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

[Hephaistos]

Hello Jochen........

First of all, much thanks for your comments. With your guidance, I hope I can prevent the MiniMOB from being called the LeadMOB.

I just checked the pitch movement at a roll-throw of 15 degrees as you suggested. The rotor-spindle is in fact moving forward a little. It’s moving forward for exactly 0.32 degrees. With moving the connection-points from the rotor-head-lever to the connection-rods upwards, I can reduce the pitch-movement with full roll to zero. To my surprise, it’s still below the middle of the spherical-joint. It’s just the question if 0.32 degrees are big enough to be remarked as a bad thing. I expect the mechanical freeplay to be in the same range or perhaps bigger.

As you can see, the simulation of a movement is not problematic. The level of detail I spend for the CAD is poor in comparison to the CAD of the Minimum, but I spend lots of work on the parameters inside instead. Switching the complete construction from a 10x10 mm to 15x15 mm square-tube can be done in a few seconds: Just change the parameter for the tubes size and her goodness Catia will do it..........

I know for sure that the flexibility of the construction is really needed. Until now, the weights of the parts I already modelled match with the "shopping-list" for the complete assembly. The critical part will be the trimming of the centre of gravity at the end. I have done it several times on the CAD before. If I can just change the parameter "battery distance from mast", then click "OKAY" (…to terminate, the Catia-fellows round her will understand) and after a few seconds the new centre of gravity is displayed, it’s okay.

But I have to finish the CAD to know exactly if I have missed the target weight of 500 grams or not. The mechanical limitations of the spherical joint are reached now with 18 degrees roll and 8 degrees of pitch. Perhaps it would be better to go for a spherical-joint one size-step bigger. I will think about this, but as you can guess, it will be nothing more like changing a few parameters……….

Bye
Mike

[Hephaistos]

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

I reworked the design of the head-joint again to achieve more possible movement. Mechanically it´s now possible to have up to 18 degrees of roll and 14 degrees of pitch at the same time. The screw I use for the rotor-axis has to be epoxied now to the ball of the spherical-joint, but this small change made the difference. The fixation of the connection-rods has also been modified, now there is no more unwanted pitch output when applying full roll input.

With a lever-arm on the servos of 17 mm lenght, I can have 15 degrees of roll and 10 degrees of pitch at the same time with a servo throw of 45 degrees. Increasing the servo lever-arm to 20 mm, it will be 17 degrees of roll and 12 degrees of pitch. By adjustment of the connection rods I can get some static pre-trim until the mechanical limitation are reached.

The head-joints middle for pitch is now on 15 degrees, the mast stayed on 10 degrees. This made it possible to move the servos a little more to the front. The motor can move a little to the front, too. I hope to have only a small distance between the prop and the rotors rotation-axis, so the pushing-effects can be reduced.

The orange-colured device is a pendulum to measure the hang-angle. I only needed to modify the servo-plate a little to create a scale and make some small extension on the head-joint-flange to hang the pendulum. I think its practical for the first flights and trimming experimentation: I just hang the pendulum to its mount, hold the MiniMOB at the rotor, and I can measure the hang angle.

BTW, I grabbed one of the last 2,4 GHz RC´s Graupner had in stock in Germany. Its the new JR-IFS-3, they started to sell them in Germany two weeks ago, and now they are sold out. As I want to visit the Inter-EX this year, its good to have 2,4 Ghz and no trouble with the channels.

Bye
Mike

[JochenK]

Mike,

the pendulum for measuring the hang angle is a nice idea. Why not use an elongated hole at one end of the spherical bearing, then you can adjust the whole bearing for optimal pitch movement.

Jochen

[Hephaistos]

Hello Jochen......

I don´t understand where you propse to have the elongated hole. I attached a screenshot where I could imagine an elongated hole for adjustment. But I have to say that I dont like them, because they are problematic to adjust. I thought about an elongated hole for adjustment of the lateral offset of the prop, but I prever to have some "nonius", it only works in certain steps, but its easy to reproduce without measuring, so I think its better for changes on the flying-ground.

Yesterday I started to read the manual of the TX o the www( I hope it will be delievered today). They write I can extend the standard-servo throw of 45° up to 150% or 67,5° at the transmitter. I did not knew about this, because I tested my servos with a 25-years-old 2-channel TX. I looked at the manuals about the possibility for mixers etc. before ordering, but not at this page. Doing this, I can have the full mechanical movement of the spherical joint with the 17 mm lever-arm on the servo.

For your Minimum, do you have 100% of servo-throw or did you adjust the TX for more than 100% ?


