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Author Topic: How many G's  (Read 4712 times)

Offline Allan Perret

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How many G's
« on: January 08, 2016, 08:03:59 AM »
I am testing a new rotor hub design for Otto the Gyro.  I have heard this design can actually perform a loop.  So the question is how many G's will be on the rotor hub when performing an old time loop say at 50mph,  starting from level flight with the top at 60 elevation, on 60ft lines ? 
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Offline Tim Wescott

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Re: How many G's
« Reply #1 on: January 08, 2016, 10:21:57 AM »
Not a hell of a lot?

I can't remember numbers, but I think 5 g's is the minimum -- I've done tests with accelerometers on board, and in square loops a flapless stunter doesn't exceed 10g -- round loops, obviously, are less.
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Offline Allan Perret

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Re: How many G's
« Reply #2 on: January 08, 2016, 07:52:19 PM »
Not a hell of a lot?

I can't remember numbers, but I think 5 g's is the minimum --
Did you mean to say maximum ?

Was hoping one of the aero engineers would do a calc on this.
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Offline Tim Wescott

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Re:
« Reply #3 on: January 08, 2016, 08:59:22 PM »
Minimum. I'm getting 6g doing the math in my head, with enough rounding that it may be 5, it may be 8.

Sent from my SM-G900V using Tapatalk

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Offline Howard Rush

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Re: How many G's
« Reply #4 on: January 09, 2016, 03:50:29 AM »
I get 6.7, counting 1.15 for gravity.  Check that, please, Tim.  I actually talked to Fast Richard about whether this thing would do a loop.  He decided it wouldn't.
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Offline Chuck_Smith

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Re: How many G's
« Reply #5 on: January 09, 2016, 05:46:40 AM »
Seems to me that with the spinning mass of the rotors you'll have a combined loading situation because of precession too. I don't have the figures but my suspicion is it's non-trivial.

It's a tough question because it really depends on how round you make the loop and how constant you keep your velocity. A constant radius or airspeed isn't a given with Otto the Gyro...

If it was me, given the fact that even at 20 g's the loads aren't very high I'd just over-design it. I'd worry more about adventurous landings and impulsive loading than sustained g's in a loop. Of course, we could build one with a swashplate coupled to the elevator and fly it inverted...

All IMHO and YMMV.
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Offline Allan Perret

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Re: How many G's
« Reply #6 on: January 09, 2016, 08:05:40 AM »
The Otto is capable of a loop, but just barely.  See reply #3 in this thread:   

 http://stunthanger.com/smf/open-forum/otto-the-giro-(gyro)/

Jim Lee tells me he has also done a loop with one.

I figured that the loop would be the highest G load it would ever see so I would use that load x 3 for safety factor.  
Think I will test it to 10 G then put it in the air.
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Offline Tim Wescott

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Re: How many G's
« Reply #7 on: January 09, 2016, 10:42:58 AM »
I get 6.7, counting 1.15 for gravity.  Check that, please, Tim.  I actually talked to Fast Richard about whether this thing would do a loop.  He decided it wouldn't.

This is more for a generic stunter, but it should be in the ballpark:

Assume 20 meters from center of circle to center of gravity.  That's pretty reasonable for 60' lines.

Assume v = 25m/s airspeed -- that's close to 5s laps with a 20m radius circle.

Assume a loop that goes from level to 45 degrees.  That means that the radius of the loop will be

rl = sin(22.5o)(20m) = 7.7m

Ignoring gravity, acceleration toward the inside of a circle is velocity squared divided by radius:

al = v2/r = (25m/s)2 / (7.7m) = 82m/s2 = 8.3g.

This seems way too high, but I'm not sure where my mistake is -- I get 5.7g with six-second laps.
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Offline Serge_Krauss

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Re: How many G's
« Reply #8 on: January 09, 2016, 10:03:00 PM »
Here's mine, with no additional explanation. Last time I got flak. Dimensions and speed as requested.

Offline Tim Wescott

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Re: How many G's
« Reply #9 on: January 09, 2016, 10:19:08 PM »
Hmm.  The included angle between two equal sides of a triangle is 60 degrees, yet the angle between one of those sides and the last one is 65 degrees.

So, do you experience strong tides in your neighborhood?

Assuming that the results are roughly the same for those of us living in places where geometry is much closer to Euclidean, it looks like your numbers are at least roughly the same magnitude as mine.
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Offline Serge_Krauss

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Re: How many G's
« Reply #10 on: January 10, 2016, 12:05:55 AM »
You'll need to excuse my hurried "0" that looks to me more like "6" than "5". The large triangle is of course equilateral, and the angle apparently in question is 60 degrees, as used in the computations (and I did write '30-60-90' triangle). The math is correct. I think you'll get the same results with the numbers requested and used. Believe it or not, this aging math teacher actually knows the sum of the angles of a triangle, but won't ask why anyone might think otherwise. No,we're not being dragged under by the tides here on the "north coast." - SK

Edit: If anyone really wants to split hairs, the 60' line length hasn't been adjusted to include arm or half-span. I thought this math model was "close enough."
« Last Edit: January 10, 2016, 12:40:47 AM by Serge_Krauss »

Offline Chuck_Smith

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Re: How many G's
« Reply #11 on: January 10, 2016, 04:06:09 AM »
You'll need to excuse my hurried "0" that looks to me more like "6" than "5". The large triangle is of course equilateral, and the angle apparently in question is 60 degrees, as used in the computations (and I did write '30-60-90' triangle). The math is correct. I think you'll get the same results with the numbers requested and used. Believe it or not, this aging math teacher actually knows the sum of the angles of a triangle, but won't ask why anyone might think otherwise. No,we're not being dragged under by the tides here on the "north coast." - SK

Edit: If anyone really wants to split hairs, the 60' line length hasn't been adjusted to include arm or half-span. I thought this math model was "close enough."

