ddemarco

Hi everyone!

I'm new to the forum, I'm 37 and from Buenos Aires, Argentina. I'm almost finished with the building of a Top-Flite 63" Spitfire Mk IX. I've been working on it on and off for about a year, and I'm now very close to finishing it. I went with flaps and retracts (Robart 605 & Robostruts). The build itself was very interesting, sometimes challenging, but I'd say that it was very pleasing overall. I only used Monokote on the control surfaces, and glassed the rest. I'm very happy with the result after painting, and the cockpit interior kit is a very nice touch.

This being my second scale plane, and my first warbird (the previous kit was a J3 Piper Cub), it turns out I should have paid more attention to the CG, and specifically to the tail weight. Here are the facts before I do any adjustments:
1) Engine: 4-stroke OS .81FS alpha.
2) Retracts: Robart 605+Robostruts.
3) Glassed and spray-painted (primed first).
4) Cockpit interior kit (negligible weight, tough)
5) No special modifications, other than routing two LEDs to the wingtips, one near the antenna mast, and another one on the tip of the rudder.
6) Current weight without adjustments: 4.955 grams (10.92 pounds)
7) Weight that needs to be added to the nose to balance: 750 grams (1.65 pounds).
8) Total weight after balancing: 5705 grams (12.57 pounds)

So I evidently got carried away with the painting (primer + spray paint + clear spray coating). And I should have eyed the CG closer during the build. Sadly, I haven't...

I think I can get it down to 12 pounds with litthe effort by:
a) Moving the battery pack to the engine compartment.
b) Moving all servos forward as much as possible.
c) Changing the 6 standard servos (45 grams each) to micro-servos (12.5 grams each)

If necessary, I could also:
d) Move the air tank forward, replacing it with two smaller tanks if necessary.
e) Rebuild the control surfaces, especially rudder and elevators, making them as light as possible.

Does anyone have experience with this issue? I've seen other threads, but can't see anyone with a 12.5 pounds Spitfire.
Would 12 pounds be enough? Should I cut more weight?
Do you think an OS .81 FS alpha would be enough if I stayed on 12 pounds?

Any help with this will be very appreciated; I'll uploadpictures of the Spitfire as soon as I can.

Cheers,

Diego

All Day Dan

My Feedback: (5)

Diego, Diego, At 12 pounds, your wing loading will be 40 ounces/ square foot. That is very heavy for a plane that size plane. It will certainly fly all right but you will have to keep your speed up during landing. My planes at that size ran around 30 ounces/ square foot. Since I did not have flaps, I always landed with plenty of speed. Maybe someone will see this thread who has built that plane and knows more about it than I do will respond. Sounds like a great project. Keep us posted. Dan.

ddemarco

Thanks Dan,

I've seen in various reviews that the wing loading is supposed to be around 35 ounces/square foot. You're right that I would be at 40, so that's an additional 15%. As you mention, input from other people who have built their TF Spitfires might help here. In the meantime, I'll do several things:
1) Remove all servo trays & servos
2) Remove wing servos
3) Remove metal pushrods
4) Try to re-balance moving the battery pack to the engine compartment.

Then I'll measure everything I've taken off (servo trays, servos, pushrods) to see if there's anything I can improve on that side (for sure, I can save some 200 grams (7 ounces) by changing to micro-servos, which would be a minor change. But I'm starting to suspect that I will need to re-do the elevator and rudder. I can try removing one of them to see if it's worth the try. If all three surfaces are 100 grams or more (3.5 ounces), it may be worth trying to re-do them and maybe save 50% of the weight, as it would save a total of 200 grams (7 ounces): 50 grams saved on the surfaces themselves, plus 3 times that on dead weight on the nose.

Thanks again,

Diego

DavidAgar

My Feedback: (108)

I would not change the servo's from standard to micro's. They may not have enough horse power for your flight controls. I would move everything forward as far as possible and work from there. Your wing servo's are real close to being on the CG, so I would not remove them. Once again I would not alter any of the stock pushrods with anything that is not as solid. You do not want any flex on your pushrods as that will equate to flutter on yoour flight surfaces. As I said, I would move everything as far forward as possible to cut down on the ammount of weight in the nose. The Top Flite kits all seem to build out a little heavy, but they a great flyers. As mentioned you will need to watch your airspeed for take off and landing. Good Luck, Dave

ForcesR

Hello Diego; as wasmentioned, your wing loading is very high for the size of the wing area. The TF Spitfire that weighs 12+ lbs will be a flying brick with wings. Most who have built the TF Spitfire kit have tried to keep the weight downfrom 10 to 11lbs, still a bit heavy but manageable, flight wise.

