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Old Sep 06, 2013, 12:34 PM
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BBCC3 - 38" DeHavilland Vampire - Builderdude

Hello.
I am beginning the build thread for my BBCC3 contest entry.

My choice is the DeHavilland Vampire.
Although I have not yet decided on exactly which Marque of Vampire I'm building. I like the paint jobs best on the 2-seater versions, but I like the looks of the single seater versions the best. So it'll either end up being a Mk1 or an FB5.



The model was designed originally as a 20" Jetex free flight airplane. I am enlarging and converting it to be a 38" wingspan RC jet. I will add provisions to convert it to a small EDF, perhaps 50mm to 57mm size range; however, if I run out of time before the end of this contest, it'll fly as an RC slope glider. I'll still add the EDF, even if I don't get to it before the contest is over.

It will be a belly-lander, for grassy fields, to save weight on this build. I would like to hit a target weight of 10 oz in glider form, and naturally, a few more ounces with the EDF. It has a big wing, and as long as I don't overload it with mass, it should have excellent flying manners, and that's what I'm after.

I'm going for a 4 channel model. rudder and elevator servos will mount in the wing root with the pushrods inside each tail boom. Aileron servos will be out at the ailerons. The fuselage will clamshell in half for access to battery, receiver and EDF.

I am working on enlarging and modifying the plans now.
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Old Sep 06, 2013, 02:53 PM
UMs & parkflyers... for now.
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I'll be interested to see your approach to interior ducting and your ideas for fastening the clamshell closed. I've not built an EDF-powered plane before. My profile Harrier is my first, and I chose it partly because it had no interior duct to speak of.

In the one twin-boom plane I've built, I ended up driving only one of the rudders, which actually worked very well.
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Old Sep 06, 2013, 06:31 PM
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The key to this one is light weight. At least I'll try. I intend for this Vampire to be a park flyer, which means it cannot end up being a winged brick.

I've modified one other FF rubber plan for RC once before. It was a Curtiss P-55 Ascender, and it turned out really beautiful. However, the approach I used on it ended up causing it to be heavy. The P-55 is a canard airplane, and the front canard has a lifting airfoil. However, the problem I ran into on my first attempt at RC-conversion of a different plan was that it had a twisting instability along the fuselage. I built up the interior pretty heavily to alleviate the twisting, and some of the interior reinforcement doubled as battery tray, servo mounts, etc. And I put a VERY powerful Cermark motor in the rear pushing a 3-bladed prop, and even had a custom spinner cut for it. The wing was also built very strong. It will definitely fly, but until my skill approaches that of jet pilot, it will remain on static display.
I could have entered the P-55 into this contest, but instead decided to learn from my mistakes on a second conversion attempt...thus this Vampire.

Please forgive the diversion, but I probably should follow up with a picture of the P-55...
I will relate it to this build thread however. This was a converted Earl Stahl plan.



If I was a more skilled pilot, I'd enter this P-55 into your contest. But I'm not there yet. I calculated a stall speed of around 21 mph and a top speed of about 74 mph. And it has the power to back it up. It's just a very small airplane to be that fast, and it's going to be awhile before I can fly it. So for now, it hangs from my office.

ANYWAY, how that relates to this second attempt at a conversion is how I can build it light and eliminate the twist instability of a converted smaller, lighter plan.
This time, I am going to sheet the fuselage in 1/16" balsa. That worked on my most recent plane that had a top-heavy T-tail creating a nasty wobble in the fuselage. And had I applied that to the P-55, I wouldn't have needed all that heavy internal fuselage bracing.

The way I'll handle the twin rudders with one servo is by hiding a pair of bellcranks in the horizontal stabilizer, so as one rudder is actuated by the servo, the other rudder will be too. I used a similar design method on the P-55, but again, I learned from my mistakes. It worked but with very little rudder movement. (So the P-55 will be more of a bank and crank airplane). I will fix the idea on the Vampire this time.
The pushrod will go right down the middle of one of the tail booms, through a carbon fiber rod (for rigidity). The other tail boom will carry the pushrod for the elevator. Both of those servos will reside in the wing, inboard of the tailbooms. The ailerons will be conventional.

The way I'm going to handle the split fuselage design is using rare earth magnets to hold the removable top half in place. I've used that with good success before. I might use a secondary failsafe, like a pair of pegs into receiver holes. But using at least 6 rare earth magnets on each fuselage half should be enough.

The internal ducting for the 50 or 57mm EDF was going to be simple manila file folder paper, glued in place. However, I might decide to use 1/16" sheet balsa instead. I'll use a plastic tube rolled up for the exhaust nozzle. If I can keep it light enough, it has a LOT of wing area and should have a pretty low stall speed. That'll be my challenge this time.