Bye
Mike

[JochenK]

Mike,

upper right hole. I have my servo-throws limited to 100%, but that's the standard setting, not something I did on purpose.

Jochen

[PeterO_UK]

Mike,

According to the working out at the bottom of this page http://www.peteronion.org.uk/Autogyr...mum/index.html
my elevator end points are set to 118% and aileron end points are set to 89% to give the correct tilt angles.

PeterO

[Hephaistos]

Hello........

First of all, much thanks for the quick answers. I don´t think I will use the full 150% of servo-throw-extension the TX offers, but its good to know that the 45 degrees of servo-throw aren´t as fixed as when I was a little boy. Some thought about RC-components stayed inside until now.
I remember exactly how the resistor-type speed-regulator of my RC-car smoked up when I fixed a bigger motor to it. I solved the problem with four 30-Amps relais from a real car making some foreward-backward and serial-paralell switching of the batteries, controlled with some micro-switches and the old servo. Only two speeds, but it never smoked up again.

Jochen, the adjustment of the middle-position for pitch is not only done with the elongated hole you proposed. The middle-positon affects the design of nearly the complety assembly I have set up until now. The spherical-joint allows for a pitch from 1 to 29 degrees. I hope this should be more than enough. I attached a screenshot, where all the parts affected by this change are displayed in red colour. The servos etc have moved, because the connection-rods are designed to attack at 90 degrees at the middle position.

Peter, I have to commit that I did forget that you wrote about the servo-throws at the end of your build log. Thanks for remembering me.

Bye
Mike

[Hephaistos]

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

As I don´t need to go to work the next days (outch, the world-wide-crisis is affecting me) because my boss asked me if I could spend the overtime-hours (I got much of them) I collected until now at home until my next job starts next week, I have time now to design the MiniMOB all day long.

I examined the mistakes in the movement of the head joint caused by extended servo-throws closely. They are all in the zero-point-zero-something-region. So nothing to be scared about.

So I freezed the design of the head-joint with the following settings: roll 17 degrees and pitch 13 degrees with a servo-throw of 60 degrees, thats 134% of possible 150 %. The middle for pitch of the head-joint is still 15 degrees, and the servo-levers are optimized for a length of 17 mm.

The next thing to work out is the prop. I decided to have a Graupner-C350S-8.4 volts Outrunner. It offers a KV of 1160 turns per volt and a very broad spektrum of power with maximal efficency. I estimated the output-power to be about 130 watts. Graupner recommends a 7.5"x5.5" two-bladed-prop for usage with a three-celled-lipo-battery.

I wanted to have an adjustable pitch for the prop, so I called Mr. Ramoser, the manufacturer of the VARIOPROP on the phone. Fortunately, I got him on the first try and he had some time to help me to choose one of his props.

When I told him that I want to build a Gyrocopter, flying mostly in the low-speed range (compared to the usual fixed-wing), and what motor I want to use, he advised me to try a three-bladed-prop with a mount of the "8B-size" and blades of 7.9" or 8.9" diameter of the type "D". As he said, the three blades are better to have the maximum of thrust at low airspeed, and at the same time the prop-diameter will be reduced. He also advised me to make some experimantation with a balance to measure the static thrust and to determine which of the airblades he proposed will work best with the outruner I have choosen. I will do this before I start to build the MiniMOB, to get the right prop-diameter into account. The next steps I will make with the bigger prop, because I think its easier to reduce the diameter than to enlarge it.

Jochen, now I have another important question. I assume that the two checked bowls in your CAD-file shall represent the centre of gravity. I measured the distance the prop-thrust-line is placed above the checked bowl, its something about 40 mm. Do you think it would be okay if I design the motor-mount to move from the thrust-line being inline with the CofG 40 mm upwards in steps of five millimetres? I know that the prop-thrust-line has to be placed a little bit above the CofG to compensate the rotor-drag, but not to much for not creating a power-push-over.
I would be happy if you could post something concerning this question.

Bye
Mike

[SafeLandings]

Mike,
Just a little info on my thrust line position which may or may not help, mine measures 25mm above the CG position and I have had no problems flying. The only thing that I noticed was when I was taxing around in the snow, as soon as I applied power the nose would 'dig in' which was easily overcome by the nose ski angle being moved, but you wont be flying in the snow I guess.
Another thing I have noticed is my pusher performs better with a large diameter propeller, I think it helps to keep the air moving over the tail surfaces. As soon as power is reduced to say 1/8 throttle the rudder becomes in affective. This is probably the fuselage blocking the airflow..I dont know, I did try to make my gyros fuz a thin a profile , the markII version has increased vertical fin area. Looking at the way yours is coming along airflow will not be as restricted as mine.
Just my experiences so far..hope it helps in your design..keep up the great work.


