James c harrell

My Feedback: (18)

Can someone suggest a link to find the CG on a swept Flying wing? I am attempting to re-use a wing from a Tri-star with a front mounted engine and no canard and no tail.

combatpigg

My Feedback: (3)

http://www.geistware.com/rcmodeling/cg_super_calc.htm

James c harrell

My Feedback: (18)

I keep coming back to the tail measurements. I need to find one that calculates a flying wing (no tail).

combatpigg

My Feedback: (3)

Just put zeros in to the tail column.

In the results table, it will still tell you where the MAC is. They will tell you that the MAC chord line is X inches from the root chord [the wing's centerline in the overhead view]

The MAC line runs straight ahead, parallel to the line of flight. On a swept wing flying wing it is very important to lay this line out accurately on your wing]
Place your CG mark 15% of this line's total length back from the leading edge and this is where your flying wing will fly at as a starting point.

James c harrell

My Feedback: (18)

Thanks. I will try that. I take it from your screen-name you fly combat?

combatpigg

My Feedback: (3)

Not so much in the last few years, but I'm always ready to go. In the last few years I've tried my hand at designing and building flying wings for speed.

The novelty of seeing them zipping around never wears off on me.

Lnewqban

Some additional links here:

http://www.rcuniverse.com/forum/fb.asp?m=8708615

Considering adding some reflex close to the tips.

James c harrell

My Feedback: (18)

I love the look of a flying wing and kind of want some speed out of one as well. While this one is still in the makes, I hope it will provide a little speed. It has a K&B 40 and as you see a pipe. Think she has a chance?

Jezmo

Mr. Harrell, one thing you might want to look into is the tuning of that pipe. It's hard to tell from the pic but the header looks a little long for all out speed. Again it's hard to tell from a photo but in case you decide to check it out there are tuning instructions on the Macs pipe website.

James c harrell

My Feedback: (18)

That too will be a learning expierience. Never ran one befor but I have ten different pipes different lengths and such and have been doing some reading and research on them. The one on it now turns 12500 w/ a 10-6. I could get more with a shorter pipe but it is the longest thing I have and it is where I wanted to base the length of the fuse. If it fits the short ones will too. I really don't want to start the flights of this thing at max speed any way. Kinda want to build up a little at a time. I will look at the Mac site for some more info though. Thanks

combatpigg

My Feedback: (3)

That should be a cool looking bird when you get done. If the engine is happy with that pipe and prop combination, it should give you gobs of low speed thrust for trustworthy launches while you are getting the plane dialed in..

Think of the pipe and the prop size as a matched set. When you start looking for more rpm, the prop diameter gets shorter and so does the length of the exhaust system. It is good to know what the engine's reliable upper limit is for RPM and to also know what a realistic prop diameter is to match the drag of your airframe. The plane you show looks like it will perform well with a 8x8 prop if the engine can turn it at least 17,000 ground rpm. As a bench mark reference, a OS .32 with Macs pipe will turn a APC 8x8 about 17,500 or higher.
If the K&B .40 can't be tuned to deliver that, then you might try a 9x6 or 9x7 and evaluate how solid your launches feel and how you like the all around performance. No set up for sport should be so highly stressed that the engines lands too hot to touch. The engine should be cool enough to handle after making your deadstick landings.

James c harrell

My Feedback: (18)

I hear you on the prop sizes. I just tried my combat SPAD that has a .32 O.S. with a 8-8 prop. I had been running a 10-6 but the 8-8 really lit her up. I haven't put the tach to it yet but the test flight was a "hang on" type of ride. Any faster and I think it is going to come apart. Best thing is there is no bog after a tight turn>

combatpigg

My Feedback: (3)

The OS.32 runs so good with a Mac's pipe as it comes out of the box that I've never been tempted to mess with the porting. It's just too good at what it was designed to do. I ran it a couple years in a row against Nelson .36s in AMA control line combat and it was as competitive as running a FOX .36 would have been.

James c harrell

My Feedback: (18)

I too have been impressed with mine. Given to me from a friend who crashed his twin, I replaced my GMS with it and it runs with the guys who slip in the 40's. I run the stock exhaust on mine but if it comes apart again I am gonna get Mac for it.

combatpigg

My Feedback: (3)

By saying "Macs Pipe", I mean the tuned pipe without the silencer....just in case there's any confusion.
The black, one piece pipe they sell is just an un-tuned muffler with no performance gain over stock.
Unless you're going for every last tenth of a mph or just like the sweet music, the cost of a full tuned system is questionable.
IIRC, the OS.32 / 8x8 prop went from 16,500 stock to 17,500 with the pipe. The actual mph difference in the air might make you wonder if Ben Franklin's quote about certain people and their money has some truth to it.
The engines that make the most sense to pipe for speed are usually rear exhaust. Otherwise Jett Mfg. makes a real neat replacement muffler that does provide a tuned pipe effect for side exhaust engines.
Back to your plane, the old saying of "keep the aft end light" gets flipped around with a flying wing and you might be looking for ways to get the front end lighter. The sooner you do a trial fit of all your gear to get an idea how the CG will work out, the better. The cleanest solution is to move the engine back if it turns out the plane is too nose heavy.

rcguy59

My Feedback: (8)

Hey C.P.! What's your opinion on washout on a swept flying wing? I'm currently working on a Joe Saitta 82" Me-163 Komet. I've decided to go foam-core on the wings and can order them with or without washout. The airplane will be turbine powered, which will put it in the 140+ mph range. I've got some own-design 1/2A swept flying wings that do just fine without washout, But the Komet calls for 3 degrees of twist. What gives?