I am in the process of enlarging the Jetex plans now. Then structural modifications will be done.
(I like designs that challenge me...so this is a good one).
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Old Sep 06, 2013, 06:45 PM
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Another point I'd like to touch on is the paint job.
The best paint jobs for these Vampire jets are on the two-seaters. However, I like the looks of the single seat cockpit better. That limits my choices. The Brits used mostly camouflaged paint jobs with a light eggshell belly. That would be excellent for when it's up high in the sky. However, I really loved the metallic paint job of the P-55 and would like to do something similar.
The Canadians had a really attractive paint job on their Vampire single seat jets, so I'm contemplating painting it with a Canadian paint scheme. Those are silver with orange accents. The only downside to that paint scheme is that the top and bottom look the same, so when it's up there, it might be hard to tell which side is up.

This weekend, I'll be working on enlarging my 20" wingspan plans up to 38", so that I can be at exactly 1/12 scale (1"= 1 ft). I will also think about which series of Vampire to make it, as the different marques had different fin designs.
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Old Sep 06, 2013, 07:51 PM
UMs & parkflyers... for now.
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I've had to learn to balance flightworthy versus crashworthy. I converted Mr. Stahl's Blackburn Skua and went through all the thinking about handling the engine torque and thrust forces, drag loads on a wingtip if she cartwheels on landing, building the wing to handle 8 g's positive and negative... but I'm learning the practical value of low momentum (mass x velocity). Low weight is it's own crashworthiness... and, I endorse the use of carbon fiber. I'm getting to the point where I can just focus on flying the plane... BFCR (build, fly, crash, repeat).
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Old Sep 06, 2013, 07:55 PM
The "pro" in procrastination
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Always liked the Vampire. I'll be following closely, since the Vamp is in my build bucket, and if you can pull off a small EDF version, I'll bet there will be lots of us keen to follow your lead.

Steve
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Old Sep 07, 2013, 09:22 AM
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My love for the Vampire began during the first Desert Storm war. My squadron, flying F-16s, deployed to Jordan to fly against Iraq the first time. I wanted to go but was one of a few people held back because my daughter was having surgery and the boss didn't want to send me at a time like that. So half the complement of 28 F-16 jets went to the Middle East with a vast majority of the ground support crew, and the other half stayed behind to participate in Operation Noble Eagle, which was initiated right after 9/11 attacks on the twin towers. The remaining 14 fighter jets were responsible for patrolling the skies with orders to shoot down any hijacked airliners, air planes, whatever. I was one of only 3 jet engine mechanics left on base. And oh boy were we busy! We were far busier than the mechanics that went to war.

Anyway, one hot summer day, this DeHavilland Vampire, in private ownership, was very low on fuel and got permission to land at the Air Force base. He rolled up the tarmac to the Jet Engine shop, and we secured the airplane. We then fueled him up with JP4, and in a few hours, he was ready to go again. That tiny little jet, about the size of a Cessna, used a cartridge starter and I performed ground crew duties while he started up and prepped for take off. I still remember noting the unusual high pitched whistle of its jet engine. And then the beautiful little vintage fighter jet took off and left.

I don't know why I've never thought of building a Vampire for RC and ducted fan before this. So this is a very interesting and exciting project for me.

(Yes, by the way, I am now retired from the military, a Master Sergeant with 27 years. I'm now working as an engineer).
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Old Sep 08, 2013, 08:30 PM
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Okay I finally think I got the original Jetex plans enlarged about right. It took a number of tries on the copy machine, because it uses 11x17 paper as its largest size. One problem with the copier is that there is some very slight optical distortion when blowing up an original print, so it took several attempts to get it both sized right and handle some of the distortion I was running into. I wanted 1"=1' scale, which would give me a 38" span. I ended up having to set it to 1.900:1 scaling to grow it from 20" wingspan up to 38". One version came out about 45" which was too big, but I was tempted to go with that because I really liked the size.

I found that when I scaled up the P-55 (above), it took a certain amount of weight to reinforce it for RC and E-power, along with the equipment, and growing it in size would not have changed the weight of the plane that much. In other words, if I had gone with a larger upsize, I would have lowered the wing loading so that it wouldn't need to be so fast in order to fly.
I am trying to find that right balance this time with the Vampire.

The next thing I've got to do is start adapting the plans with my modifications and create the provisions for the EDF. I might only manage to get a 50mm EDF in this one, and even then, I'll have to enlarge the exhaust nozzle a little larger...and probably the intake too. Basically, most of the design changes will be in the fuselage, although there will be several alterations along the way everywhere else too.

I have decided to build my Vampire as a Mk III. I've also picked out a couple of possible paint schemes, but I'm still thinking about what squadron to emulate mine after. I originally wanted RAF, but to get the color schemes I like, it might end up being Canadian Royal Air Force. I've got lots of time to choose a color scheme while I'm building this.

After building a few of Pat Tritle's excellent airplane kits, I've learned some of his approaches to achieving light weight while still being adequately strong. For one thing, I had to learn to quit building airplanes to crash. Instead, build them to fly, and then AVOID crashing. Sounds simple, but being an engineer, I always tend to try to over-design things. I'm convinced there is a happy medium, and I'm hoping to find it with this build.
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Old Sep 08, 2013, 08:49 PM
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Here are two of the paint schemes I am considering. They both are the same except that one of them has a Vampire head painted on the nose. If I do that, it would have to be hand-painted. I don't think I'd be able to get Callie to successfully make a decal that would fit over that kind of a curved surface. Otherwise, I like the silver with the reddish orange accents.

