Build Log The MiniMOB ~ RE-DESIGN STARTED..........

[PeterO_UK]

Originally Posted by Hephaistos

Hello.........
Peter, what do you want to say with: "My experience so far is that I only ever use any "down" when doing a dead-stick landing or flying fast". I struggle with the grammar a little. If I write: "My experience so far is that I only use "down" when doing a dead-stick landing or flying fast", do I write the same or have I changed the sense of the sentence?

You have correctly understood what I wrote. The "ever" just adds emphasis!
PeterO_UK

[JochenK]

Mike,

what sort of file types can your CAD program import? I can offer you
- AP 203 (*.stp)
- AP 214 (*.stp)
- ACIS R13 to R18 (*.sat)
- STL (*.stl)

Let me know what kind of files you can use, and I'll send you a copy of the Minimum CAD files.

Jochen

[Hephaistos]

Hello Jochen........

The CAD of the Minimum would be perfect to have some reference-data. STEP AP 214 ISO would be perfect. I mostly work with STEP for conversion, it seems to work best with all programs.

BTW, I decided yesterday to size-up to the Minimum, when I asked myself if I should take 1,5 inches blades like on the Micromum or 2 inches like the Minimum. A new outrunner is cheaper than one set of the wrong-size Aerobalsas with shipping to Germany!

Bye
Mike

[JochenK]

Mike,

the CAD file is on it's way. Two inch blades for the Minimum.

Jochen

[Hephaistos]

Hello..........

First: Thanks to Jochen for the CAD-files! And for the others: They are really incredible! Every detail is perfectly worked out! If you take some clouds as background for the CAD-program its almost real.

Second: For those who cannot open a CAD-file on their computers, I could convert the Minimum-CAD to 3Dxml-data. Its based on the CAD-data, but you can watch it with a special viewer you can download for FREE at DASSAULT-SYSTEMES, the manufacturer of CATIA, the program they build the Airbus with. I thought to do like this if I have finished my CAD, but for the Minimum I do only with Jochens explicit permission, of course.

Third: I decided to take the dimension of the Minimum for my construction. No more juggling with dimensions! The center of gravity needs to be placed the same, et cetera, et cetera. But I can order some Aerobalsas and I know that I order the right ones! So I will change the title of the thread accordingly and put some edit to my first post to explain.

Fourth: As only the mob steals without feeling a shame ( I took a look to the meaning to the native speaker for this combinations of my initials ), I decided to name my clone of the Minimum the MiniMOB. The best name for a stolen idea....................

Fifth: Now its time to go for the CAD again, put the servos on their place. Perhaps its time to terminate another Tannenzäpfle, but thats something completely different!

Bye
Mike

[JochenK]

Mike,

I've got a 3D pdf file of the Minimum CAD model. Unfortunately it's nearly 7 MB and thus too big for publication on the forum (3 MB limit). If anyone would like to have this file - or any other *mum file, I've got them all - p.m. me your e-mail address and I'll send it over.

Jochen

[Hephaistos]

Hello Jochen...........

I just converted the complete Minimum to 3Dxml to have a look at the size of the file: Its only 660 kilobyte! I did not expect the file to be this small. The colors have all gone, but this is only a question of the conversion. The models I converted before had the same clours like on the CAD. With a little work this problems should be solved. For me its much easier to understand the technical drawings if I can view it in 3D, turn it and watch it from all directions.

Bye
Mike

[Hephaistos]

Hello..........

I finished the servo-mount today and tried to adjust the servo-throws. I made the lever-arms on the head-joint asymmetric, double lenght for pitch ( 50 mm ) than for roll ( 25 mm ), exactly the same relation like on the Minimum ( 40 mm / 20 mm ).
The lenght of the servos lever arm is 14 mm ( the minimum is 10 mm ).
Next I tried to have 10 degrees of pitch and 15 degrees of roll on the rotor at the same time: It did not work at all, the servos lever arm is to short.........
To solve the problem I had to make the servos lever-arm about 22 mm long, then I had a servo-movement of 40 degrees.

Now my question: Are the maximal movements for the rotor measured indipendly? Like 15 degrees roll with zero pitch, 10 degrees pitch with zero roll ? And if I give full pitch and roll on the RC at the same time, the rotor doesn´t reach 15 degrees roll and 10 degrees pitch because the servos reached their limit of about 45 degrees to each side?

Or have I made some mistake???

Bye
Mike

[PeterO_UK]

If it's all set up and working correctly, there should be no coupling between roll and pitch.
So it should be able to achieve 10 degrees pitch and 15 degrees of roll at the same time. This assumes you only use 40% of the servo movement to get 10 degrees of pitch and 60% of the servo movement to get 15 degrees of roll.

