AcroWot Built up wing

[Martyn K]

I managed to break my second foam wing on my Acrowot at the beginning of December. I spun the model into a tree and sacrificed the wing but (mainly) saved the fuselage.

I couldn't go back to Chris Foss and ask for another wing = only 3 months since the last one was dispatched so I decided to build up a wing. It wont be any cheaper, but hopefully, it will be lighter and I will also be able to add a couple of mods that I wanted to try out that will be incorporated into the Magic.

Basically, I want:

Snap flaps, Flaps that go up and down in sympathy (but reverse direction to the elevators.

Ordinary flaps with crow brakes. To try and slow the model down a bit, I want to be able to lower the flaps independently and lift both ailerons simultaneously to act as air brakes.

Finally, I want to be able to disable the flaps as well.

To get the flaps in, I needed to reduce slightly the length of the strip ailerons. As it is the inboard area that is affected I do not think it will have serious impact on the models roll rate - which can be very fast if you are enthusiastic with the throws.

I thought that I would show a demo for PROFILI PRO showing how a wing panel is created.

So the Profili bit first...

Step 1.

On the opening screen for a new panel, you input the basic measurements (up can select imperial or metric from the preferences option). The Acrowot is definitely a good old imperial design..

Step 2

We set the rib spacing. You can have ribs set equidistant - as they are here or choose specific spacing starting from the root and working outwards.

Step 3

Choose the wing section. I imported the section from the wing tip (and hoped it was the same at the root). I don't think its a standard section, but you can choose different tip and root sections and the software will blend the intermediate steps

Step 4

Add sheeting (or not as the case may be). I will be using a LE 'D' box with webbed spars and cap strips over the rear of the section, so I have selected effectively a 1/16" all over sheeting. Not that this sheet will appear to overlap the LE and TE sections and you will need to compensate for that.

Step 5

Add a LE - I am planning to use a 1/8" false LE plus 3/8" real LE - a total of 1/2". Note that you can taper this and or choose different shapes if required

Step 6

Repeat for the TE. The ailerons on the Acrowot are 1.25" wide plus 1/4" false TE - total of 1.5" from root to tip.

Step 7

Add Upper and Lower spars - you can add as many as you want and taper them if required. What is not shown is the 1/16" web which will be added later. Note that the spar is very slightly raked forwards from root to tip.

Step 8

Add some lightening holes. The ribs will be laser cut from 3mm poplar lite ply so big lightening holes

Step 9

Add a building tab (or two. I settled on one tab at the TE with no height offset. You can set the width and various distances to get the tabs positioned just how you want them..

Step 10

Print them off. I prefer the old obsolete way. Note that I have set the paper size as a sheet of Lite ply. There will be ample space for 2 complete sets on one sheet of wood.

Click finish

The ribs are then exported to DXF then edited (using Draft sight) to remove the sheet skin and other bits that are not required.When happy, email them to your friendly laser cutter and 2 weeks later you will get back a full set of perfectly cut ribs. (I used Belair).

This was back in early December, I got the ribs back just before Christmas and started the build on Christmas eve.

More to come.

Martyn

[FastFlyer Smyth]

Any chance we could see the build Martyn ?

[Dave Miller]

Why not just mix the standard ailerons to give the flap movement that you described, it would be easier than separating them and save two servos. I think the section is a modified Wortmann, it certainly looks like it to me.

Edited By Dave Miller on 28/12/2013 19:27:35

[Martyn K]

FF, I lost a couple of the build photos when the camera died so you will have to visualise what I am explaining - sorry. Build info below..

Dave, yes, I think that I could do that, but as I explained, I am trying to replicate the configuration used by Hanno P on the Magic, I think it's worthwhile persevering with - if my transmitter (Futaba T8J) is capable of the mixes required...

----------------------------------

I started work on the wing on Christmas eve. I originally planned to demonstrate how to produce a disposable single use plan - a worthwhile exercise before starting building as it allows a sanity check before you start cutting. So I did another step guide, produced lots of photos and then the camera threw a wobbly and trashed the memory card. Sadly, I lost all the images (including some early build shots). However, if there is enough interest, I'll do it (produce a plan) again (as a retrospect) to show how quick and easy it can be.

The plan was produced on Christmas wrapping paper - nice, cheap and plentiful at the time of year!

Start by selecting 4 medium hard 1/4" square balsa spars and pin one down over each half of the plan.

Place each rib in the correct location and check that it is square (upright). Glue them (I use PVA) and allow to dry. At the stage, omit the centre ribs. They will go in when the dihederal is set.

