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Old Oct 12, 2005, 03:02 AM
Scotland
rcminiman's Avatar
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How much G force does a model aircraft encounter?????????

Hi guys, Im wondering just how many G's our planes can actually pull. Does anyone out there know? I would like to install a g force gauge, but the selection is poor. I have found one that can read up to 6G, but dont know if thats enough? Any help is as ever, very much appreciated. Barry
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Old Oct 12, 2005, 03:29 AM
AJH
AJH
UK
Joined Jul 2004
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I made a system using an Analog Devices ADXL210E (I think) which does up to 10g. The trace is from a test flight in a Twin Star. I assume the highest loading came from pulling out of a loop.

Hope this helps.

AJH
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Old Oct 12, 2005, 03:52 AM
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I'd say stres to at least 5g, but you can pull a LOT mre than that with a high speed plane.

My guess is up to 20g.

Parkflyers OTOH maybe 2-3g max.
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Old Oct 12, 2005, 04:55 AM
Giz
Tony Rogers
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Bath, UK
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Here is my take on it from my rather rusty knowledge of aerodynamics.

In balanced (no sideslip) level flight in a banked turn, g force is 1/cos(angle of bank). (I'll call it g force but really g is an acceleration and is a multiple of acceleration due to gravity which is 9.81 m/s/s).

So at 60 degs, g = 2
75 deg, g = 3.86
80 deg, g = 5.76
85 deg, g = 11.47

Whether any particular aeroplane can fly at a particular angle of bank depends on how much lift it can produce. 10g basically means that the lift force is 10 times the weight of the aeroplane. The ability to generate this much lift depends on airspeed and coefficient of lift, which is closely related to angle of attack.

When you try to turn or pull up too hard what you are doing is increasing the angle of attack up to and beyond the stall angle and when the wing stalls lift is reduced. Maximum lift is achieved at maximum airspeed and at an angle of attack just below the stall, provided of course that the wings don't fall off .

As an example, my Jazz weighs about 90 oz. Motocalc says (very rough) that at maximum lift at full power is produced at an angle of attack of 7 deg and lift is 520oz. 520/90 = 5.8g.

Now I expect Motocalc's modelling of lift is a bit conservative but the figures are of the right order. In a dive, airspeed would be higher which would increase lift but I doubt if g force would get beyond about 7g.

What enables an F16 to pull 10+g and a Cessna 172 only about 2-3g (other than structural factors) is basically power. The Cessna wing stalls long before it is able to increase the lift force to 10 times weight. With our model aeroplanes, we can achieve very high power to weight ratios (Cessna 172 models that can climb vertically!) so perhaps we can get towards 10g with fast aircraft.
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Last edited by Giz; Oct 12, 2005 at 05:00 AM.
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Old Oct 12, 2005, 05:34 AM
Scotland
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Very Interesting answers guys. Thanks, Barry.
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Old Oct 12, 2005, 07:50 AM
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Robert v.d. Bosch (came 2nd at this year's WC F3D pylonracing, Dutch team as a whole came first ) had a accelerometer in his ship that could measure up to 35G. It tipped the scale when cornering the pylons

Vriendelijke groeten Ron
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Old Oct 12, 2005, 08:45 AM
Deletedfor proving Nauga wrong
Joined Mar 2005
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This discussion comes up occasionally...

The dynamic soaring guys can "peg out" the Eagletree data logger's +/-38 G meter (both ways)

In the late 1970's or early 80's a test was done with a G logger in a Pattern competition model and it had some maneuvers hitting 25 and 28 G's.

What you are doing has a large efect on how many G's the model will pull.
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Old Oct 12, 2005, 10:05 AM
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Giz,

your assessment is right-on! The amount of g-force an airvehicle can withstand is totally dependent upon it's structure, the amount of g-force an airvehicle can generate is dependent upon it's speed and aerodynamics (CLmax), once an airvehicle exceeds it's CLmax it will depart or destruct. At extremely high speed it is much more difficult to get to CLmax than it is at low speed, SR-71 for example!