Don't forget the flight path is inclined at the lattitude of the center of the circle and that the center of the circle is in fact rotating around an axis a few thousand miles away, that said axis wobbles due to the earth-moon gravitational interactions and gyroscopic precession. This means we need to include forces from the Coriolis effect.  This axis itself is revolving around a star, and that star is itself revolving around a supermassive black hole at the center of a galaxy in an expanding universe. Add to that the fluid medium through which Otto is navigating is forever in turbulent motion with unstable velocities and directions.

Remember all that next time you bounce a landing... when you look at all the vectors involved it was still a triumph of the human spirit! At least that's what I tell myself.

Hmm, does Spektrum make an accelerometer with telemetry interlink? Maybe it's time to gather some data, if nobody has already.
« Last Edit: January 10, 2016, 06:26:05 AM by Chuck_Smith »
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Offline Tim Wescott

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Re: How many G's
« Reply #12 on: January 10, 2016, 01:17:30 PM »
You'll need to excuse my hurried "0" that looks to me more like "6" than "5".

I _was_ just yanking your chain.  I don't know my relativity well enough to be sure, but I'm pretty sure that if you were somewhere that an isosceles triangle were 60-65-65 it'd be very close to a very massive star, and the tidal forces between your head and your toes would be somewhere between perceivable and ripping you apart.
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Offline Serge_Krauss

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Re: How many G's
« Reply #13 on: January 10, 2016, 02:21:12 PM »
'could be. My general relativity was never that strong mathematically, except for good ol' E = Mc2.

I just added 3.5' to the line length for arm and half-span lengths to see how much things would change. It worked out to 6.14 or about 6.1 g's.

That's for the same speed requested, but if you adjusted speed to obtain the same lap time, then the number of g's would increase from that original 6.5. I'm presuming though that we just want a ball-park figure. Barring "figure 9's," It looks as though the autogyro is safe enough.
« Last Edit: January 10, 2016, 02:38:57 PM by Serge_Krauss »

Offline Tim Wescott

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Re: How many G's
« Reply #14 on: January 10, 2016, 02:40:59 PM »
Hey!  How about experimental data!  I dredged up the data from my flights on 10 19 October 2014, and extracted the accelerometer data.  Here's the inside loops.

The acceleration does get up to 6 g's -- almost 8 in one spot -- so I guess we're in the right ballpark.  Note how much the acceleration sags at the tops of the loops -- this is because the speed is sagging badly, as can be seen by looking at the blue trace (outward acceleration) and drawing some conclusions.

(the z-axis acceleration isn't much more in the square maneuvers in this flight data -- partially this is because it's a flapless plane, but I think it's also partially because the squareness of the square maneuvers is largely illusion.)

edit to correct date of flight
« Last Edit: January 10, 2016, 04:40:21 PM by Tim Wescott »
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Offline Serge_Krauss

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Re: How many G's
« Reply #15 on: January 10, 2016, 04:26:44 PM »
Now that's really cool! I've tended to think of the speed at the bottom as the same as the speed going into the loop, but despite the speed loss on top, we do have thrust as well as gravitation accelerating the plane down. So it can acquire more KE than it had to begin.

Near the end of our season (it's 'blizzarding' outside now), I asked a competent club combat flier with seriously fast planes to do several consecutive ultra-tight (radius estimated at 7' - 8'), high-speed loops so that I could get a good enough clocking to use in computing g's. I was really impressed with how much the induced drag slowed his plane. In level flight, he was doing something like 110 mph, but in the loops, the average was under 75 mph. That cut the number of g's by more than half. The figure was still very impressive. Unfortunately the figures I get now don't match what I got at the field; so I've forgotten something. His radii were much smaller than any stunt corners I've seen though, even at that speed.

Offline Air Ministry .

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Re: How many G's
« Reply #16 on: January 11, 2016, 08:56:35 PM »
Zee Tecanique , like finding your braking point for a corner / hairpin , of hitting FULL UP , at say 20 Ft ( + ) in the wingover , where you see what altitude the pull out is ,the shifting the point you hit the brake, Er . . . Up , gets you the tightest corner .  S?P LL~

Advice from Jackie Stewart was to wait till you should hit the brakes , count to ten , then, Hit The Brakes. . . REALLY HARD .
Testing a sucker I reckon it should hang on a wire ( full up or down ) for at least two turns. Somell do it indefinately
with a bit of corection to correct for drift .
Most yump & wallow a bit, porpoising in five ft loops . But it seems indicative of it it'll stay out there when the goings tricky .

How much are these accelerometers &from where , please.
« Last Edit: January 12, 2016, 04:40:59 PM by Matt Spencer »

Offline Tim Wescott

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Re: How many G's
« Reply #17 on: January 11, 2016, 09:09:26 PM »
How much are these accelerometers &from where , please.

The actual chip is the ST H3LIS331DL.  But that's nearly useless to you unless you're a circuit designer and embedded programmer.

The measurement was taken with a TUT.  http://atomiczombieworkshop.com/tut/tut.html
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.


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