Cut the false LE (from 1/8" sheet) and glue

Cut the false TE from 1/4" sheet and glue. Note that false LE sits flush with the rib extremities, the false TE is 1/6" thicker top and bottom to allow the cap strips to but into place. This is now a very good test of the build accuracy, personally, I left mine 3mm top and bottom so I could trim back down. Seems safer...

Lower spar, false LE and building tabs...

Notice the hand drawn and annotated plan..

When dry, add the top spar. You will need to trim this to length. The original instructions for the AcroWot says prop the centre up by (IIRC) 3/4". Note - this is not the same as propping the tips up be 3/4" (due to the taper of the wing) - ask me how I know that,... You will need to trim the LE, the TE and the spars so that all sit correctly when propped up by the correct amount (do this without the centre ribs in place, Make sure the LE sweep is correct (a good reason for drawing a plan).

When happy, glue the 2 centre ribs together, glue the spars and LE and TE and slot it into place and leave it for 24 hours for the glue to set properly. As there is no bracing at the moment, the wing will be quite fragile.

The single dihederal brace is made from 1/16" ply. I simply placed one wing flat and then measured the height of the upper and lower spars and marked it up accordingly. The brace goes out 2 bays each side of centre.

Carefully cut through the centre 3 ribs and take out a slot 1/16" wide. It isn't difficult, in fact it is easier (once the cut behind the spar has been completed, to remove the rib and remove the remainder of the slot using a scalpel.

Glue it all back together and clamp firmly. There is a lot depending on the integrity of this joint. Let the glue dry..

Add the full depth 1/16" webs behind the spar.

Solid block (from 1/2" balsa sheet) is added at the centre TE. This will support the wing mounting bolts

The LE is braced at the centre with 1/16" ply doubler (behind the LE) and 1/2" sheet behind that - these will support the wing mounting dowels like the original.

Servo mounts are 3mm lite ply, inlaid 1.5mm into the ribs (leaving 1.5mm proud) which will be at the same level as the LE sheet and cap strips.. There is a 3mm doubler across the underside of the servo trays to give the screws something to bite into. You can just see this at the front of the orifice in the photo below. Make sure it is well glued, you dont want these to come adrift when trying to fit the servos

Repeat for the flap servo. The layout is that the centre 2 bays are solid TE section, the flaps are 6" long (2 bays out each side) and the ailerons will go right out to the wing tips. You can also see the cheapo flap torque tubes as well. The ailerons have been salvaged from the original wing and will be trimmed and reused.

More to come

 

Edited By Martyn K on 28/12/2013 21:48:32

[Martyn K]

Now add the LE sheeting. Along with the webs, the front structure now forms a tube and it will get most of its torsional rigidity with this combination. This is the reason why I always try to put webs at the rear of the spars for most impact.

Lots of pins.

The real LE (cut from 3/8" sheet) is added after the sheet has dried and has been trimmed back. I use a sanding block with 80 grit paper.

The 1/2" wing tip blocks have been added as well, simply glued to the end rib and LE. The AcroWot had quite elegant swept wing tips, this is the 'clipped wing' variety, I did contemplate raking it out by about 25mm by laminating then cutting the wing tip from 2 pieces of sheet, but decided not to. It will have to do..

The centre sheeting has been added, 2 bays out on the underside, 1 bay each side on top of the wing

The excess is cut back when the glue has dried. Note the bands and clamps etc are to hole the LE in place while the glue holding that dries.

More to come (but probably not much more). If anyone wants more info on the wing with a view to building one, I can let you have the DXF files and some pertinent measurements as well.

Best wishes

Martyn

[Masher]

Martyn

This is a very informative step by step guide which I have found very useful. I for one would be grateful if you recovered the lost information you mention.

I actually made this which flies just as well as the kit and ARTF Acrowots (which I love). The wing plan gives some good info and various options but it doesn't do the step by step guide which you have provided. I find this type of "teaching" is great for us "improvers"

Being a bit cheeky, it would be useful to know rough idea of costs to get the laser cutting done.

Thanks, I look forward to more!

Martin

[Bob Cotsford]

I've got to admit that when I needed a new wing for my AW I sent BillKits a drawing of the root and tip with appropriate dimensions and got a set of balsa clad wings back. I'd also be interested in hearing the cost of a one-off cut rib set for comparison.

Will they be straightforward flaps or combined flap/airbrake (ie with the top surface extending forward of the pivot line)?

[kc]

Presumably Bob meant a balsa clad foam wing.

