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

[Hephaistos]

Hello Mickey.........

Yep, aircrafts have been designed long times with the trial-and error method. I would prefer this method too.
As I have decided to build the gyro with a simple airframe made from square aluminium tubes held toghether with some simple GF-plates to make the experimentaition as easy as possible, I need some plans before I start the build. The time I spend now on lots of details is not wasted, because it creates some deeper understanding for this details. I would like to start the trial-and-error tests better today than tomorrow.............

But as you said, some things can only be prooven by making them work in reality. And by discussing some little details, you wrote something about yawing the wheels on the UC. Before, I would have fixed the wheels rectangular to the airframe. Now I will try to fix them a little bit yawed, so the front of the wheel points a little outwards. As long as both wheels touch the ground, nothing happens. As soon as the right wheel will lift off, the left wheel will drag the gyros tail to the left, and the right wheel will move downwards again.

Bye
Mike

[PeterO_UK]

Originally Posted by Hephaistos

Hello Mickey.........

Yep,Before, I would have fixed the wheels rectangular to the airframe. Now I will try to fix them a little bit yawed, so the front of the wheel points a little outwards. As long as both wheels touch the ground, nothing happens. As soon as the right wheel will lift off, the left wheel will drag the gyros tail to the left, and the right wheel will move downwards again.
Bye
Mike

Interesting idea, but it sounds like you are trying to counter a lift-off roll by appying yaw ? Will that work ?

I'm also no sure that it won't make the model skid (or worse swerve) as the front wheel will no longer be pointing in the direction which the model is moving.

Following on the "over analysis" theme, I would say that it would be better to get a model flying so you can learn how to take off without rolling to the left rather than spend time trying to devise a way to fix a problem that won't exist once you've got the flying skills !

PeterO_UK

[JochenK]

Mike,

look at the attached drawing. On the left is your proposed gyro, on the right a usual one. The dotted lines show the axis over which the gyro is going to topple, the leght of the lines going out from the c.g. shows the lever arm of the c.g.s toppling over the dotted lines. Now, which solution are you going to choose?

Jochen

[Hephaistos]

Hello Peter........

If the right wheel will lift off, the left wheel will yaw the tail to the left. You are right when you say this will not balance the roll induced from the rotor.
But at the same time the bottom of the airframe is moving to the left performing the turn, and as the center of gravity is placed above the wheels, a roll to the right is also induced. And this roll works against the roll to the left coming from the rotor.

I cannot judge if this all will be working like I hope, but I am quite shure that the ROG of the gyro without the rotor will be more stable when the tail-wheels point to the outside. For the RC-cars I know for shure that playing with the jaw of the rear-wheels affects the stability. I think its worth to try, just a simple test with the finished MiniMOB without the rotor can proof it. I can also imagine that the yawed wheels improve the stability when running over some bumps on the ground.

Having the skills to deal with rolling to left during take off or not, a good UC is nothing bad.........
But to get the kills I will have to crash the MiniMOB a little. So there will be more than enough possibilities to modify some of the parts when replacing them.

Bye
Mike

[Hephaistos]

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

Thanks for the sketch you posted. It put me on the right way again.

Having a closer look at my sketches concerning this problems I made yesterday, I discovered the basic mistake. I have drawn a undercarriage with two wheels in front of the CofG. In this case, as long as the rear wheel is in the air, the lever-arm to protect the gyro from rolling over is as half as long as the UC´s width. When the rear wheel is touching the ground, the lever arm is getting shorter, smaller resistance to roll-over, and the ROG-roll-over comes near.

Having a closer look at the Minimum, I discovered that your layout has got a longer lever-arm if the front wheel lifts of and the tail-boom touches the ground! Once again, I have to spend some credits for the clever design.

After this is cleared, I can concentrate on the motor-mount. I will keep the idea in mind to try with yawed wheels on the UC, although I will mount little back from the CofG, this trick sounds still promising to me.

For the motor-mount problem I played with the idea to use two motors in a side-by-side configuration. I know that two smaller motors and two smaller props are not as efficient as one. The advantages of this configuration are the following: The motor-torque don´t needs to be compensated anymore, with using two separate ESC and the second V-mixer I have on the TX I can realize a rudder without a servo, good ground-handling without steerable front wheel, and last but not least: the two small outrunners with matching ESC I already have would match with a target weight of about 500 or even 600 grams.

An other interesting detail is that the plane of rotation is almost on the centre of gravity. Perhaps this fact could create some kind of "all-wheel-drive" , nothing like "slipping" or "skidding" in the turns, just something neutral in between...........

