Finding C/G of Delta
 

Anyone know how to do this?

RE: Finding C/G of Delta
 

Go 1/3 of the span out from the center rib, then balance the delta at 25% of that chord, you'll be very very close to optimum CG.

RE: Finding C/G of Delta
 

You can do it the same way as a conventional aircraft, but the tail volume is zero. In this case, you work out the aerodynamic center of the wing, and since there is no tail surface the NP is there as well. Then you move ahead from that point with your desired static margin.

I used 15% since it is generally a pretty conservative setting and keeps pitch sensitivity sane-o for first flights. Usually.. barring other factors. IIRC Lennon generally recommended 12-15% for deltas (where is that book)? Maybe it was 10-12%.. in any case, that's what experimentation is for.

Soarrich's method comes out farther back but ahead of the NP, at least on this example - the taper ratio will affect it's position relative to the calculated one, but if it looks like a typical delta you'll be pretty close! In this example, it seems to be maybe what, a 7-8% margin or so in this case? That will work and will fly more efficiently for sure, though I am not sure I would go there on test flights, that's a personal choice. Optimum position is the rearmost one that gives you the stability you can tolerate - IOW, the closer the CG is to the NP, the more efficient since the download to counteract pitching moment becomes less and less. So does your stability margin.

Here's a crude picture, plus a scan from a handy reference - all of these layouts can be worked out the same way as a conventional aircraft. Find AC(s) - locate NP - add static margin.

MJD

RE: Finding C/G of Delta
 

Ok thanks for replies. I came accross this too, what do you think?

10% infront about.

I notice this attatchment the tips are not pointed and have a cord instead. i guess you just omit this?

RE: Finding C/G of Delta
 

Pointed tips = all bad, lose them! They contribute nothing but drag and bad manners, except maybe the Buck Rogers look.

Although if you keep them, relative to the identical planform but with clipped tips the pointy one will have a slightly more rearward NP due to the increased rearward area, so I would use the same location for starters. If you wanted, you can do the workup precisely the same way, it is simply that the tip chord is zero so you plot from the LE apex to the extensions lines at the tip. Same thing as the attachment, except the extensions lines at the root are zero length.

Clipped tips are much better. Try to keep reasonable chord at the tips so the Rn is not too low. 4"? 5"? or so depending on size.

10% is fine. Unless something else gives you grief, like excessive throws or what not you ought to be just fine. Don't horse the elevator at launch! Actually don't horse anything. If you are used to aircraft like this go for it. If not, like any other model if unsure lean towards nose heavier of course. That's what uptrim is for, until you get it the way you like it.

MJD

RE: Finding C/G of Delta
 

Here is a cg calculator to make it easy.
http://www.scaleaero.com/CG_Calculator.htm
My Diamond Dust, the delta I designed and sold (Screamin Demon) and C Roundy's SlipStream all flew well 17% MAC. Good place to start.

RE: Finding C/G of Delta
 

You mean at the 17% of MAC point, right? (as opposed to a 17% static margin) That would be 8% static margin for a delta. Good to know. Pitch sensitivity was okay?

MJD

RE: Finding C/G of Delta
 

Yes. I would consider this the center of the cg location so pitch sensitivity was good.

RE: Finding C/G of Delta
 

You guys are all correct but I do it a quicker and easier way if I'm out at the field. Take the center chord + tip Chord then divide by 2= MAC (or average chord). Multiply the MAC by 15-17%. Slide the ruler out along the wing (parallel to the center chord) to where the chord = MAC and measure back your 15-17% number. I've gone back as far as 20% on plank style flying wings.

Example: Center chord is 18" and Tip chord is 9", 18+9=27, 27/2=13.5" MAC
Slide a ruler out parallel to center chord until you measure 13.5" chord.
13.5X 15% (.15)= 2.025" - we'll round down to 2"
Measure back 2" from the leading edge and do the same on both sides of the wing. Make two dots or draw a line across.

If you want to balance from the center of the airplane, take 18"-13.5"=4.5" difference
4.5"+2"= 6.5" back from center chord area.

Easy fo sheezy.

RE: Finding C/G of Delta
 

Yep - that's the macho way versus the slide-rule-geek way of doing the same thing. The way I figure it, it never hurt anyone to go through the paper exercise once or twice to develop an appreciation/understanding/clue of what it all means. These methods are simpler than they first might appear once you do it a coulpe of times. Until you get to curved profiles that is.

Sounds like there is pretty general agreement that 7-10% static margin is a well tested and functional range for these deltas. Hey, we learned something constructive on this thread, son of a gun.

MJD

RE: Finding C/G of Delta
 

I use geeky spreadsheets if I'm at home.

Actually, I was amazed out at the field the other day quite a few guys didn't know how to calculate cg on basic wing shapes. These threads are good when folks are scratch building or flying a new plane without guidance on the cg. I saved a guy 3 POUNDS OF LEAD in the nose from the wrong assumed cg spot. I've also seen quite a few Chinese arfs with an unsafe or inaccurate cg.

RE: Finding C/G of Delta
 

That observation is why I spent a bit of time explaining the little bit that I think I do know, I'm really just encouraging people to get their skit together doing the same thing a few times on their aircraft. It is a valuable learning experience, and I speak from personal experience there. Spreadsheets are great, coupled with that I'm an advocate of doing things the long division way a few times so I understand it rather than just using someone else's tools. Writing your own spreadsheet is an add-on experience.

Two books were (EDIT: are] really good resources for me - Andy Lennon's "Basics of R/C Model Aircraft Design" and Martin Simon's "Model Aircraft Aerodynamics". Good reads.