If it would help I can take some video of my Minimum moving it's servos.

PeterO_UK

[JochenK]

Mike,

I just checked the actual lenght of my servo arm, it's 13.5 mm. And my servos have a standard throw of 45°.

Jochen

[Hephaistos]

Hello.........

Peter, Jochen, much thanks for your quick answers.
They really helped to solve the problem!

This morning, I looked again at the CAD. I checked the head-joint with 10 degrees pitch and 15 degrees roll for collisions. As you can see from the picture I posted, the head-joint-lever is moving inside a hole of the head-joint-fixation-plate. This has been a little complicated to design, but now it works perfect.

For checking the servo-throws, I stayed with the 10/15 degrees and got problems. If I reduce the head-movement to the recommended 8 degrees pitch and 12 degrees roll, It works like it should be working. I only had to scale the servo-lever-arm Jochen measured with 13.5 mm up to 17 mm to fit with the dimensions of my longer head-joint-lever-arms.

With 8 degrees pitch and 12 degrees roll I get a servo-throw of 42 degrees now.
I expect to finish the head-control today, and I will post some screenshots then.

Bye
Mike

[JochenK]

Mike,

don't limit the roll-wise movements of your spherical joint beyond those 18°. What you have not taken into account yet is roll trim. And you need several degrees of roll trim to the side of the retreating blade to make your gyro fly straight. Make your control lever a little shorter to get full throws and put it above the joint if you have problems with your arm-through-hole design. A control lever soldered to a wheel collar which you can slip over the rotor spindle should be alright.

Jochen

[Hephaistos]

Hello...........

Now I hope its okay. With using the full servo-throw of 45 degrees I can get 8 degrees of pitch and 14 degrees of roll at the same time. I can adjust the connection rods a little asymmetric, and I get up to 3 degrees static pre-trim for roll. Thats 17 degrees of roll towards the retreating side and 11 degrees of roll to the advancing side, still with a servo-throw of 45 degrees. I hope this is enough to realize the roll-trimming.

I reduced the lever-arm for pitch to 45 mm, for roll to 22,5 mm, and stayed with 17 mm for the servo-lever-arm. Quite different relation than for the Minimum, but I don´t doubt the values I measured on the CAD.
The head joint has also been slightly reworked, a half of a millimetre here, another half there, and the mechanical limitations change. The devil is living in the details!

The arm-through-hole design is not principially problematic, its just a little work to get the right dimensions for the hole. If I would move the lever-arm to the upper side, I get away from the trouble with the hole, but I have one additional part, slightly longer connection-rods, and less distance between the connection-rods and the rotor, or I would need a longer axis. Quite enough facts to go for the hole..........

The servos for the rotor are both mounted in front of the mast. The mounting-plate, made from GF, is also used to connect the both tubes of the airframe. I think about fixing the mounting plate with rivets, they have to be drilled-out for replacement, but they are not as heavy as bolts.
Placing both servos in front of the mast should be good for placing the centre of gravity, too.
There will be a similar plate on the other side of the airframe, carrying a third servo. This servo will be for rudder in combination with a steerable front-wheel.

The connection-rods are made from CF-tube with reinforcements on their ends, a piece of a screw will be epoxied inside to connect them to the ball-joints. I will use IGUBAL-ball-joints for this purpose.

Bye
Mike

[Hephaistos]

Hello...........

Its got quite around here. As Jochen pointed out the problems of my layout very fast until now, I assume that the servo-throws are okay now.
But I would feel better if I could read something to confirm my assumption. So, Jochen, as you are the expert for this design and dimensions, please post some statement about the actual layout.

Bye
Mike

[JochenK]

Mike,

don't limit yourself too early in the construction, you'll never know what's going to happen. Design your control mechanism so that it can use the full limits of your spherical bearing, at least roll-wise. It's very easy to reduce the throws afterwards, the other way round it isn't.

Can you simulate movement with your CAD program? If so, check the forward or rearward movement of the rotor spindle at full roll throws. Does it stay in place, does it tilt backwards or does it tilt forwards? The first possibility is ideal, the second one can be tolerated, the third one not. You don't want down pitch as a side-effect when you're rolling. If this happens, you'll have to move the control lever up above the joint.

Keep your construction as flexible as you can, when you get to flying the MiniMOB you may want to do some final adjustments. I guess you'll have to use more rotor back tilt than I do because your gyro will be heavier than the Minimum - the Minimum isn't called Minumum for nothing - and then you'll have to move the c.g. forward to keep it in front of the rotor thrust line. Allows for things like that.

Jochen