I was fortunate to have read two other build threads on the TF Spitfire before I started my build and from those two threads, I knew that the kit built tail heavy. I reduced as much tail weight as possible without jeopardizing structural integrity of the kit. I have pneumatic actuated Robart LG, 605 struts, aluminum spinner, Saito FG-20, 4 cycle gas engine and the Spitfire is covered in Monokote. The weight came in at 9lbs, but to get the recommended COG, I still had to add 8ozs ofleadto the nose; all up weight is 9.5lbs.
It will not be easy to reduce the weight of the plane since your build is already completed.I wish you good luck in finding a way to reduce the weight of your Spitfire. In your case, the ideal weightto shoot for, in my opinion is 11lbs.

<o></o>

Roger

ddemarco

Thank you Dave and Roger,

Tommy_Gun

Install a slightly heavier more powerful four stroke. Mine did fine at around 12 lbs.
[youtube]http://www.youtube.com/watch?v=VmpaROieHck[/youtube]

ForcesR

Diego, there is little you can actually do to lighten the wing, as you know from building the wing; itis quite robust but fairly light anyway. The fuselage is where you need to concentrate your efforts to reduce weight.When I get homeafter work, I willweigh the wing and fuselage separately and post the weights tomorrow. This shouldgive you a starting point so you can formulatea plan to tackle your weight issue.<o></o>

Roger

Shaun Evans

Diego,

RELAX! I'm sure I'll get some push-back on this from others, but I think you're overreacting to the problem. I did not build mine, but I finished it after buying the built kit from a customer when I worked at a hobby shop. I used it as my glassing tutorial, and added a ton of weight because up to then, I'd only glassed all-foam airframes which don't absorb the resin like balsa does. I used Spring-Air gear with Robart struts and in compliance with the U.S. law that said you had to overpower everything, I shoe-horned a Thunder Tiger 120 2-stroke in it. I never weighed it, but it felt VERY heavy when compared with my other planes of that size. I didn't have to add much weight to the nose because of the bigger engine, but what little I did add, I added it to the cowl itself in the form of #8 lead shot mixed with 30-minute epoxy that I poured into the chin, directly behind the spinner. This was a fiberglass cowl from Fiberglass Specialties (if I recall correctly). Again, I never weighed it, but I was not at all conscientious about weight when I built it because I didn't really know any better at the time.

The airplane flew FINE. It was fast, and it landed a little hotter than my 60-size Mustangs (Top Flite and Model-Tech), but it flew great. It did not fly like a lead sled or a brick with wings. It flew like a slightly heavy, GREAT FLYING warbird.

If you do decide to make mods and rebuild things, I'm sure it won't hurt, but I also think it's more trouble than it's worth if the solution is far greater than the problem it's solving. Just be prepared to carry a bit more power on final and use your flaps. You'll love flying it!

john josey

I could not agree more.

I have built many spitfires over years Diego and they have all been on the heavy side. If you keep this in mind for future spit builds that for every extra oz at the tail you will need 6oz at the nose. Finish your beautifull model and go and enjoy it. Of coarse it wont float in on landing, but it will be absolutly fine to fly.

Go and have a ball with it and let us know how it goes, john

ddemarco

Tommy,

Is that your Spitfire in the video? I'm asking because it clarly says "10.5 punds".

Cheers,
Diego

ddemarco

Hi John and YellowAircraft,

Thanks for your comments. Indeed, I think I'll just have a cold beer, cool off, and rethink the problem. I will move as much as possible to the front (which is just a few hours of relatively simple work), rebalance, and then go flying. I'll keep you posted on the progress I make, and the final weight before I go flying.

I won't be able to fly next weekend because I'm attending the final of the World Gliding Championship, which is hosted here un Buenos Aires for the first time, some 400Km from where I live. (see http://www.wgc2012.com.ar) I'll try to do as much as possible before I get there, and then hopefully go flying the weekend after. I'll keep you all posted.

Cheers,
Diego

ddemarco

I would really appreciate that, Roger. Thank you!

Cheers,
Diego

Me410

I built the same plane several years ago, but don't recall the exact wieght, around eleven pounds i think, give or take alittle.

I did have to add a fair bit of lead and what I did was build a plywood plate that was epoxied to the firewall /box and it ran forward and alongside the engine. The engine I used was a OS 91 4s,it flew it great. I was able to attach the lead to the most forward part of this plate, where I would need as little as possible and it was firmly attached. Putting lead right on the cowl was not an option as I used the supplied plastic one that would not tolerate the weight. It would crack under vibration.

You could also use a heavy hub that would hold your prop and may fit under your spinner, depending on what spinner you use? If you use a spinner with a center attaching nut, you could have the heavy hub drilled and tapped to accept that bolt.

You can use a light foam tailwheel to save wieght at the back.

Having had many 60 size warbirds, some heavy, I think I would not start rebuilding tail/wings/ etc.
just do what you can to move the battery forward, servos forward, etc. The air tank is light so I would not worry about that part.

