Tommy_Gun

Need to be careful going too far forward with the CG.
l have test flown planes balanced in this manner that were ill handling excessively fast landing lead sleds.
With the CG set beyond the forward limit, power reduction and the drop in airspeed that comes with it will require a larger up elevator deflection to control the rate ol descent.
This will lead to higher approach speeds on final and the possibility of forcing a stall or running out of adequate up elevator to arrest the sink rate before touch down.

SteveC68

One of the problems with the Top Flite Spitfire is that the tail plane is 35% oversize compared to scale. Specifically the horizontal stabs, but the vertical is also over sized. Not sure why Top Flite chose to enlarge the tail surfaces so much, but they did so what you end up with is a kit that is very easy to build excess weight in the tail.

sensei

Sounds like there are many improvements that can be made to TF Spitfire kit while in the building process. There is allot more planning and detailed work involved in building a truly lightweight high performance airplane, and when properly setup, will out perform its heavier counterpart in all aspects of flight, even in high or gusty wind. Heavier does not fly better, especially when they are smaller platforms like this one, Reynolds numbers and cubed loading are both working against you the smaller you go and there is no changing that.

Bob

Tony Iannucelli

My Feedback: (193)

Light flies better, always. In my opinion it doesn't have anything to do with how it 'feels' in the air or on the sticks. It's physics. With most ARFs and kits you can hit the high end of the given weight range with gobs of epoxy, full cockpits with pilots, Robart wheels instead of the modern lighter wheels, bigger fuel tanks, extra paint, decals, etc. You can hit the lower end if you are careful with all of that stuff. The lighter (relative) weight will definitely be faster, land easier, climb further, roll faster, etc. ALWAYS. Assuming you use the same and proper power plant. If your plane does come in heavy, use a lower pitch prop to manage the landings better. Ditto on balance. Get it on the money, lighter or heavy doesn't matter on balance, just BE balanced.

OldScaleGuy

My Feedback: (2)

Light weight or a heavy weight, it has to be balanced to fly properly. If it is balanced and every thing is right, you will have a plane that will fly.

sensei

Obtaining a proper C/G is a given and never in question in my book, but truthfully you can place enough power on an aerodynamic brick and make it fly, but fly good in all aspects is a very subjective argument and I guess it really depends on what measuring stick your using...

Bob

ddemarco

Hi guys,

I'm sorry I haven't been very active this weekend, but as I told you, I travelled 450 Km to attend the final of the Wortld Glider Championship here in Buenos Aires. I know it's off-topic, and that a glider is hardly anything like a warbird, but I can't resist sharing a few pictures with you (the two kids in the last are my boys). It's been simply amazing, more than one hundred and twenty gliders on the same runway, one being towed away every 20 seconds (yes, all of them were together in the air in an hour).

Seventeen planes used to tow these lightweight beauties, and more than 5 hours of flying until the final was complete. Proudly, two Argentineans won two of the categories (3 in total). This is probably not going to happen again in my country any time soon, so I've been very fortunate to attend. A friend of mine took much better pictures, (including some amazing landings). So if you want me to post more, just let me know.

And now, back to the spitfire. I'll continue the work and keep you posted for more of the wonderful help you've been giving me so far.

Cheers,

ddemarco

Back to the Spitfire, I did two things:
- Weighted the long iron pushrods.
- Removed an elevator to weight it.

It turns out that replacing the pushrods for plastic ones can save some 60 grams (a little over 2 ounces). On the other hand, the elevator weighted 30 grams (1 ounce), and I'm quite sure I can rebuild it below 15 grams. Assuming I can (seems quite easy), that would save some 120 grams (a little over 4 ounces).

Doing these two things would save a total of 180 grams (4.5 ounces). It's not really a lot of work (replacing the pushrods is trivial in terms of effort), and I'll try building a "light" elevator shortly. If that works, I'll go ahead with that, and do the same with the rudder, which is much heavier, and that would get me close to 12 punds.

Finally, I'm still carefully considering replacing 5 servos with microservos (all but the flaps, which I think may need the extra strength). I'm currently using regular 3.3 Kg (116 ounces) servos, and would like to switch to 2.2 Kg (77 ounces) microservos. It seems to me that it's worth it, as I would save 170 grams (7 ounces).

Applying these to changes would produce a total reduction of 11-13 ounces, and I'd probably stop there, with a total"all up" weight around 11.6 pounds.

Finally, a very debatable thought, I'd like your input about: I have access to CNC machining, relatively inexpensive. I've figured that moving some of that weight to the spinner would also help reduce the weight. How do you feel about that? Has any of you used spinner weight? Does it make any sense that I build a 500 grams spinner weight instead of placing 550 grams of it within the cowl, which would also require building a special mount very close to the front end? I can certainly make this weight "close to perfection" in terms of it being balanced, thanks to the ultra-high precision of the CNC machine, but I wonder if there's any other drawback. I'm attaching two views of an example of how such a weight could look.

Thank you all!

Shaun Evans

Diego,

You have beautiful kids! I just got my 7-year-old his first trainer, and he's not doing to badly with it on the cord. We're putting the TF Mustang together as a father/son project, too. As for your Spit, I still think the solutions are far greater than the problem!