But how an autogyro could be called that is neither a "tractor" nor a "pusher". "tractor" plus "pusher" divided by two is "trasher"! Okay, that sounds a little by like a bad prophecy. Well, I could fix some definitely political incorrect silhoutte-pilot to the MiniMOB, like a punk with some strange haircut in bright colours (never forget the visibility), paint a "A" inside a circle to the rudder and live with it.

, I should have taken the advice two start with a proven design first. Now its to late to stop.........

Bye
Mike

[SafeLandings]

Mike,
Surely you will need extra amp hours in your batterry pack? thus extra weight..2 motors, 2 esc, 2 propellers.. I really think you are trying to over engineer your gyro and will have too many 'possible' things to go wrong and break.
I agree with what everyone else is saying, try a proven design first, get some flying hours in then carry on with your creation. You dont know but you could fly one and think ..'this is not for me'
I also think that you will need a rudder as we all have a little rudder trim dialled in, using motors is not the way to go, as the voltage reduces your yawing thrust will alter??
But I have to take my hat off to you, you are a very creative man


Regards Rich

[Hephaistos]

Hello Rich.........

Well, its some "occupational disease" for me that I always try to re-invent the wheel. My boss exactly knows which jobs are not made for me, its the kind of job like "we have an very exact idea about how the construction should look, we only need someone who makes the CAD"!

BTW, I go to the Sinsheim-RC-fair tomorrow, if somedy is selling the rotorshape, I will grab one. I read about the particular problems of this gyro, but for just gluing the parts together and then crash it, due to absence of expirience, it will be okay.

The extra weight created by the twin-motors should not be so big. I expect something about 20 grams, equal to the weight that falls off by forgetting about a third servo and a rudder. The changing yaw-trim with decreasing battery-voltage can be compensated with switching the ESC´s to their heli-mode (they have it). Then I don´t control the amperage anymore, I directly control the RPM of the props.

Bye
Mike

[PeterO_UK]

Originally Posted by Hephaistos

Hello Rich.........
BTW, I go to the Sinsheim-RC-fair tomorrow, if somedy is selling the rotorshape, I will grab one. I read about the particular problems of this gyro, but for just gluing the parts together and then crash it, due to absence of expirience, it will be okay.
Mike

Mike,

If you get a Rotorshape, have a look at my two pages about them on my web site http://www.peteronion.org.uk/Autogyros/

There are some tips on construction and adding some extra strength to stop it falling apart quite so easily There's also ideas on bigger wheels and adding a rudder.

PeterO_UK

[PeterO_UK]

Originally Posted by Hephaistos

Hello Peter........

If the right wheel will lift off, the left wheel will yaw the tail to the left. You are right when you say this will not balance the roll induced from the rotor.
But at the same time the bottom of the airframe is moving to the left performing the turn, and as the center of gravity is placed above the wheels, a roll to the right is also induced. And this roll works against the roll to the left coming from the rotor.

Have you considered any gyroscopic effects ? Trying to tip the rotor back to level by rotating the rotor shaft may not have the result you expect.

PeterO_UK

[Hephaistos]

Hello Peter........

Until now, I just looked on the things happening without the rotor. Like it would be a tricycle only. For the very first state of ROG, when the rotor is not producing much lift, the yawed wheels could add some stability. I remember that on motorcycles with a side-car the wheel on the side-car is yawed a little to add stability.

I thought about the gyroscopic effects, too. But the situation is quite confusing. I remember Mickey´s theorie that you cannot make a gyro fly a turn with rudder-only, because yawing of the mast produces a nose-down-attitude only. I hope I remember his conclusion the right way.

In this special state of ROG where a roll-over is quiet near, a nose-down moment induced from the cylic effects via yawing the mast should be positive: It would decrease the lift a little and so gives more stability again.

But there can be some other effects coming along, so for this question I suggest to simply try it. Attaching the wheels to the UC in a way they can be adjusted in yaw should not be a big technical problem.

Bye
Mike

[PeterO_UK]

Originally Posted by PeterO_UK

Have you considered any gyroscopic effects ? Trying to tip the rotor back to level by rotating the rotor shaft may not have the result you expect.

PeterO_UK

When I read this again it didn't quite say what I wanted it to say....

Obviously tilting the rotor shaft will eventually move the rotor back to level, but it will happen due to the cyclic pitch that will be applied, not because you've forced it back by moving the airframe back to level.

PeterO_UK

[Hephaistos]

Hello Peter.........

