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Wot 4 Foam E Mk2 - CoG

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John Roberts 919/09/2018 15:19:45
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167 forum posts
167 photos

Hello,

I have just finished assembling a Wot 4 Foam E Mk2 which I bought to use as a winter hack.

Although it went together very quickly and without any issues I was a bit surprised to find that I needed to add 70 grams of nose-weight to get it to balance at the recommended 70mm point. I am using a 3S 2400 Hacker LiPo (185g) which is probably a bit heavier than the 2200 recommended by Ripmax so the need for additional nose-weight was doubly unexpected. The instructions say it ought to balance correctly with no adjustment.

I suspect that the model will happily fly with a more rearward CoG and the instructions mention adding tail weight to "provide extra manoeuvrability for experienced pilots" so I will probably find myself removing some or all of the extra weight after its first flight!

Anyway, I am just curious to know whether any other owners of this model have encountered the same issue. A friend who owns the Mk1 version confirmed that he didn't need any extra weight to balance so it might be just a Mk2 issue.

brokenenglish19/09/2018 15:28:12
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342 forum posts
24 photos

Ripmax have been very bad on this. Their web site states that there's an error in the instructions, and that the CG should be at 80mm from the LE (not 70). This has been known for the last year or two, but they still haven't changed the instructions!

You'll find there are two ballast weights under the fus. at the rear end (keep them!). I diligently removed these, to get the 70mm CG, and it cost me the model...

John Roberts 919/09/2018 16:12:19
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167 forum posts
167 photos

Thank you Brokenenglish smiley. I appreciate you taking the time to post.

I agree that it is very poor indeed for Ripmax to continue selling the model including instructions containing such a fundamental error.

I am pretty sure I know the answer to my next question but I will ask it anyway - did Ripmax replace your lost Wot 4?

John Stainforth19/09/2018 18:13:41
223 forum posts
38 photos

I had the same problem, and I added a little weight to the front. Without that the CG was too far back for my liking (even though I usually like my CGs fairly far back). Overall, I do not rate the model very highly for the price.

brokenenglish19/09/2018 19:30:31
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342 forum posts
24 photos
Posted by John Roberts 9 on 19/09/2018 16:12:19:

I am pretty sure I know the answer to my next question but I will ask it anyway - did Ripmax replace your lost Wot 4?

I'm ashamed to admit that I didn't even ask, or even inform the seller (Inwoods), who give great service and it's not their fault.

Furthermore, it's the second time that a gross kitting error by Ripmax has cost me a model.

Dave Hess19/09/2018 20:27:38
148 forum posts
15 photos

I have one and use a 2200mAH 3S pack. Mine's the BNF one that comes with motor and servos so I didn't bother to measure the C of G. i just put my fingers under the wings about 1/3 of the way back to see if it was there or thereabouts, which it was. It flies perfectly. On low rates, it flies like a trainer, and on high rates it can fly a 10ft vertical square.

I just measured it at 90mm from the leading edge.

As a general fun plane, I think it's pretty good, but it could do with a bit more power and maybe a servo upgrade to unleash its full potential.

Edited By Dave Hess on 19/09/2018 20:31:32

John Roberts 919/09/2018 22:10:59
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167 forum posts
167 photos

Well I have now removed all the extra weight that I had painstakingly hidden within the tight confines of the cowl and the plane balances at the 80mm point. This ought to be a safe starting point given that Dave Hess' version flies well at 90mm. Just need a calm-ish day to get it maidened and trimmed out.

For what it is worth I intend to take Ripmax to task about their failure to notify purchasers about the error in the manual. How hard can it be to put a note or sticker in/on the box? Relying on a statement on their web site just isn't good enough.

Thanks to everyone above who responded to my postsmiley!

brokenenglish20/09/2018 07:06:57
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342 forum posts
24 photos
Posted by John Roberts 9 on 19/09/2018 22:10:59:

For what it is worth I intend to take Ripmax to task about their failure to notify purchasers about the error in the manual. How hard can it be to put a note or sticker in/on the box? Relying on a statement on their web site just isn't good enough.

Thanks John, It's clearly unacceptable that Ripmax should consider that anyone buying, and then carefully assembling and flying a model, must consult their Web site to avoid a disaster.

Peter Christy20/09/2018 09:07:17
1064 forum posts

I had the same problem and it cost me a fuselage! I added weight to the front to get the CofG right as per the instructions, and it kept trying to dive into the ground. Whilst frantically reaching over to dial in a load of up trim, I managed to switch off the Tx!!! Being a computerised tranny, by the time it had come back up, the model was already in the deck!

The replacement fuselage came with a sticker attached warning of the correct CofG which was spot on without any nose-weight!

I'm not a fan of the model - the Foam-E version, anyway. Its OK for stick banging, but is divergent in pitch, presumably due to the VERY floppy tailplane. I bought it as a hack for testing suspect radio gear, but the radio compartment is too cramped for anything but the smallest of receivers. I won't be buying another when this one finally goes....

--

Pete

John Roberts 920/09/2018 11:37:52
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167 forum posts
167 photos

I have dropped a polite but forthright email to Ripmax regarding this issue. It will be interesting to see their response. Some on this forum have found Ripmax to be both quick & helpful when responding to queries/issues whereas others say they hear nothing.

From time to time all companies will encounter faults with their products - the true measure of a good company is how they react & respond when it happens.

David Mellor20/09/2018 12:22:28
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950 forum posts
339 photos

One option is to use an on-line CoG calculator for the entire airframe (it takes into account the horizontal stabiliser shape, size and moment arm).

As a generalisation, most designs on plans and models publish a CoG that is conservative - that is to say it uses a big static margin for safety's sake on the maiden flight.

This CoG calculator lets you choose your own static margin so you can very accurately confirm the proper range from the initial maiden CoG (typically 15% static margin) to whatever you feel comfortable with based on experience (say, 4 or 5% static margin):-

**LINK**

Using the calculator takes just a few minutes and could save your model........

Edited By David Mellor on 20/09/2018 12:24:10

Mike T20/09/2018 12:25:06
353 forum posts
27 photos

I don't usually do repeats but in the W4f-e's case, I'm on my fourth. The other 3 were lost to radio problems or excessive "exuberance"...

All were assembled and flown out of the box - I didn't pay any attention to the CG as I was using the recommended battery size (3s 2200) and all the other gear was 'in situ'.

All were slightly nose heavy requiring a little up trim - my latest more so as it has been 'up-gunned' with a 3542/1250 motor and 60A ESC. This required a little more up trim for it to fly S&L hands off. I've never had any pitch divergence issues. My tails have not been floppy, but they were/are all 'flexible'. The reinforcing spar does its job and the retaining screw must be well tightened. All used full-size JR (35) or FrSky receivers, which fitted snugly in the space provided.

A more versatile model I've never had and I suspect its sales indicate many others think the same. That's not to excuse any failure to amend critical information, BTW, it's just that I never found it to be an issue.

As usual, YMMV...

Mark a20/09/2018 13:46:10
319 forum posts
3 photos

I don't think I even bothered to balance mine just chucked in the recommended battery and flew it trimmed it out and that was that, been my hack ever since.

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