Expo... why?

[Heli_Chris]

Hi,

I've been flying my Blade MSRX since the New Year. Initially I set my radio up with quite high dual rates and Expo with the aim of reducing the sensitivity of the controls whilst I learned a few basic things like taking off, landing and hovering.

Ever since I have kept the dual rates the same but gradually brought the expo down, and I find it much easier to control with less expo. With a healthy dollop of expo I found the mid sticks fairly unresponsive. When I got into trouble I had to keep pushing the sticks as nothing as happening and then all of a sudden an over correction happened and then I was into the seesaw of trying to regain control again whilst my helicopter lurch unceremoniously all over the place.

I've yet to dial in no expo, as I'm just taking it down gently in case it all goes wrong, I'm currently at 10 and going to try 5 on the next flight. I'm beginning to wonder why and when I should use expo? Does it come into it's own with a CP heli?

Best wishes,

Chris

[vinny155]

i think its down to yourself how much expo you use ,if you are smooth on the sticks without it and happy with no interaction apart from which input you give it,then keep bringing it down

[MarkG_R1]

Expo... why?

I think in the early days expo is a big help when you're unfamiliar with the sticks and what they do and as you improve your ability to control the Heli you'll find you need less and less until eventually you might not need any at all

I've hovered a couple of heli's for others and found the expo to be way too much but when I look at the tx settings they aren't using any more than I used to

[scoobystig]

the amount of expo will be dependant on the linkages
Expo should be used to equal out the rotational movement of the servo into the lateral movement of the linkage, it shouldnt be used "to make mid stick soft"

when a servo arm has a linkage at 90 degress to the horn, the first 10-20 degress of rotational movement of the arm produces the most lateral movement of the linkage,
as the servo rotates through its arc, then its effectiveness to produce lateral movement is reduced

EXpo is dialled in so that a servo produces equal lateral movement through out its entire arc of rotational movement

therefore the amont of Expo required will depend on the servo horn, size, amount of movement through the arc and the angle of the linkage connected to the horn

[Andy from Sandy]

I am with you Steve but unfortunately on this forum you are completely wrong. lol.

As explained to me it is for making the servo travel linear with stick movement.

[scoobystig]

well we know Andy

the common misconception seems to be that Expo is for making the centre of the stick "Soft or Mushy", when in fact what you are doing with the expo is equalising the the linear movement of the linkage to the transmitter stick
i have been handed some things to fly sometimes that have 80% expo on them !!! so 3/4 of the stick has no effect on the control surface and then all of a sudden its off n a new direction

[vinny155]

interesting reading,because i am one of the ones that thinks it deadens midstick,however this is down to ignorance on my part,can you explain how you would set this,would it be used mainly for fb helis then as when you set the swash level with the fbl unit,v-bar in this case,it gives you equal movement of the swash through out the pitch range

cheers
vinny

[Andy from Sandy]

interesting reading,because i am one of the ones that thinks it deadens midstick

That is the effect of what is happening. As Steve pointed out around centre you get the most travel of the linkage from servo horn to swash. By adding expo you are reducing the amount of travel around mid-stick. Continue adding expo and you get the other effect of reducing travel which is then the effect you feel in flight.

Expo can be used regardless of fbl or flybar because it is the servo that needs "fixing". As you know with v-bar you can set it in the controller or your TX.

[scoobystig]

vinny

i use Expo on all my models be they fixed wing or Heli
i cant comment on FBL as i dont have any and so have never set any up

there is no set rule for saying you must put so much on, as i say it all depends on the linkage and movement of the control surface, on my fixed wing the ailerons may have 20%, elevaotrs 25% and the rudder sometimes as much as 35-40%

its very simple to understand when i draw it out, but i cant do that on the screen so i will try my best

imagine a servo lying on the table with the servo horn point straight up - we will call this 0degress and the aileron stick is centre, as we move the stick to the fully right the servo horn will rotate to the right to say 90degress from its original position so now is pointing straight to the right

now back to mid stick and we will look at the furthest hole out on the horn, as we move the stick to 1/4 way across the horn will have travelled 1/4 of the way round its arc 22.5 degress, but look at the distance the hole has travelled in a lateral direction to the right it may have moved say 10mm, now the stick moves another 1/4 across to halfway, the horn rotates another equal 22.5 degress to 45 degress, but the lateral movement of the hole may be 8mm
now we move the stick another 1/4 to 3/4 distance, horn rotates a further 22.5 degress, but the lateral movement of the hole may be 5mm, now move the last quarter and although the horn is still rotating a equal arc the lateral movement of the hole may only be 2mmto the right

these are all speculitive numbers and it will all depend on which horn is used and how far from the centre the horn hole is

so when setting up the control surface, we are less concerned with equal rotational movement, but equal lateral movement of the linkage

so in a crude manner yes it does soften the centre of the stick, but only because physics dictates that it is more sensitve in the first place

hope this helps, as i said its easy to draw it

[vinny155]

yes get it,thanks,ill stick to the crude mannerbut good to understand why

[450man]

I am certainly not arguing with the experts here that say expo is to compensate for the rotational movement of the servo vs the linear movement of the rod... however...

There is a significant benefit having a bit of expo I found when learning. I found that it did give me a bit more control and stopped me from over compensating in the early stages. So whilst I agree that it is for allowing for rotational servos I think it is a bit misleading to say that is only what it should be used for. It is a feature of the transmitter that does a specific thing and if that helps learners then why not use it.

I do agree though that it should be used sparingly and with the knowledge of why and there it is probably a good idea to reduce expo once the basics are learned.

[scoobystig]

but thats why dual rates are used, this then gives you a setting that gives you less travel of the servo per travel of the stick, but will still also have expo applied to equal the lateral movement
although the expo on the lowe rate will be less, as there is less rotational movement to need correction

the lower rate then gives you a softer feel to the sticks, but still with equal lateral movement

[Heli_Chris]

Well I was living with a misconception of the whole Expo thing, and really appreciate knowing that it is intended to compensate for the rotational movement of the servos.

This being the case, would I be correct in saying that there is no need to use Expo for the Blade MSRX, Nano, 130X (or any helicopter that has liner servos) as there is no rotational movement in the servo to compensate for?

Thanks, this has been a real eye opener for me and makes a lot of sense.

Best wishes,

Chris

[Hawke]

I couldn't stand any expo when I started flying. I could never understand why someone would want to move the stick without it doing anything. Thanks to this thread I now understand what it is actually used for, cheers.

When I started flying I removed all expo and just reduced the D/R. Once I was able to confidently control the heli I just increased the D/R a little bit every few weeks.

[Mark_T]

Expo can be used regardless of fbl or flybar because it is the servo that needs "fixing". As you know with v-bar you can set it in the controller or your TX.

Just thinking through this though, with most FBL systems you are commanding a rotational shift rather than directly moving a servo, so by default the rate of reaction you get should be linear if you have zero expo. It is the control loop in the FBL unit that has to correct for the non-linear servo movement.

Adding expo to the FBL system really is just changing the sensitivity around the center rather than dealing with servo geometry correction I'd have thought?