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Phase 5 Remix / Removing the "All Moving" Tail


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Scruffmeister

I've got a fully built "standard" Phase 5, and a Phase 5 plan here. As a Winter project I'm thinking about building another Phase 5 from the plan but with a conventional rather than "all moving" tail. The main aim here is to simplify the build, but first I would like to understand the consequences of this choice.

Can anyone enlighten me in the advantages of an "all moving" tail and thus why Chris Foss would have chosen this for the original? Would changing it to a conventional style would be likely to impact flight performance? Would the horizontal stab possibly need resizing for the redesign, and is there a rule of thumb that could be applied to find the "correct" area of the elevator surface in the redesign?

Many thanks.

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Phil.Taylor

my 2p

Won't make any difference at all - will fly just the same - not worth the bother

Phil.

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wookman

The position of the Foss5 tail lends it's self to being all moving. I'm with Phil on this one. It ain't broke, don't mess with it. The Foss 5 is generally accepted as being one of his best. You  are not likely to improve it. 

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SilentPilot

Doesn't an AMT mean you don't have to worry about decalage issues?

I'd keep it :)

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pete beadle

Hi Scruffmeister

I understand that the main reason that fixed tailplanes were used in preference to AMT's was simplicity and ease of building

If you look at the other Foss aerobatic designs, from trainers to advanced aerobatic designs, you will see they are mostly (all?) fixed tails - Middle Phase, Phase 6 (Sport and Advanced) 

A large part of this is building-in simplicity,-  ease of manufacture, ease of assembly and that old favourite - minimising things that (potentially) go wrong - fixed tails are easier to build, need/have no special setup requirements, and are simpler and easier to  make and set up - AMT's are adjustable and/but require greater skills to build accurately with  minimal "slop" in the linkage

So, the Phase 5 was/is a bit of a break from Chris Foss Designs traditions

In my opinion (and Phil Taylor and/or wookman's - if it ain't broke, don't fix it - but you're not doing that, you're building another Phase 5 from scratch - so it's really up to you.....build it how YOU want, but please consider these answers, as above, before you commit to all this work that, the consensus, so far, thinks will be wasted effort

Good luck with deciding which choice you will eventually take, it IS a very personal choice:thumbsup::)

Regards

Pete

BARCS1702

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Scruffmeister

Thanks all, that's very informative and an excellent point @SilentPilot about the advantages from a decalage point of view. My aim isn't to improve the design and I wouldn't even pretend to be in this category of ability, but more to look at simplification for a quicker build. As @pete beadle notes, none of the other Foss designs use an AMT which is what got me thinking along these lines.

@wookmanthe other part of this thought process is around me learning about design decisions. You note the tail plane position lends itself to  being an AMT, is it typically the case that an AMT would be set slightly higher relative to the main wing? I'm looking at a Phase 5 vs Phase 6 and don't see a significant difference in tail plane position although the Phase 5 does appear higher.

Thanks for all the responses so far!

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mikef

The fixed tail/elevator advantages

easier structurally - loads are not concentrated at the pivot/horn - potentially lighter

servo is potentially less stressed - potentially lighter

you get a camber change as well as an incidence change

 

Ask yourself why Chris’s Foss went back to a tail/elevator set-up on Phase 6.

What's a decalage issue please?

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oipigface
43 minutes ago, mikef said:

What's a decalage issue please?

Strictly speaking ‘decalage’ refers to the difference in incidence between the top and bottom wings of a biplane, but it is widely used to refer to ‘longitudinal dihedral’ -  the difference in incidence between wing and tailplane. Since the incidence of an AMT is easily variable, so is the longitudinal dihedral, whereas with a fixed tailplane, surgery may be required. I have never had a Phase 5 or 6, but I would imagine that the two surfaces are both set at zero, so perhaps it’s not much of an issue.

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Phil.Taylor

One big advantage of an AMT is that it can be made removable for "flat pack" storage & transport. I had a Phase 6 some years ago - the one piece wing & fixed tail were a pain to transport & store.

Ph.

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mikef

 

1 hour ago, oipigface said:

Strictly speaking ‘decalage’ refers to the difference in incidence between the top and bottom wings of a biplane, but it is widely used to refer to ‘longitudinal dihedral’ -  the difference in incidence between wing and tailplane. Since the incidence of an AMT is easily variable, so is the longitudinal dihedral, whereas with a fixed tailplane, surgery may be required. I have never had a Phase 5 or 6, but I would imagine that the two surfaces are both set at zero, so perhaps it’s not much of an issue.

Got it.  The actual issue is that, if you move the CG, you may end up with a separate elevator out of exact alignment with a fixed tailplane.

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Gary B

Just for info the Chris Foss glider designs that use a fixed tailplane and elevator (in no particular order) are:  Mini-Phase,  Force Four,  Phase Two,  Phase Four,  Middle Phase and Phase 6.

Many  of the latest competition model  designs have recently moved toward fixed tailplanes and elevator, the Doroshenko Edge has a moving tailplane and elevator, one servo for each.

 Gary

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oipigface
7 hours ago, Scruffmeister said:

is it typically the case that an AMT would be set slightly higher relative to the main wing?

That’s probably true. The reason is that AMT’s are usually mounted on a bellcrank with the joiners and pivot all in a row. The arm of the bell crank is preferably at right angles to that row. It is also best if it is quite long, because the movement of an AMT needs to be small. The last part of the explanation is that pushrods or snakes are best kept straight. These three requirements are usually resolved by having a straight snake running along the fuselage and driving the bellcrank arm, which means that the pivot of the bellcrank (and of the tailplane) has to be the length of the bellcrank arm above the line of the snake.

With a fixed elevator, the same considerations arise, but the elevator arm is usually short.

If you do decide to fit an AMT it needs to be carefully engineered. It’s easy to make a floppy one.

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Scruffmeister

Thanks @oipigface, that's an excellent explanation. And thank you everyone who chipped in on this - plenty of food for thought...

 

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