James c harrell

My Feedback: (18)

All my rough set-ups with gear show that it is going to be a little heavy in the nose. I am working on how to fix this without heavy mods. This is another reason I wanted to know what the exact formula for the CG was. I did the MAC and figured out a rough guess as to the CG from there and I have some work to do. I hate to think the firewall will have to be moved but it may be inevitable. Battery all the way back and moveing the throttle servo back may help. What would the % from Mac for the CG be at the max for you? Never mind the last question. I just went back and reviewed your earlier post. 15%.

combatpigg

My Feedback: (3)

You might get away with 20%, but I think that is the aft limit. The lighter you can keep the model, the less critical the CG...or in other words the wider the flyable CG range is. The aft limit is where I like to fly them at and I do not use very big throws. The wing doesn't have to work as hard with a rearward CG and can devote all it's attention to going fast.
None of my wings fly with any measurable up trim [reflex] needed.
Moving the engine back and keeping the thrust angles 0-0 is pretty easy if you bolt the engine/firewall to a flat plate [cheap plastic cutting board] and situate the rest of the model so that it is plumb and square every which way you check it off your building table. Any left thrust is a very bad thing for flying wings, so always err on the side of right thrust if there is any question about where absolute ZERO lies. It's still a juggling act to do a perfect re-glue job of the firewall, but this method has worked for me while using a quickly rigged holding fixture to steady the plane until the glue dries.

The model in tonight's photo has been through some rough times and this is a good excuse to check it while doing some repairs.

combatpigg

My Feedback: (3)

Good evening rcguy.....those are neat looking little birdies..!
I can't answer your question about the Komet, that size and weight is out of my league.
If you build the plane light enough you shouldn't have to resort to that...but this model might end up benefitting from it. It would be nice if you could get input from guys who have flown this plane with a similar RTF weight.

rcguy59

My Feedback: (8)

Thanks, C.P., I'm flattered that you like them. I thought you only liked the really extreme stuff. The smaller one has a Norvel .074 and weighs 14.5 oz. dry. The other has an OS .15CVA and weighs just under 30 oz. dry. Fast enough to be fun, and nimble as hell. The reason I asked about washout on the Komet is because I have a smaller one (.46 size) that flies fine with no washout at all. The full-size one had 7 degrees of washout. Quite a range. I thought you might know which was better for higher speeds, less washout or more. Any thoughts?

James c harrell

My Feedback: (18)

Maybe I am staying up a little to late on this thing but let me make sure I am doing this right. I measured out the MAC and end up with a line about half way of the wing. Line is paralell to the fuse. I measured 15% back down that line from the L.E. and the same on the other wing and there is the CG. If I got this right I am nose heavy by about an inch. The firewall is already at the wing so if I go any further I will have to cut into the wing. Think I will do some re-arrangeing and see if I can get it right.

rcguy59

My Feedback: (8)

Assuming your MAC figure is correct, yes, you've laid it out correctly. Both of the flying wings in the pic I posted were nose-heavy even with the firewall so close to the LE of the wing. The RX batteries are as far aft as I could get them and those huge vertical fins are solid balsa sheet, all in an effort to get the CG where it belonged. Sometimes you've got to get creative on balancing these things. Good luck.

James c harrell

My Feedback: (18)

I am getting as creative as I know how. I just weighed this thing and I am at 3.5 lbs now. I think I need to re-evaluate things a little!!

rcguy59

My Feedback: (8)

A little heavy isn't always a disaster, it just means the airplane will do everything a bit faster. Finding a way of getting your fuel tank on the CG will help your cause as well as minimizing trim changes in-flight. A pump or crankcase pressure can make that possible. Also, splitting the RX battery sometimes makes the two halves small enough to fit farther aft, where a one-piece pack won't. Is it too late to lengthen the fuselage? It wouldn't add much drag at all and would provide a home farther aft for the battery. I place my elevon servos about mid-span as close to the hinge line as possible, again to get the mass of the servos aft. This also reduces the risk of flutter vs. having the control horn at the elevon root, which invites the elevon to twist.

combatpigg

My Feedback: (3)

If it's any consolation, I've had to move a few firewalls back and the more you practice...the better you get at it.........

I'll bet the red plane in my earlier post needed that done. That's what razor saws are for, fine tuning model designs.