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Old May 17, 2001, 05:01 AM
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Multiplex Cargo

There's a topic running in the 'Scale' forum at the moment about the MPX Cargo.

One comment which I find rather disturbing is that the instructions apparently say the model requires a large amount of down elevator mixed with the throttle! Can anyone confirm this?

We've just had the Twin Jet which - great though it is - gave the distinct impression it had been rushed onto the market before it had been fully sorted (even the first batch came with an 'amendments' sheet, and MPX are already shipping a modified version). We're talking basic things here, like the amount of reflex on the wing and elevon pushrods that didn't reach the horns!

Now it's not exactly unusual for a model with a low thrustline (like the Cargo) to pitch up when power is applied. The remedy is simple - you add downthrust. Just like a pylon mounted motor on a glider normally has upthrust to prevent pitch down under power. This is basic aeromodelling and something you would expect to be sorted out in development of the design. Telling people they need a computer radio with elevator/throttle mixing to 'fix' the problem (if this is true) surely is fundamentally wrong?
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Old May 17, 2001, 04:17 PM
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Austin, Texas USA
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I agree. Seems like a pain in the @$$ to mandate a computer radio. The Terry comes with downthrust, as do most other electric planes that require it.

I guess you could also just add half a pound (1/4 kilo) of weight to the nose and that would fix the problem, too!

Shawn
eflightpower.com

[This message has been edited by sf_247 (edited 05-17-2001).]
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Old May 17, 2001, 04:51 PM
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I think my TwinStar has considerable UP thrust and RIGHT thrust, not down, I could be mistaken though....

Rob
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Old May 17, 2001, 05:06 PM
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Austin, Texas USA
Joined Mar 2001
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You are absolutely right. The Twinstar actually has UPthrust to help with hand launching.

I meant my TERRY has downthrust, and requires no trim change to fly well with or without power on.

I have changed the above to reflect Terry, not Twinstar.

Thanks.

[This message has been edited by sf_247 (edited 05-17-2001).]
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Old May 17, 2001, 05:10 PM
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Oxford(ish), England
Joined Feb 2001
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Yeah, TwinStars have UP thrust. Bare in mind, however that it's motors are mounted above the CofG, and thus rotate the aircraft nose DOWN when thrusting, unless angled up to counter this (which works because they are mounted in front of the CofG as well as above it).

Owning both TJ and TS I can confirm that they BOTH behave admirably when flying, whatever else you may think about Multiplex Foamies you can't deny they are pleasent little beasties to fly!

As for the cargo needing mixing... Well I've not heard that, and I doubt it is right. In any case, you can buy 3rd party onboard mixers for not much. From one point of view the TwinJet needs a computer radio for it's elevons, but I used an onboard mixer with a conventional radio, and it works fine. This would be a possible solution, But I doubt the reports are true. Another solution would be to manually "fly" the thing through the trim changes yourself, of course!
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Old May 18, 2001, 05:27 AM
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Quote:
Originally posted by gdb1973:
the TwinJet needs a computer radio for it's elevons
Many basic radios include delta and v-tail mixing, for example the Pico Line tx which is (from memory) only about 50. Even some of my old Fleet transmitters have it!

But throttle/elevator mix is a bit more specialised. You wouldn't want to use an onboard mixer as you may well need to switch it in and out (as the columnist in AMI who put it on his Twin Star found out - each time he went to hand launch it dived into the ground ).
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Old May 18, 2001, 05:38 AM
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Quote:
Originally posted by gdb1973:
As for the cargo needing mixing... Well I've not heard that, and I doubt it is right.

This was the comment I was referring to:

Quote:
Originally posted by Gordon Tarling:
the instructions say that a large amount of down eleavtor needs to be mixed in with motor (50% I think I was told) FWIW, the recommended amount was WAY too much for the first flight attempt, but it still needed some.
Gordon was there when this model was being set up and flown. This was a UK kit, from West London Models.
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Old May 18, 2001, 06:19 AM
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Actually,.. noseweight would exacerbate the problem. There are three "major" things that we use to control trim,..... decalage, thrustline, and CG. All three can affect a pitch attitude change with airspeed. If the CG is too far forward, the plane will pitch up with any additional speed over the speed it is trimmed for,..as the CG is moved back,.. the pitch trim becomes more consistant throughout the speed range. If the cargo has this problem, it needs a thrustline adjustment first and foremost, then possibly a change in incidence of the wing/tail. You can't correct a decalage problem or a thrustline problem with noseweight.

Quote:
Originally posted by sf_247:
I agree. Seems like a pain in the @$$ to mandate a computer radio. The Terry comes with downthrust, as do most other electric planes that require it.

I guess you could also just add half a pound (1/4 kilo) of weight to the nose and that would fix the problem, too!

Shawn
eflightpower.com

[This message has been edited by sf_247 (edited 05-17-2001).]
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