Calculating Offset Changes

[daveb]

How do you calculate changes in the offset between the VCG and thrust line when you move mass either higher or lower?

For example on a HTL machine I’ve moved the battery from below the seat to above it. How do you calculate what improvement there has been on paper? What happens if the seat is moved up a few inches or maybe the fuel tanks? Is a four inch change that much better than a three? What happens if you take off the electric prerotator and use a drive type instead? :eek: :eek:

It would be good to work it all out first ...but with my maths abilities :confused:

Dave

[Victor Duarte]

Dave, i asked exactly the same question, and in a thread, Aussiepaul explained how he does it.. i didnt get it right but, he takes pictures of hangtests, then puts the gyro on the wheels and takes other pictures, then he draws some lines..
Maybe you should search this, i dont remember the thread.

the method i thought about, is to hang the gyro by its side on a slider bar, then slide the rope and stop when this bar (or the mast+keel) is (are) horizontal, the CoG should be under the rope.
If anyone has a simple method to do that I TAKE IT.
Thanks

[gyromike]

DaveB,

It's not that hard to figure.

Let's say we have a gyro with an All Up Weight (AUW) of 600 lbs. (pilot, fuel, and machine). This machine has a thrust offset of 3 inches above the Vertical CG.
Let us also say that the battery, located on the keel, weighs 10 pounds. It is located 12 inches below the current CG. We want to move it up 12 inches to be even with the current VCG.

Battery weight (10 lbs.) / AUW (600 lbs.) = 0.0166

Multiply 0.0166 X the distance moved (12") = 0.1992"
The VCG has moved upwards 0.1992 or ~0.20", reducing the thrust offset to 2.80" above the VCG.

Now let's move the battery again, but this time we'll move it 24" above the new VCG putting it just under the cheek plates. AUW and battery weight is the same so we'll use the same %:

24" X 0.0166 = 0.398 or ~0.4 inches.
The thrustline is now ~ 2.4 inches above the new VCG.

Next, the pilot (170 lbs.) and 8 gal. seat tank (50 lbs.) are going to be moved up 6 inches.

(170 + 50) / 600 = 0.366
0.366 X 6 inches = 2.196 or ~2.2 inches.

We have further reduced our thrustline offset to 0.2" above VCG.

So now you can see the effect of mounting the engine up and the gearbox down, also moving the pilot and fuel up. They're the heaviest parts of the gyro, and will have the greatest impact on VCG with the smallest movement.

(All weights and dimensions are for example purposes only, and the actual VCG should be calculated before any changes are made.)

[gyromike]

Victor,

Paul's method works for finding the initial VCG.
After you know that, you can then calculate any changes made from moving components up or down.

You could also do a double weight and balance, with the gyro tilted at two different angles. Same as the double hang test.

[gyromike]

DaveB,

To calculate the effect of swapping a heavier part for a lighter one, find the weight of the heavier part, and it location above or below VCG.

Example:
10 lbs. elec. prerotator motor @ 24" above VCG,
5 lbs. hydraulic motor @ 24" above VCG.

Remove the 10 lb. electric motor and replace with 5 lb. hyd unit:
-10 + 5 = -5

-5 / 600 = -0.0083
-0.0083 X 24 = -0.1999 or ~-0.20 inches that the VCG would have been moved down.

Adding heavier parts above the VCG moves it up.
Adding heavier parts below VCG moves it down.

Use the opposite for substituting heavier parts for lighter ones.

[skyguynca]

I think I might have what you are looking for. I got it from Ralph Taggart's site. You weigh the acft with all aboard, fuel and rotors and level then you put something under the main gear and weigh it again and enter the height of the device and the thrust line and it will tell you in reference to the thrust line where the center of gravity is both vertically and horizontally.......the link for the software is ftp://taggart.glg.msu.edu/cgcalc.exe and the link for the page that explains it is http://taggart.glg.msu.edu/gyro/cg.htm it works pretty good

[daveb]

Victor,
We initally used the pic method, Paul took the pictures... then needed Photoshop to get the scaling identical for the comparisons. Next time we will use a fixed camera on a tripod! Then it was easy to work out a proportional scale for the drawing and get the offset.

Thanks Mike for explaining the simple sums so very well. Exactly what I was after. Moving the battery gave .98" improvement, but I'll have to sit on the prerotator to get CLT :D ...I can feel the stab getting bigger already.

Dave

[Brent_Brown]

Dave if the photo next to your name is your gyro. move the seat up 12'' and you will be much better off. Again why do we still make gyro with the seat like this?

[daveb]

Dave if the photo next to your name is your gyro. move the seat up 12'' and you will be much better off. Again why do we still make gyro with the seat like this?Well that would give me a 3.5" improvement, still not perfect. The gyro was built some years ago. The reason for wanting to be able to do the calcs was to help decide the best way to go, I may be better off dropping the engine a little. It is a very comfortable ride as it is, and I can get in it easily :rolleyes:

Having just lengthened the boom, a more effective stabilizer is next. Even now, it handles very rough air surprisingly well.

Dave

[quadrirotor]

Double hang test.

[Victor Duarte]

Andre, damn simple, the thing is that your rope must be attached to the CoG yet !

[quadrirotor]

The wheels don't touch the ground!!!

[Brent_Brown]

blades have to be on

[Victor Duarte]

Yes i know, but is you hang it too high or too low, you won't be able to figure where the horiz. CoG is, or am i wrong ?.

[quadrirotor]

Intersection of the two verticals!!!

[gyromike]

Victor,

It doesn't matter how high or how low you hang it.
Of course you want it low enough to snap a picture of it.
So just off the ground is high enough.

Then draw a line from the anchor point down the cable through the frame. The CG will be at some point on that liine.

Hang it again from a different location, and do it again.
THe CG will be at some point down the second line.

Where the lines intersect is where the CG is, both vertical and horizontal.

If you wanted to be really precise, you could perform each series with a full tank, a half tank, and an empty tank.

I would just do it once with a half tank, myself.

[Victor Duarte]

Ok, i was misunderstanding, excuse me
thanks andre and mike