Regards Rich

[JochenK]

Mike,

the red c.g. is the one I got from the CAD program, the black one shows the measured position of the c.g. Steps of 5 mm are just what I'd choose.

If I were building your gyro, I wpould not use that Graupner motor (same thinking as Rich's). A prop of 7.5"x5.5" is too small and has too much pitch. Have a look at this Flyware motor:
http://www.flyware.de/wde/produktinf...x.php?navid=32
This one can drive a 9"x4.7" prop at 8000 rpms, which should be alright. And it's even cheaper then the 350S. Use a three-bladed prop only if you have difficulties with the prop's dimensions.

Why don't you visit the Sinsheim fair next weekend (Thursday to Sunday)? Flyware (Wema) is going to be there.

Jochen

[Hephaistos]

Hello........

Once again, much thanks for the quick answers!

The prop Graupner recommends for the C350S is for a fixed-wing I think. 5.5 inches of pitch are really to much and only for high airspeed. The three-bladed VARIOPROP I have chosen should be adjusted to something in the 3"~4" pitch region to get the power out of the motor, I hope that sounds better.

I am a little in doubt if its better to have two or three blades for the prop. From the things I know until know, a two bladed prop has more efficency on full airspeed than a three bladed prop of the same diameter. But the three-bladed prop should produce more static thrust due to its lower pitch if both props are set-up to match with the same motor. And as the gyros fly most of the time with half throttle, a three-bladed prop could be interesting.

Other possibility is that I only prefer the look of the three-bladed prop. Even though the things You have seen until now perhaps make the impression I hang to the theory that the form is following the function, some small details look like they look, because they are looking better this way.

Your comments about the prop-size concerning the airflow on the rudder make me think about something else: If the efficiency of the rudder is bad if its not working in the props airstream, wouldn´t it be better to place the elevator (or horizontal stab) as low as possible and to put the moving fin on the rudder in front of it, where it can be a little longer? Just a thought........

I played with the idea to place the elevator (or horizontal stab) below the airframe, make it from GF-sheet so its not broken when the tail strikes the ground. And if its already there and stable, I could fix the two tail-wheels to the sides of the horizontal stab. In combination with a steerable front-wheel the ground handling will not be a problem. From watching Rich´s videos of the RPG I got the impression the tail always tends to touch the ground when starting or landing.

Jochen, the motors you linked are looking good, and they are cheaper than the Graupners. Only thing about them I don´t like is the 3.175 mm shaft. I was lucky with the 4 mm shaft of the C350S, because I can replace or modify it without problems, what will be needed to fix the hub of the VARIOPROP.
I just had a closer look at the Minimum-CAD. One CofG bowl is 27 mm, the other 37 mm from the motors thrust-line. But they have both the same coulour, I think they got lost during conversion. So which one is the measured CofG?

Bye
Mike

[JochenK]

Mike,

the upper c.g. is calculated, the lower one measured.

How about Sinsheim?

Jochen

[StephanB]

Mike,
the disadvantage of three-bladed props is one more blade to break and a smaller chance to get away with an unbroken prop in case of not so perfect landings. A two-bladed prop may just turn away from the ground with a little luck, but a three-bladed prop has always at least one blade showing down to mother earth.
Stephan

[Hephaistos]

Hello.......

Jochen, I have to talk about Sinsheim with my girl before I can decide. We go for ski-holidays the following weekend, so I guess she wants me to stay home a help her to clean the appartment and do other things before we go. But I will ask her. Will you visit Sinsheim too? She did not kill me when I ordered the new TX, so I have to be gentle. My father (he builds RC-ships, because they are not as easy to crash as airplanes) was in Sinsheim last year and he told me that its very interesting.

Stephan, your explanation of the disadvantages of three bladed-props is really good. For a tractor-style I would now definitely use two blades. For the pusher breaking blades seems not so problematic to me. The prop is protected by the airframe, and if I crash the prop, I have to crash the rotor before. In this situation replacing some blades on the prop ( 2.30 Euros each) will not be dramatic. The battery placed in front of the airframe for trimming scares my lot more. Time to think how it could be protected.

I wonder nobody shouted loud about the madness to fix the wheels to the horizontal stab. Is it just unusual, but possible, or is to insane to talk about?

Bye
Mike

[JochenK]

Mike,

Originally Posted by Hephaistos

I wonder nobody shouted loud about the madness to fix the wheels to the horizontal stab. Is it just unusual, but possible, or is to insane to talk about?

Nobody understood what you meant.

Jochen