Sep 08, 2014, 01:11 PM Registered User United States, MA, Middleboro Joined Jul 2007 605 Posts Actually this is one question I know the answer for... Sorry to tell you but your 6.4 G figure is far too low. Because of the speeds and tight turn radius our models can achieve the actual limit you want to use for structural stress should be 30-40 G's. We have had this discussion regarding G forces several times on another message board, and engineers have repeatedly shown us figures that reach or exceed these ranges, even using trainers as examples. I have folded several flying wings where I used 30 G's as the design limit, twice when pulling out of shallow dives I would not have thought I was even close to this limit. Latest blog entry: The third time is a charm!
 Sep 09, 2014, 11:18 AM InceCreations Joined Jul 2009 732 Posts My God, y're kidding ! things to consider .. Let me do my same calculation on a classic spar setup, this G figure should be a good basis to compare to the carbon former G value .. So then the G values are to be considered relative to eachother and not absolute . Latest blog entry: FZ5: design phase
 Sep 09, 2014, 01:36 PM Registered User United States, MA, Middleboro Joined Jul 2007 605 Posts I know a lot of DS guys have pegged the eagle tree limits of +/- 38 G's on a regular basis, they may have actually pulled way beyond that. It's pretty dependent on speed and turn radius, I know many years ago they did a test on pattern ships that were pulling over +/- 25 G's back in the late 80's. Latest blog entry: The third time is a charm!
Sep 09, 2014, 02:14 PM
InceCreations
Joined Jul 2009
732 Posts
Ok, let's do a simplified math.

I just want to compare a usual classic spar design I always use, with the carbon former i plan to use.

1) The classic spar as a reference:
- Suppose you have only one spar system, with one spar top and one spar below, and webbing in between.
- the spars have 5x3mm cross section (15mm2 area)
- center spar to center spar distance is 25mm

2) Max compressive strength data of spar material: 40N/mm2

--> max compressive strength of spar = 40N/mm2 x 15mm2 = 600N
--> max moment on spar system = 600N x 25 mm = 15000Nmm

3) Take wing span of 1100mm and suppose that lift takes place a wing tip (=worst case approach), then the max allowable force at the wing tip would be = 15000Nmm /550mm = 27N.

Thus the max. lifting force would be 2 x 27N = 54N
Suppose the plane is 1.7kg heavy, the max G = 54N/(1.7kgx10) = 3.2G's

So what does this mean ?
That my classic spar setup, existing of 2 spars 5x3mm can pull about 3.2G's,
meaning the carbon former setup is stronger with it's max 6.4G's.

I cannot refer to the G's you mention, but only compare to the classic spar design I used several times before, which did fine.

Vincent

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Latest blog entry: FZ5: design phase
Sep 09, 2014, 09:54 PM
Registered User
United States, MA, Middleboro
Joined Jul 2007
605 Posts
The G levels I was talking about is the forces that can be generated during maneuvers at speed. During a discussion regarding the German Speed Cup rules in order for an aircraft to enter the course you have to make a turn with a radius of less than 50 meters at the 300+ mph these aircraft pull something like 50 G's. Now keep in mind these are extreme high performance airplanes but if you have a "quick airframe" and make a tight turn you can really pile on the G's quickly, granted this is more of an issue the faster you go. Your design has potential to be a fairly quick and nimble airframe that will generate G forces the like of a jet fighter rather than a Cessna. One thing I noticed you didn't add the strength the skin added when calculating your strength and it really adds a lot of strength we sometimes forget about.
I would say your delta probably has gone over 30 G's in some of the flying in your video. Either way what you have been doing has worked thus far. Heck that delta took a serious hit and came away with pretty minor damage.
Quote:
 Originally Posted by podavis I gave this some thought when the recent 'holding wings on with magnets thread' was active. I came up with a way to estimate it g's in a loop based on the classic formula centrifical_acceleration (aka radial acceleration) = velocity^2 / radius_of_turn, which is a variation of omega^2*r. Both of these are things we can estimate with practice, preparation and visual cues. A frame-by-frame analysis of video would be more accurate. A loop at the bottom of a vertical dive will test the plane's capability and can be performed square to the observer for better accuracy. I put together a little table of the solution for the the radial acceleration versus various velocities in feet/sec and turn radii in feet and the results suprised me, it took some pretty tight loops at speeds over 50mph to get past 10 g's. The table is at home, I'm eating my lunch at my desk right now, if anyone is interested I'll post the table when I'm home.
Latest blog entry: The third time is a charm!
Sep 10, 2014, 12:13 PM
InceCreations
Joined Jul 2009
732 Posts
What we all use frequently is a carbon rod, or 2 of them, for joining wings.

I'm surprised their strength is not so high:
http://en.wikipedia.org/wiki/Bending

Formulas:
SIGMA (y) = M* y / I
with:
- SIGMA(y)= stress at location y (N/mm2)
- M = bending moment (Nmm)
- y = distance to neutral bending line of cross section (mm)
- I = moment of area (mm4)

--> max bending moment is then:
M_max = SIGMA_max / y_max * I

I of cirkel cross section = 1/4 * pi() * r^4

Material data:
SIGMA_max for carbon = 720 N/mm2

Resulting max. bending moment:
Rod diameter = 8mm --> I = 201mm4--> M_max= 720 / 4 * I = 18.096Nmm

The same for the classic spar setup of before gives = 15.000Nmm

The same for the carbon former results in = 39.690Nmm

Summary:
- A carbon rod of 8mm is as strong as a classic spar setup
- A carbon rod is 2 times weaker than the carbon former.