For comparison with Martyn's built up wing the weights of Acrowot wings I have are:-


Chris Foss kit foam wing , covered in film with decals and 1 servo exactly as flown 0.595 kilo

Home cut foam wing obeche veneer covered with tissue & wallpaper paste then hand painted with Japlac including 1 standard servo 0.625 kilo

Be interesting to see how much a balsa wing can improve on this.

Edited By kc on 29/12/2013 14:41:29

Edited By kc on 29/12/2013 14:51:45

[Martyn K]

OK - more progress

Additional blocks for the hinges. I am using 'proper' SLEC hinges with wire pivot pins.

LE now fitted and planed then sanded to shape. TBH, the wood is a bit soft - cut from 1/4" sheet. I wish that I had something harder available, its resistance to hanger rash is the primary concern here

I have used a 1/16" ply plate to bring the servo leads through. This also has an additional benefit in that it is a good bearing surface for the fuselage side - the lite ply on the other side will also do the same thing.

The lower sheeting is now completed. - all going together very well..

More to come...

Martyn

[Martyn K]

Continuing...

I used the hinge slotter for the hinges. I have never really got on with installing hinges no matter what tool I used, but this time it seems to have worked OK.

Test fit the hinges - 2 for each flap, 3 for each aileron...

Then a test fit for each flap and aileron... I have extended the flaps by about 1/4" - if it is good enough for a Curare then its OK with me...

The joint at the wing tip to TE has been boosted with some carefully carved 1/4" sheet gussets. Note the grain goes down the longest side. They need to be trimmed to fit under the cap strips.

A test fit against the fuselage side. I am not a million miles out with the wing section. I have worked out the section is probably about 3mm thicker than the original so a bit of sanding of the wing seat is required to get a nice fit and keep the same wing incidence.

And another shot.....before sanding..

 

 

A bit of a gap while I covered the wing.... (Oracover top and bottom)

Not a million miles from the original..

 

And the underside.

The AUW of the wing with 3 servos fitted (Cheapo Futaba S3003 I think)

Somewhere between 660 and 670 grams... Quite pleased with that..

 

A short vid will follow..

Martyn

Edited By Martyn K on 01/01/2014 15:01:58

[FastFlyer Smyth]

Very impressive and very informative Martyn.

[Martyn K]

Thanks FF

I'll do the plan demo shortly.

A set of 3mm poplar lite ply ribs cost about £40 plus postage. If I was building this from balsa for the ribs and hand cutting, I would need about 8 sheets of wood at about £2.00 per sheet. But they would be no where as accurate as these.

I'll also update the DXF files and leave them for download from my website. (There is a minor error in the original set so may as well get them right).

Hope that is OK.

Martyn

[Bob Cotsford]

Oi! Slow down will you. You're putting me to shame on the Mustfire build though I do have the excuse of having spent the last week decorating.

I'd be lost without my Robart hinge slotting kit, they really are brilliant.

And yes KC, it was a balsa covered foam. Probably took me as long to complete as Martyn took to build this from scratch

[Martyn K]

Here is the youtube video of the flaps in operation. It was much easier to set up than I expected. (Apologies for the dreadful audio)

 

 

 

I have reprogrammed the flap channel to operate from DT6 rather than the VR. Apart from that, there are 2 standard program mixes which are operated from one of the 3 position switches, Moving the switch up activates the coupled snap flaps, moving the switch down activates the crow brakes and down flap. With the switch in the centre position, the flaps are disabled.

 

I haven't measure the deflection yet, but the snap flaps seem to move up and down about 15 degrees with the elevator and the flaps go down about 45 degrees and ailerons up about 20 degrees in brake mode.

 

It was a bit of a tight fit getting the flap actuation rods under the wing mounting plate, but it doesn't seem to be binding..

 

Martyn

 

Edited By Martyn K on 01/01/2014 15:52:00

[Martyn K]

Sorry Bob

I missed your post yesterday... My wife broke her wrist at her Christmas works bash just before Christmas, so apart from being saddled with all the household chores (I normally do the cooking), I have managed to get quite a bit of shed time as well. I would have missed this model, its my normal daily hack and I was hoping to get it ready if we had a decent flying day. Sadly, the only two decent days that we had over Christmas was boxing day (12 round for dinner) and the 29th - my sons birthday bash. So no flying over this Christmas break at all

Anyway, its complete now and ready for yet another maiden, I'll try and keep it in 1 piece a bit longer this time.

Back to the Magic..

Martyn

[Martyn K]

Zipped DXF file can be downloaded from here:

AcroWot wing ribs

Regards

Martyn

[Jon Laughton]

I'm very impressed Martyn - well done!

