I'm building a foam flying wing type of park flyer, with a 35 size motor, and only elevators for climb and turn. Its a home made job, so I'd like some advice on where the center of gravity should be? Do you do it with an empty fuel tank? Thanks people.

If it is a conventional type plane, figure about 30 to 35% back on the wing with an empty tank for starters.

The CG has to be in front of the point where the lift of the wing is centred. This is roughly at the thickest part of the wing. If the wing is straight then it's easy to figure. If the wing is swept so the high point varies from root to tip then a bit more guesswork is needed. I'd go for maybe 5cms in front of the wing high point to begin with but be very cautious on the first flight while you figure out how it's reacting. If the CG is too far back then you're guaranteed a very interesting first flight

He said it's a flying wing.

Most flying wings have the CG set around 10-15% MAC as a starting point. That's way forward from where a conventional plane would be.

30% back still seems to make the thing too nose heavy. I'll give it a launch this weekend and see how it goes. By the way, Downunder, I'm from Adelaide too.

Montague has given you very accurate information.

For any flying wing to acheive positive pitch stability (a must) two things must be present. First CG must be further forward than a tailed aircraft and the second there must be aerodynamic reflex. The reflex can be acheived in the airfoil itself or can be done simply by rigging the elevons with an up reflex in neautral.

The amount of reflex and forward CG is directly related to the sweep of the wing. A straight wing flying plank will require the most most forward CG and up reflex and a delta will require the least.

The 10 to 15 percent starting point is indeed a good starting point for a straight wing. 30% percent will without a doubt be uncontrollable. Now I think there is some confusion about MAC (mean aerodynamic chord) going on here. On a straight wing its a no brainer 25% MAC is indeed a quarter of the way back from the leading edge but if there is any taper (leading, trailing edge, or both) or sweep then the 25% percent point is not going to be a quarter of the way back at the fuselage. There is a simple way to figure MAC graphically and someone I am sure can help you with that in the aerodynamics forum.

The last wing I did was for some Public RC shows and was simply a conventional forty size trainer with the tail wacked off and reattached without the elevator and rigged with a release mechanism the CG was move forward to around 13% and an additional servo added to the ailerons to enable elevon with the ailerons reflexed up twenty degrees. For the shows the tail was dropped on T/o and the flight was completed without. Was quite a show stopper.

thats a helluva park flyer.
not the size to be considered a 'parkflyer'.
parkflyers are usually small and electric.
if its a flying wing start at 25 % back from l.e and add 10% root chord- its always worked for me

yep

cabanestrut2002, you should see the size of the parks around here! Seriously, since I'm new, some of the terminology you guys use, such as "10% root chord", is out of my depth. I'll attach a photo of my beast later and repost it here. See ya' soon.

If you're going to fly it this weekend you'd better hope this weather clears up real soon

Where will you go to fly it?

MAC is the "mean aerodynamic chord". Basically the average chord. So if you have a tapered, swpet wing, you average it out and take 10-15% of that.

Here's a short hand way to do it (assuming constant taper and sweep, no funky curved surfaces). Take the tip and center chord measurements and average them. So a 10" root and 6" tip would give you 8". Then find the location on the wing that has an 8" chord line. That's your MAC. If you take that point on both the left and right wing. Then go back from the leading edge 10-15% and make a mark. That's your CG. Connect the marks, and you can see where the CG is at the center of the wing for easier balancing.

The proper amount of reflex will be determined experimentally. After all, we are talking about elevons. When you get it trimmed for level flight, you have the right amount of reflex.

It's very common for the first few flights of newly designed wings to be really really REALLY short ones. I've seen the instant power dive to the ground, and the immediate loop of death. The reason is that the reflex was wrong, making the pitch trim way off. Mix that with a rearward CG, and it can be really crazy. Also, even properly tuned wings tend to be really sensitive to elevator commands, so you really don't need much throw in that department.

Here's a picture of the mongrel. Hey Downunder, the weather should be clearing up on Sunday, so its gonna be make or break. Hopefully this thing flys like a bird.....and I don't mean a chicken. I'll fly it in a paddock out at Mallala and scare a few cows. But seriously, where do you think the cg. will be? The dimensions are 1100 long and 900 wide.

Never had it explained that way before. I like it cause it seems simple. Thanks Montague.

this should be about right, nice plane, looks a bit like a pbf

Wow...I reckon you're going to have a handfull there! Don't forget where the throttle stick is

But looking at it, I'd think the CG should be about where the first joint is on that top sheeting. If you're flying at Mallala then make sure you're about 5k away from the Constellation field. Doesn't worry me cos I don't fly there but they might get upset if you shoot them down. Actually, seeing you're so close to that field it might pay you to call in there and get someone to check it out. Let us know what happens

thats almost a third of the chord back! it would last all of 2 seconds with a c of g that far back
30% back is okay for a normal plane but this is a flying wing

I'll second cabanestrut2002's image, that looks about right.

If you want some other references on the CG for that kind of design, look at some of the planes on www.spadtothebone.com Even if you don't like spads, there are several variants on the PBF/DOT/StopSign type design, and you should be able to easily check those plans to get a general concept of where there CGs are.

its been a while, did it fly??