracts also seems flimsy when going through grass.

    For second flight, i reduced the aileron throws, made some expo on throtle curve and made a careful adjustment of wheel toe and nose wheel position.

    On second flight, the taxi run on full throtle seems stright as an arow, althought the slow taxis vere veering on one side.

    Reading a Bob Viloet sugestion for take off for jets, i taxi down the runway, turn in to the wind, aplly brakes, flaps about 1/3, and half up elevator. When the turbine reachs over 60% rev, i deployed the brakes. The taxi was stright but since i was runing slightly cross the runway, when i tried to strighten the plane down the runway, there was apsolutely no reaction to rudder output.

    After 200ft i hit one bush on one side of runway (of course the only one), with full right rudder holding, but without airplane showing any tendency to react on my input.

    I supose that the problem was that the F 15 raised its nose soon, so i lost the front wheel steering.

    The rudders was simply unefective, so the steering was imposible.

    Now for the damage...

    As the fuselage is arow like, it protrude through the bush, but the leading edge of wing suffers all the pounding.

    I ruined almost entire right wing, left wing leading edge, some minor damage to front part of fuse and after the plane roll on its back, there was a slight damage to one of vertical stab tip.

    The worst damage was on the wings.

    Hawing spent many heures adjusting and measuring everything on this plane, i must admit that the measuring of final weight of plane was not done.

    I measured all components and astimasted weight should been 11 kg.

    But, after the crash, the actual weight measurment was 12700 grams.

    It looks to me that i overshoot the manufacturers weight.

    It certainly was overweight and that might be the reason for very fast landings and generaly high flight speed. I had the impresion that it was hard to to make a bank turn during landing, because the plane would slip to a side.

    After building and flying over 100 airplanes in past 35 years, i knew that those vere the signs of heavy and maybe a little nose havy airplane.

    I spoke with my flying buddys who were ther that day, all of those was not noticeable to the spectators, except for the evident high speed of F 15.

    Since i had many problems about ordering a new pair of wings, where the shiping was more expensive than the price of wings, i started thinking about making them.

    That opened the idea of making the diet and general enlitening of airplane.

    Twin battery pack (470gr) –gone, all the cable extension holders and shirms (around 250gr) gone, starting gas equipment ( almost 500gr)...gone....but the heaviest part was the vertical stabilizer and rudders.

    Before making a new wings, i started with my idea on stabs.

    I made something like this 15 years ago as a student.

    This constuction is made of honeycomb construction from 2mm balsa.

    I mounted the two aluminium airfoils on each side wich vere used as guides during sending the honeycomb.

    I planked the whole construction with 1.5mm balsa.

    The conclusion was that this was a very lite and rigid construction. The skin integrity was great because the free area between ribs ( honeycomb elements) was mutch smaller than on classic constructions.

    When building the styro foam wings, i hate the amount og glue that is needed to atach the skin on core. In this case, even that was mutc better.

    Now, 15 years latter, with computers an laser cuts, i made this construction

    mutch faster and more precise.
    This is how we started








    I have my good frien helps me in drawing the entire construction by SolidWorks.

    The orientation of elements was sutch that as many elements reach from the stab root to tip.

    Also i carefulu incorporated the trailing edge of stab, wich holds the hinges, so it does not cutt any of those elements.

    I cutted all the elements, by lassercut, from 2mm balsa and planked it with 1.5 mm balsa.
















    The leading edge is made from two layers of balsa. First, the false leading edge , is made from 1.5 mm balsa and it helps during planking, because i simply glued the plank on it.







    After planking, i aded one 4mm leading edge.

    On trailing, i used 10 mm as leading edge of rudder and 5mm on stab trailing edge.

    >>

    I instaled the carbon tubes wich is going to hold the aluminium tubes for attaching the stabs to a fuselage.

    The squares is 30x30mm.

    I made some infills for attaching Robart hinges, thre on each rudder.

    The thin HS servo is located at the bottom at the stab.

    Arround the big opening for servo, i made one frame from 1.5mm ply, so i could distribut the loads evenly.

    >>

    Most fillets is instaled to help distribute the loads from carbon tubes to planks.

    >>

    I planked one side with ca, wich is used for all the glying, but has to use white gly on other side.

    I also made the tip forms from balsa and ply core.



    I used three hinges per rudder and i made a small block dubler for mounting the rudder horn.

    Also i made small fairing on stab for hiding the hinges from rudder and to help airflow flow over gap.



    >
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    >

    When it was finished and sended i cover it with 25gr glass cloth.







    At this time, every stab weighted around 90 grams, with hinges, but without servo. The weight of original stabs, painted, with servo, was little over 400 grams, each.

    >>

    The final measurements is still to be made but the weight loss is evident.

    This will allow me to get rid og plenty equipment i installed in nose, so i hopes it will greatly effect the final weight.

    >>

    The tosion stiffnes is very high.

    Also when you hold the stab in your hand, you cant ask yourself about how lite and rigid the constuction is.

    >>

    At the moment of writing this text, all components are glassed and sended, waiting for details and painting.

    Now im building a wings by same method.

    For the end, let me say that i did made 1.5 degrre convergent (wash out) on stabs tips.

    That means that the center line of tips airfoils of vertical stabilizers is meeting itselfs at some point in front of the airplane.

    This is common on multi engine configurations and i hopes that this will help F 15 tracks even better.

    Any opservation as comparing of other F 15 of that size is greatly appriciated.



  2. #2

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    RE: jet legend F 15 diet

    Impressive construction of the stabs. I've never seen something like that, I can only imagine how light and stiff it is. A+ job, I think you are on the way to a great flying plane here, I'll be watching!

  3. #3
    KC36330's Avatar
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    RE: jet legend F 15 diet

    impressive work!!! my JL F-15 weights in at 29 lbs and flies great, balance is center of forward wing spar, from watching your video it looks like you were more nose heavy then that. i do not use a gyro and my steering works great both on take off and landing roll. the only issue i've had with mine was the nose wheel hung up on me a couple weekends ago at an event (issue has been fixed), as you can see even in a 15-20 mph 90degree crosswind she was steerable.

    http://youtu.be/fxhHLsKXNDc


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