Make sure your CG is not at the rear limit, better at the forward limit for a heavy plane.
And be very gentle on the controls, but keep yor speed up.

Good luck

ddemarco

Here are a few pics I've just taken. The canopy is not glued, as I'll leave it out until all is well, so it doesn't get scratched. One of the pictures shows how I hid the on/off switch inside one of the radiators. All valves (fuel, air, and needle) are flush with the side of the fuse. I haven't glued the command stick inside the cockpit yet.

jet22b

Hello Diego;
Come over to the Spitfire Brotherhood thread here on RCU. There you will find a lot of help and info on this build.
Here is the link to this thread below!!
http://www.rcuniverse.com/forum/m_10880219/tm.htm
Sonny
aka
jet22b

jet22b

Diego;
You did a great job on your build!!! Like I said before, come over to the Spitfire Brotherhood and join us, We will be happy to have you with us. Right now we have over 150 brothers!!!
Sonny
aka
jet22b

ddemarco

Right away!

flycatch

My Feedback: (26)

Sounds more like an ESM ARF. Nothing but a lead sled.

sensei

First let me say you have done a beautiful job on building and finishing this airplane, what seams to be constant across the board is that everyone that builds this airplane, heavier or lighter still need to add some nose weight after completion. All this tells me is that the kit manufacturer missed the proper wing location in the first place for the engineered material kitted to build this airplane. I mean really, would you rather add lead or a larger more expensive power plant to your airplane while upping the wing loading, or would you rather fly it with the wing placement slightly aft or a slightly longer nose and a lower wing loading? Do the plans or instructions state that this platform will come out tail heavy when completed, or offer any guidance to reduce the weight aft of the C/G during the build? Finally, if you knew that this was going to come out so tail heavy; would you have purchased and built it in the first place? just curious.

Bob

Tommy_Gun

Yes it is mine. The weight stated in the video was a typographical error. It was actually 12.5# . As you can see it flies great.

Tony Iannucelli

My Feedback: (193)

Your building skills are excellent. The plane looks very very nice. Mine weighs a bit under 10 pounds and flies well. I used Robart struts initially with Hobby King electric retracts. I kept the retracts which work perfectly (are lighter that mechanicals), but took off the struts in favor of wire struts and gear doors which look better and are lighter.

Here's a few dare I say radical ideas, contrary to what others may offer.

The larger engine route is one way to go. I put a 120AX two stroke in a TF P-47 and that did the trick. The P-47 and Spitfire land about the same, with the ground handling in favor of the P-47. Use a 16-6 prop to help slow down the landings. You might only pick up a couple of ounces this way and it might not be worth the effort. You'll have to deal with more torque, more vibration, and other side effects. But two other ideas might help.

I use fender washers screwed to the firewall with sheet metal screws for balance. One biplane I had had stacks of five washers in several spots. As I became familiar with the plane's handling, I took 2-4 washers off at a time since the manual stated the balance point on the safe side and I wanted the bipe to be more neutral. Works great. I then put a spot of CA on the screws to hold them to the firewall permanently. You add weight with this one, but it's not permanent weight unless it has to be. Lighter wing loadings of course are the goal.

Another assist in balance is to BALANCE THE PLANE WITH 1/2 tank of FUEL IN IT. I know this is NOT the way to do it, but you are using a four stroke. If the plane is flown with a 14-16 ounce tank, you will get 7-8 minutes when the tank is full... and can land with a half tank. I've done this several times and it works very well.

Don't add UNNECESSARY WEIGHT unless you have to. And in the future, just a thought, only use epoxy on the firewall and the main spar, and use it lightly. If you build to withstand a crash, you will get to test the theory. The tail section only requires CA on the joints. The manuals and other fliers are wrong if they tell you to gob on the epoxy. Those stabilizers aren't going anywhere. Epoxy is completely unnecessary. One ounce in the tail needs three-four ounces in the nose to balance out. Yikes!

Your plane is magnificent. Best of luck with the maiden and subsequent flights. -Tony

sensei

It looks like it flies great as long as your on the pipe, but it also looks like it is a handful when it slows down and unfortunately that is when they get torn up.

Bob

sensei

+ 1

Bob

SCALECRAFT

My Feedback: (13)

All my scale warplanes never equal the factory weight. Our own designs, I even build heavy, yet my friends can build our designs light.

But they all come back safely when I fly them if the CG is right.

Why? I'm not that good a pilot but I know that they are heavy and I keep up the speed. And I know that CG is more important than being light.

I have seen countless light models, from trainers to warplanes, flip over and go into the runway because CG was off. Even with some speed. The fear of adding weight to the nose is a mistake. Keep up the speed with proper CG and your ok.


Also don't fly it like it was a Cub. It will need speed to stay flying. Slope soar it to the ground the first few time. And practice, practice......................practice.

Steve