In the world of RC modeling, low Reynolds Number, you can get away with alot that you could not do in a full scale with a human in the cockpit, although many WWII era spruce built biplanes can withstand +12g's, current day high performance fighters are limited to +9/-3 g's (structural limits), the g-onset rate is what causes pilot G-LOC, Loss-Of-Consciousness, pilots train to withstand sustained high g-loadings.

The way to test your model for structural integrity is to do it the same way full scale planes are done. In a static rig, support the fuselage and load the wings with evenlly distributed weights until they fail.

AD, MSAE
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Old Oct 12, 2005, 11:09 AM
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Louisville KY
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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.
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Old Oct 12, 2005, 01:59 PM
Sussex, UK
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Crawley, West Sussex, UK
Joined Jun 2004
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> I would like to install a g force gauge

There are some simple G indicators available for sticking on items in the post
http://www.etilux.com/EN/Dep1/Emball...p?MenuItem=727
They are stuck to the object and typically contain a small weight that is knocked loose by a known accelleration.

You could do something similar by sticking a small weight to a magnet. Measure how much force it takes to pull the weight off the magnet, divide the force by the weight to give the acceleration required to trigger your indicator, change the weight or magnet until you get the value you want. Then mount it in your plane and see if it triggers when you loop.
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Old Oct 12, 2005, 02:21 PM
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Augsburg, Germany
Joined May 2005
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Hello,

the most simple accelerometer is a small mass fixed to the end of a leaf spring. A pen pointing on a fresh piece of paper will create an indication of the maximum acceleration both up and down.

Calibration can be performed by using several masses similar to the one fixed to the end of the leaf spring and making little marks for 1 g, 2 g and so on on the paper scale.

Make copies of the scale and put a new one in for each measurement flight. +-10 g full scale measurement range ist good for a start.

Gernot
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Old Oct 13, 2005, 03:47 AM
Giz
Tony Rogers
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Bath, UK
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Ron,

Good point about the pylon racers! I hadn't thought about them. And they really look like they're pulling 35g in such high rate turns, too.

AD,

I have wondered about testing the wing for strength but I thought that the way to do it might be to support the wing tips and load up the fuselage with weights, at least to test for positive g. This wouldn't give the distributed load that the wing would experience in flight though. Perhaps the thing to do would be to support the aeroplane inverted by the fuselage and then distribute weights along the wings. As of yet, I have not had the courage to do it though. I just make my wings really strong .
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Old Oct 13, 2005, 04:13 AM
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East Anglia, UK
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Quote:
Originally Posted by Giz
Ron,

Good point about the pylon racers! I hadn't thought about them. And they really look like they're pulling 35g in such high rate turns, too.

AD,

I have wondered about testing the wing for strength but I thought that the way to do it might be to support the wing tips and load up the fuselage with weights, at least to test for positive g. This wouldn't give the distributed load that the wing would experience in flight though. Perhaps the thing to do would be to support the aeroplane inverted by the fuselage and then distribute weights along the wings. As of yet, I have not had the courage to do it though. I just make my wings really strong .

On a constant chord wing, you van get the same center section loading by supporting the wings halfway along the span and weighting the fuselage..since this is the maximum stress, its a good first off check.
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Old Oct 13, 2005, 04:57 AM
Registered User
Naples
Joined Nov 2004
198 Posts
Years ago in my active times I installed an airworthy G=meter out of a 104 into a gas powered model.
Unless just cruising around it was always pegged at full scale, and that was +10 and -5 G
Hans
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Old Oct 17, 2005, 07:15 AM
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Dayton, OH
Joined Sep 2004
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Giz,

that's a good first order test, or even vintage1's method, the problem with loading the fuse and supporting the tips is that it concentrates the load at the root of the wing, the test needs to be done with the weight evenly distributed along the wing. What the heck, fly it! if it breaks repair and fly again!

AD
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