Need help with finding CG

[Batjeep22]

So i have been working with a friends and we designed a SR-72 (Dark Star). We are in final stages getting ready for flight. We need to find CG. How can we get this? There are no real life scale specs on the plan yet since it still is in production and should be done around 2025-2030. Which makes it hard to scale a model within something to compare. Ha. Is there anyone that can help figure this out?

 

[XSrcing]

Google "flying wing CG calculator" and that should get you started.

 

[Batjeep22]

I have googled for days, and haven't found my answer, so I was hoping some one in here could help.

 

[voyhager3]

If this is 100% from scratch I would say to balance it a few inches maybe behind the leading edge and do some soft tosses low to the ground. Preferably into some long grass or something to absorb the impact. This is how I’ve done it in the past for planes and wings I have created in the past. It may be brute forcing it, but otherwise I don’t know.
Best of luck, and it looks really cool!

 

[mastermalpass]

If this is 100% from scratch I would say to balance it a few inches maybe behind the leading edge and do some soft tosses low to the ground. Preferably into some long grass or something to absorb the impact. This is how I’ve done it in the past for planes and wings I have created in the past. It may be brute forcing it, but otherwise I don’t know.
Best of luck, and it looks really cool!

That's what I was gonna suggest; set up something soft for it to crash land on and do 'glide tests' until you find the best CG point.

 

[XSrcing]

That airframe is a cross between a flying wing and a plank. The CG is going to be much closer to the leading edge than the typical 1/3 of chord for a standard airframe. I would start around 10% of chord.

 

[Merv]

...How can we get this?...

I have used a Sketchup to model my planes, but any CAD will work.

With a jet like plane, look straight down on the plane make a model the silhouette. You want the CG 25% to 30% of the area of silhouette. Measure the total area, take 25% of the area, this is your goal. Pick a point and measure the area in front of your point and compare it to the goal. Move your line accordingly until you get close. I pick a line nearest my goal, using 1/4 inch increments.

With a new design I always start at 25% for a stable flight. When I get the plane trimmed out I will move the CG aft to make the plane more responsive. The location of the CG is a personal choice. If you want stability, stay at 25%. If you want a bit more twists and flips, as I do, move the CG aft. I would not recommend going beyond 30%.

 

[Piotrsko]

@quorneng suggested 25% mac and bunches of incidence or up trim over in the glider section. While I think that's a brute force method, is does have an elegance of eliminating massive stall damage. Does tend to plunk on the landing when too forward. Finding 25% mac will be tough with the taper, but doing the root might suffice. @Hai-Lee said on flying wings to pull full up and note whether it mushed or bobble stalled, bobble stall to mush transition meant it was correct.

 

[TEAJR66]

Try this cg calculator.

https://www.ecalc.ch/cgcalc.php

If that isn't what you want, I suggest you start with cg here.

 

[danskis]

I agree with TEAJR66 - Its not that much different than the F22. I'll bet the CG ends up being a little back of the leading edge..

 

[Piotrsko]

Ohhh baby an easy to use CG calculator. Does take the exciting test flight crashes out of the picture.......

 

[danskis]

If thats an EDF its going to be difficult to get enough speed to launch it. I would consider building a bungee launcher for it.

 

[DrthVdr]

http://rc-plans.com Has a CG calculator that’s I’ve used recently and worked like a charm

 

[Tench745]

Have you tried the search function on this forum yet? @FoamyDM has a darkstar he built for FF22 with a discussion on CG.
https://forum.flitetest.com/index.php?threads/sr-72-darkstar-1-20-scale-design-by-foamydm.70175/