Another paint scheme I liked was silver with yellow accents, but that was mostly on two-seater versions in the RAF, and most of their single seat versions were camoflaged...which is okay too I guess, but I've got a thing for silver airplanes. And the silver on the vampire is not a highly polished aluminum like the USAAF liked to do sometimes. It's more of a dull silver finish, which is a lot easier to duplicate with paint.

See below...
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Old Sep 09, 2013, 04:57 AM
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Okay, a bit late now I know but it might save frustration in future. If you scan (or get scanned) the plan as a bmp you can import it into CAD scale it precisely and save as a pdf. Then you can print it at 1:1 without those annoying distortions you get with photocopies. As long as you have templates of the equipment you want to fit you can drop those over the plan and see precisely what needs altering, and by how much.

Pete
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Old Sep 09, 2013, 08:54 AM
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Thanks Pete!
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Old Sep 09, 2013, 09:07 AM
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Peter, I haven't tried the method you described to convert drawings, but what I've been doing lately when re-sizing or converting existing plans is to photograph the plan and import the photo into A-Cad as a Raster Image. Then I'll Trace the plan outlines and pertinent structural locations and then scale it to the desired size. From there I'll add the rest of the structure in the usual way. The only thing you have to be careful of when you photograph the plans is that the camera is perpendicular to the plan or it'll come out distorted. So far, so good.

PAT
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Old Sep 09, 2013, 11:38 AM
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I have got to get into the CAD scene. It would make life so much easier.

Anyway, I realized I need to make provisions for an EDF inside the fuselage, which means I need to select an EDF. This really is a shot in the dark. I'm not going for speed, but I am going to look for something with some decent thrust. I need to really design for the EDF I'm going to use.

I found one which is intriguing at a first glance. Sounds like this 7 blade, 68mm EDF (2.5" diameter) would be more than powerful enough. I don't have any exact thrust figures, because I think it depends on the motor chosen. But others have commented on the impressive thrust of this particular unit. The EDF is only $18, so I definitely should get it if I'm going to settle on this one, and then build it into the fuselage. The motor is a 4050kv inrunner for $49 (if I remember those figures right). It'll need about a 60 Watt ESC and a 3 cell lipo. I have to hook the combination up on a wattmeter to see what it would pull in current to know how long my flights could be. But being a shot in the dark, this is not a bad place to start. It turns out at 1/12th scale, there is plenty of room in the fuselage for a decent EDF.

http://www.toysonics.com/2.5inch-64m...-fan-unit.html

One thing I need to watch is the weight gain when adding the power plant.
I have one very large 2200 mah lipo that weighs a whopping 10 ounces. If my whole plane weighed 10 oz in glider form, the battery alone would double the weight. So I know I don't want to go with anything that heavy. I'll go with a smaller battery even though it might compromise flight times. If I could get a couple of minutes out of it at least, I'd probably be happy with that.

The scale of my Vampire is 1"=1'. The original has a 38 ft wingspan and 262 sq ft wing area.
So mine will have a 38" wingspan and 262 sq inches of wing area, or 1.82 square ft.
If it came in at a pound, for example, it would be 8.8 oz/sf wing loading, giving it a stall speed of 14.7 mph. I think that's very realistic for a nonpowered slope glider, and fairly reasonable. If adding an EDF and a not too heavy Lipo increases it another 10 oz total to 26 oz, then wing loading would be 14.28 oz/sf, giving it a stall speed of 18.7 mph.
I guess that's not too bad. Definitely a brisker pace. So weight is going to play a pretty big role.

What I want to do is get the EDF unit, sans motor, build the airplane, and see where I end up with for weight and then go from there. If I keep the motor as close to the CG as possible, then maybe I won't need very much battery weight in the nose as ballast, and I can keep weight down better. Having a light tail is going to really help in the total equation. I feel I really need to have carbon fiber tubes in the tail booms, but the rest of the structure can be lightweight built-up balsa, including around the carbon fiber rods to get up to scale thickness in the booms.
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Old Sep 10, 2013, 10:00 AM
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There is one of these here at Mount Hope, Hamilton, Ontario.
It is part of the Cdn War Plane Collection.
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Old Sep 10, 2013, 06:05 PM
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Yes I did see that some generations of Vampire had a sharper point to the nose. But I don't know if that's still considered a Vampire or if it is a Venom, which also has wing tip tanks. Nice looking jet though.

I am still reading up on the jet's history and different variations. In fact, I'm thinking about looking for a good book on the subject.

I haven't yet placed my order for the EDF, but I probably will pretty soon. I want to make sure I'm allowing for the outer dimensions of the EDF when I redesign the fuselage. So I'll start with the wing build first. That'll give the EDF time to come in.

This brings a point to mind though. The Jetex plans show the wing spar going right through the fuselage. Naturally, this is not possible if this is going to have an EDF in the fuselage. So I am going to need a support structure that goes AROUND the air inlet and exhaust ducts. I have something in mind that I was planning on using for the future Lockheed U2 design I am planning, so I can test that idea out on this jet first.
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