MJD

RE: Finding C/G of Delta
 

Make it a sticky for reference?

RE: Finding C/G of Delta
 

I have a pair of Laddie Mikulasko Arrow's, one a 50% electric version. Both are balanced at 5% of the MAC and fly really well. (The full size glow version is balanced with the aft mounted tank full.) Really stable but slow down well for landing and have plenty of elevator authority at lower speeds. I used Troy-RCUs method of adding root chord and tip chord and dividing by two. It's important to remember though that the true root chord is an extension of the leading and trailing edges to the centerline of the fuse, which makes it quite a bit longer than it is where the wing meets the fuse.

RE: Finding C/G of Delta
 

I have a pair of Laddie Mikulasko Arrow's, one a 50% electric version. Both are balanced at 5% of the MAC and fly really well. (The full size glow version is balanced with the aft mounted tank full.) Really stable but slow down well for landing and have plenty of elevator authority at lower speeds. I used Troy-RCUs method of adding root chord and tip chord and dividing by two. It's important to remember though that the true root chord is an extension of the leading and trailing edges to the centerline of the fuse, which makes it quite a bit longer than it is where the wing meets the fuse.
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This is true, but I think it is worthwhile to think about the significance of the area submerged in the fuselage. I tend to leave that out of the mix most of the time, which at worst locates your NP slightly rearward, but the shift would be pretty miniscule in most cases near as I can figure.

MJD

RE: Finding C/G of Delta
 

Using Troy-RCU's method I checked the CG on one of my deltas. For it to come out correct I did have to use the center line of the fuse.

RE: Finding C/G of Delta
 

That I can understand - with most of our delta designs the projected planform is pretty much the real one. I guess I was more thinking about fuselaged designs. MJD

RE: Finding C/G of Delta
 

Grow some huge balls if necessary "MJD" The 17 percent (MAC) that I spoke of is quite conservative compared to what we really do. I admit right now that we do strange things...

RE: Finding C/G of Delta
 

Got 'em thanks, but thats because they're still swollen from my New Years Day Magnum "landing".. ;). Nothing 1/2 hour of fumbling in the shop won't fix, leading edges are overrated anyways.

Moving back from 8% SM doesn't worry me, I just don't think I would start that far back is all. RSI (repetitive strain injury) on those sensitive areas makes me flinch on takeoff.

MJD

RE: Finding C/G of Delta
 

Not sure what that means, but I do know some things, When a man says what should have been done by the others quite some time ago it is all on.

RE: Finding C/G of Delta
 

I visited a field where I had little hope of a smooth landing, but I figured fly it anyways since nobody there had seen a Magnum go before. Trying to horse it to the strip - and we all know how well they respond to heavy handed control - it ended up short in a bush, and I hated to open my eyes and look, but turned out to have no more than a little bite out of the LE on the right wing.

[EDIT: For clarification, this explains my prior comment about the New Year's Day landing, which was a botch up, and likely could have been prevented by running a more rearward CG to improve glide performance. It was a virtual kick in the jewels to drive all the way there, set up in the cold and snow, fly once, and crack up the LE due to marginal glide.]

What I meant was, too many OMG test flights are hard on a guy, so I lean towards forward CG then move back until it does things the way I like. But I'm not sure what you mean :) - I don't recall telling anyone what they should have done. I mentioned the way I do it, noted the similarities to other methods mentioned, and read other people's field data with great interest. The thread was quite informative from that standpoint, I thought.

MJD

RE: Finding C/G of Delta
 

MJD,...Last time I checked, this thread is about finding the C.G. for DELTA's....... delta just means triangle. It is not a complicated shape (concept) to understand.
Magnums are NOT deltas and cannot ever behave like GOOD delta'. Your experience begs the question what the heck could you possibly be thinking ?

RE: Finding C/G of Delta
 

Clipped deltas actually, since we're nitpicking today. Deltas don't have a tip chord. But yes, indeed this thread is about finding the CG.

And this is a good point, because the first post that had nothing to do with finding CG's came from you. But I couldn't figure out what you were even trying to say. It certainly seemed pointless and sarcastic, and for the life of me I can't figure out the reason. I suppose this is to be expected however, if your record of moderator edits/deleted posts, or confused and irritated replies from regular contributing members is any indication.

The original poster here got what he was looking for - some tech info on how to work out where the CG should be, plus some field observations. Other chimed in with observations, comments, questions, and a collection of useful info resulted. I don't see where anyone didn't understand anything.

Conclusions as I see them: Some folks use the graphical method, some use rulers, both of which do exactly the same thing. Some people seem to be conservative than others with their choice of static margin, and a few folks including yourself chipped in with less conservative numbers that have worked in the field and apparently are quite manageable. That's really good stuff. MF commented about using the projected planform as being preferable, I wondered out loud about the effect of projected wing shape versus exposed wing surface, but the gist is that the projected planform is more commonly used. Lots of useful info.

MJD

RE: Finding C/G of Delta
 

Right, I know Magnums aren't deltas, but hey thanks for pointing that out. And speaking of what anyone could possibly be thinking - what on earth are you talking about? I did not mention Magnums anywhere in this thread.

And you still haven't had the courtesy to explain the meaning or intent of your prior sarcastic comments. I'm still waiting for the translation. Whenever someone asks you to explain yourself you run and hide or just start pounding your chest without saying anything - shades of SpeedCup 2008. If you do not have the integrity to explain yourself after taking blind jabs at people in public, as you have done to me here and to others, then please just find some other way to amuse yourself. I am being polite - are you able to do the same?

MJD