I think there are two things happening at the same time if one wheel lifts off and the wheels are yawed: The tricycle itself will stabilize, because the yawed wheel that is still on the ground will give some input to the system that makes the lifted wheel go down again.
And at the same time a cyclic input on the rotor could by created, which ends in a nose down. And as all the expierienced gyronuts like you have stated: Never pull UP when the gyro is trying to roll over to the side, a nose down cyclic cannot be wrong.

Can you agree with this?

Bye
Mike

[Hephaistos]

Hello..........

I was out to the Sinsheim-RC-fair today. Of course, I spend some money there for things I am not shure if I will need them. The usual thing that happens when you go out and there are lots of traders with "special prices"...........

I found some special solder for aluminium including a fluxing agent. When I came there, the seller just presented how his solder works, and I thought something like: He must be working on steel, this fast how the flux works and makes the solder float. To my surprise, he worked with aluminium instead. I tried to break the parts he soldered: impossible! So I was convinced with his products and spend some money.

I have no idea how the construction of the MiniMOB will be affected from this, because I thought until today that aluminium can only be welded. I know how to TIG-weld aluminium, but for the sizes I have on the MiniMOB its nothing to think about. But I have the feeling that some parts of the finished construction could be made from soldered aluminium..........

Next important thing is the choice of the motor-prop-combination. I had a close look at different brushless outrunners. I decided to use the motor Jochen proposed.
The FLYWARE-outrunners are "Made in Germany", its something that makes a little difference concerning the details of construction and the quality of manufacturing.
The FLYWARE I have choosen takes up to 15 Amps PERMANENTELY at 10 Volts because it has some built-in cooling-vent. Its only about 61 grams.

First, I was a little in doubt of this motor, because it would not fit perfectly with the variable-pitch VARIOPROP I want to use. But the guys from FLYWARE offered to build a special motor for me with a different shaft for an extra-fee of a few bucks. Thats the advantage of products that are not coming from China..............

From the CAD there are some news, too. I have solved the problems with the motor-mount. As I have the VARIOPROP in reality lying on my desk and the drawings of the motor, the next screens will come up soon............


Bye
Mike

P.S.: Jochen, thanks for the tip with the Sinsheim-fair. Being there gave some important inputs. I hope you got home without traffic-jams.

[JochenK]

Mike,

no jams anywhere, except in the bag of things I bought.

Jochen

[Hephaistos]

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

As the choices for the motor and the prop have been made, I decided to have a closer look at my shopping list for the MiniMOB on saturday before I continue on the CAD. I took the weight from the parts I already have on the CAD, checked the sketches of the parts I only have on paper, mades some corrections to match them with the CAD, calculated their weights and summed up.....

The first result was quite a disaster: 680 grams.........
As you can imagine, I did not sleep well the night to sunday.

Sunday morning, I had a closer look again, and looked how it would turn out if I decide to size up to something in the 850 grams region: That wasn´t a good idea, the components did not match very well............

The next step was searching for the sources of the weight-problems. I found several parts that can be build in a different way to save some weight. In the end, I could reduce it to a target-weight of about 600 grams.

The gains in weight in comparision to the original MiniMUM have their sources in some modified details:

Stronger undercarriage with bigger wheels: 15 grams
2,4 GHz receiver with stabilized power-supply: 35 grams
Battery 3 cells Lipo 1250 mAh: 20 grams
Stronger connection rods for the head: 10 grams
V-style tail-surfaces with 2 servos: 20 grams

Total extra-weight from the modifications: 100 grams.

Or to say it with other words: If I would forget about these modifications, 500 grams are possible. I hope that the MiniMUM-rotor can carry this extra-weight. Perhaps it needs to sized-up in diameter a little. But as Jochen estimated correctly some posts earlier that my construction would be heavier than the original MiniMUM, I already enlarged the control-throws for the head.........

I have attached a diagram showing the static thrust versus the motors input-power for the Flyware-outrunners with different props. The different lines end when the motor gets overloaded. The maximum thrust shown in the diagram can thus be produced permanently. The lines for the 3-bladed props are calculated, the assumption was that the 3 bladed-prop needs 1,5 times more power than the 2-bladed one and produces 1,3 times more thrust. This creates a little loss in efficiency. These factors I extracted from several diagrams I found on the VARIOPROP-website. I will use the 320-1250 outrunner, its 61 grams. 700 grams of static thrust should be enough for 600 grams of autogyro. The lines for the stronger 330-1250 (91 grams) have been for the 850-grams upsized MiniMOB.

Bye
Mike