BobH

My Feedback: (2)

Spinner weights are fine so long as they are properly ballanced. One ounce at the tail translates to about 3 ozs in the nose for CG ballance.

Reading this thread its clear that some people don't understand Nose to Tail weight ratios. Its pretty simple. The CG is a fulcrum or seesaw. If one side (tail) of the folcrum is say 3 times longer than the other side (the nose) you need 3x's the wight there to make it ballance in the fulcurm point or CG.

So any calculation of how much weight you might need to add or save can be done with a ruler to establish your ratios.

Shaun Evans

Hi,

Also, the further forward you can get that weight, the less of it you'll need.

ddemarco

Thank you YellowAircraft, Indeed, I think that's every father's dream, to be able to go on a project together with their kids. Mine are 6 and 4, but they're eager to work with me. I already let them do some small stuff in the Spitfire. They're convinced they took care of the painting .

Cheers,

ddemarco

Correct, and that's how I did the previous calculations. In fact, I added 20 grams to the tail (exactly where the hinges of the elevator are located, and found that the exact factor is 3.2. But if I can move into the spinner, that becomes almost 3.5, so that's better. I haven't ever seen a 500 grams (one pound) spinner-weight, so I'm worried that I might be creating a monster here...

Cheers,

BobH

My Feedback: (2)

You have a bearing that supports the crank shaft at the front. One thing you dont want to do is to over load that bearing with a too heavy, not perfectly ballanced spinner. Better to put weight some where on the firewall or cowl..

The plane will fly even if its heavy. Now you might not like the way it flies but thats another issue. I suppose you can always keep it or sell it after that..

glazier808

My Feedback: (1)

+1

john josey

I'm assuming you are referring to the post i made with the 1:6 ratio ? The actual ratio on my DB spit is 5. something, i cant remember the exact size. I just rounded it up to make the point about huge savings at the front with small savings at the rear.

I have noticed from pictures of the TF spit that they seemed to have shortened the tail and lenthened the nose to reduse the amount of ballast needed to balance on the this scale of spit. This is obviously where you guy's are getting a ratio of 1:3 or thereabouts.
Here is a picture of the DB spit from a better angle.

bigtim

My Feedback: (13)

when I balanced my plane I used a 3" wide X 3' long Lead strip that was scrap from when I built a X-Ray room for a chiropractor, I just folded the strip until I got close to the needed weight then pounded the chunk flat then I used extra long engine bolts to secure the weight to the engine mount as far forward as possible its a sizable piece of lead I used a torch to melt the edges together,lead shot in epoxy would also work great,just make a box line it with cellophane and poor the epoxy and shot in .

what I did to balance the plane and get the required numbers was to use fishing weights in a plastic bag hooked over the back plate of the spinner, and kept adding until the proper alignment was achieved, I then used the numbers from the weights as a benchmark, but double checked my weight on a small postal scale to be sure of the exact weight approx 19oz I did need to add a little extra weight but it was minor.

alanc

Machining a spinner weight is an excellant idea, something i do all the time, i use brass, and dont go beyond 40mm, then taper it down, or step it down toward the spinner front, i also drill and tap the end for the spinner bolt, it is imperative to get the thing running true on the shaft, i have not yet done a design for multibolt hubs, but will get around doing this one day, mine have been for single bolt prop drivers, and have had great success, as mentioned, if they are not running dead true, the bearings will suffer, i am a high precison engineer, i have access, like you, to all sorts of machine tools, not something everyone has a chance to use, but,they work, and have done for me for years,

you also mention the weight of your pushrods, try the pull pull for the rudder, its more accurate than a snake, and is really easy to do,and lighter even, than a snake, i have also used pul pul for elevators, these are now included in the kits for YT/ESM models, they work, are slop free, and lets face it, full size use cables and pullys, dont they?

lastly, i dont like the idea of using mini servos, its a 12lb warbird, it needs standard ones, little gears have little teeth, use decent servos, moved forward on the CG,

ddemarco

Hi alanc,
Thank you, this is quite interesting. Is there a practical limit you would suggest for an OS .81FS alpha? I'm hesitant to go all the way to the 500 grams I'd need. What's the heaviest you've used?

Cheers,

alanc

to be honest, i never weighed them, just took metal off till the thing balanced, ive made all shapes,

BobH

My Feedback: (2)

I didn't have anyone specific in mind when I wrote that but some people seem confused about how the aircraft length and CG relate to one another.

1:6 is a lot. But it is what it is..

john josey

It is a lot bob, and i will say that my own measurements were from the 'back' of the spinner to the back of the rudder as that was my usuable area as spinner weights make me nervous.

BobH

My Feedback: (2)

John, did you ever get it flying? And did it fly ok?

john josey

I've had a dozen or so flights with it Bob and it flys superb and the landings are super sweet even with no flap and that 2lb of lead at the front of the cowl. It could do with a little more power than the Zenoah G38 has though.

carlbecker

My Feedback: (3)

The only warbird I have built that was nose heavy is the Bucker Jungmeister. It still came out heavy and I was very surprised about the CG. Make the tail as light as possible, pull-pull helps, servos forward and I sometimes put the battery next to the engine. Heavy props like APC can help. I don't use prop weights anymore as I am not sure they are easy on the bearings. I use epoxy on the tail only for fiberglassing, Ca or white glue works for the wood.