[Martyn K]

Thanks Jon

It will be interesting to see just how effective those flaps are.. I have never tried a model with flaps before

Martyn

[Jon Laughton]

I think that's a really great addition for a great model and await the results of your flight trials with anticipation!

I think your design might prove popular...

[Jon Laughton]

And if I had my old built up Acrowot (sold circa two years ago) I'd have been tempted to follow suit...

[IanR]

Very interesting Martyn. I am hoping to fit 2 aileron servos in a Precedent Hi Boy foam wing instead of the one centrally mounted servo. I had planned to fit flaps, also, in the same way you have done but a couple of respondents to my questions on the forum have made me give up on that idea, suggesting that the Hi Boy doesn't need them and that I should keep things simple. A couple of questions, please.

I note that you've used standard size Futaba S3003 servos throughout. Since the original design calls for a similar standard size servo to be used centrally, for both ailerons, do you think you could have used smaller, less powerful servos, for each aileron as the load has effectively been halved? Ditto the flaps - could you have used a smaller servo there? I'm hoping you will say "no problem" to smaller servos since I have now placed my order for HK 10g servos which are supposed to give up to 2.5kg torque.

The flaps - presumably their leading edge is the same thickness as the ailerons' leading edge. Since they are 1/4" wider, does this not alter their profile compared to the ailerons? If so, will this not alter the wing's profile during normal flight?

Really looking forward to your flight report. I am sure there are some who might say that the Acrowot doesn't need flaps or 2 aileron servos but the way you have set yours up, on your video, will surely add a bit of interest and spice to your flying sessions with the model.

Ian

[Martyn K]

Hi Ian

Some good points.

I suspect that you could get away with smaller servos if required, however, this model (with a .61 up front) doesn't hang around and I have edged on the side of safety. Chris Foss specifies two standard servos for the later version of the foam wing, so I have stuck with that. Additionally, it's what I had available... Regarding torque settings, unless you are really way out with the size and speed of the model, most servos on the medium range will do for most medium sized models, I think its more important to select a servo based on its perceived ruggedness rather than an arbitrary torque setting which I don't think will ever get really tested. Blow black is more likely to occur with flexing of the control surface than a servo unwinding. But bear in mind - I am not dealing with extreme flying here, just average clubman stuff.

Regarding the profile, the section on the original design is a little unusual, the TE at the point of the ailerons reflexes slightly - top and bottom. You are correct to note that the section at the point of the flap is different - the false TE hasn't changed here, but I suspect that the effect will be minimal. I have severe doubts as to whether the flow over the wing is still laminar at the TE anyway, in which case it wont make any difference at all.

We will see - all I need is day when its not blowing a gale and I am not working. Today would have been good, but I am back at work..

(typical!)

Martyn

[IanR]

Hello, again, Martyn. Thanks for the reply. I've got up-to-date Wot 4 and Acrowot instructions, including the 2 servo wing addendum, but I can't see any reference to servo size. I do have a Wot 4 Mk 3 kit, however, and what I find puzzling is the fact that the pre-made aileron servo cut-outs are too small for standard size servos.

Also, Andy Butler's recent excellent Wot 4 build blog shows how he had to increase the size of the servo cut-outs to accommodate standard size servos. I'm wondering if Mr Foss isn't suggesting that we should use smaller size servos, or is it just that he didn't have a larger diameter router bit!

I'm going to try and get in touch with the man himself and ask him what size servos to use.

Ian

[Martyn K]

Hi Ian

The servo cut outs in the standard (AcroWot kit) wing have rounded ends - I presume this is to make manufacturing easier and when lined with 1/16" balsa, they are (I agree) too small for a standard servo. However, I simply enlarged mine when I squared off the ends. I think a midi servo would probably be OK, however, I would definitely stay away from the mini servos for a model this size.

It really all depends on how violently you fly, if you fly sedately with gentle manoeuvres then it should be OK with a lightweight low torque spec servo. However, for a model this size, its not worth worrying about. My AstroHog has a pair of MG996 servos fitted - they are very (55g) heavy and way out in the wing tips - just where you don't want the weight/inertia. I am thinking of replacing those with something lighter.

 

Martyn

Edited By Martyn K on 02/01/2014 13:12:16

[Martyn K]

Posted by Martyn K on 02/01/2014 09:48:59:

Zipped DXF file can be downloaded from here:

AcroWot wing ribs

Regards

Martyn

Hi folks

Apologies, I have just discovered an error in version 2 (incorrect TE length) version 3 is now corrected The TE width was incorrectly set at 1.25", it should have been 1.5".

The DXF file has been corrected and reissues on the same link (above) but now zipped as v3.

Regards

Martyn