Jump to content

Kwik Fli 40


Martyn K
 Share

Recommended Posts

Posted by kc on 13/10/2016 19:00:26:

My view is that at least the stub end should face backwards not forwards. Facing backwards it would push into the rib. But vertical is much better in dealing with the force.

Edited By kc on 13/10/2016 19:12:22

I am not sure I agree with that point

It is true that the rotation will try and lift the anti rotational saddle clamp. However, if the arm was facing backwards, then the whole undercarriage will try and lift - have a look at the last 2 pages of Danny Fenton Chipmunk build to see what happened

I think that your idea of adding a hardwood block in this area will suffice.

Martyn

Link to comment
Share on other sites

Martyn, I'm with kc on this. I've used vertical arms for years and they've survived some pretty horrendous landings. The ultimate is to set a ply plate under the top skin to take the end of the vertical part of the leg as this gives the best moment to hold it steady. Next is a vertical block attached to a ply rib. I also happen to think it looks neater, especially if the horizontal part of the leg is set in flush with the wing skin but maybe that's just me being picky.

Link to comment
Share on other sites

The problem I have is that I am trying to avoid birch ply wing ribs.

The undercarriage block would sit about 1/3 back between the front and rear spars = and there is no easy bond position for this without adding lots of weight.

I have added a 12x6mm birch block that is bonded to the lower front spar and sits under the ply plate. The front anti-torsion saddle is screwed into this. The birch block goes across the wing and links both u/c legs

How does that sound?

Martyn

Link to comment
Share on other sites

It sounds ok, but if it were me, I'd still go for a partial ply rib doubler between the spars to take a vertical block - I've had models with a horizontal torque arm and while they worked most of the time, there were incidents using them.

Each to there own, at least Danny's experience gives a pointer as to what not to do!

Link to comment
Share on other sites

This is what the wing root looks like:

wing-root.jpg

Two main spars with doublers going out (tapering over the last bay) to W4

1/16 ply brace centre 2 bays

1/16 balsa webs from W2 > W10

12 x 6 mm birch block linking the 2 u/c legs

9mm triangle bonding the block to the lower spars/webs

Ply U/C plate from W2 > W4 as shown on the plan

If that breaks its because you have crashed

Martyn

Edit. There is a 3mm balsa infill (under the birch block) between W1 and the edge of the ply plate to accomodate the dihedral angle

 

Edited By Martyn K on 14/10/2016 13:09:02

Link to comment
Share on other sites

By using a central rib of ply and then extending the u/c wires to the centre you can have a longer torque rod ( must be better) and attach the vertical block direct to the central rib. Centre rib could be 1/8 on each wing half which makes 1/4 and is the right size so a slot takes the dowel. Then the dowel is also clad either side with lite ply, just as in Peter Millers Ballerina etc. Note that Peter says install one half rib but leave the second half unglued until joining the 2 wing halves, then it is mated to the first half at the correct dihedral.

Only reason for the original Kraft design to have the shorter u/c wires is to leave room for a central aileron servo, Martyn is using 2 servos out on the wing so no need to stop the u/c short. Of course slightly longer wire means slight extra weight but the strength of the construction surely makes that worthwhile.

Link to comment
Share on other sites

Are the spars balsa or spruce or cyparis etc?

Reinforcing the u/c blocks to spars may have a disadvantage. In a really heavy landing it may be better to have the u/c rip the central rib apart rather than the spars.   Also having both vertical blocks together strengthens both.  ( Vertical blocks dont have to be true vertical or the u/c can be bent to the dihedral angle instead of 90 degreees.)

Edited By kc on 14/10/2016 13:28:15

Edited By kc on 14/10/2016 13:30:44

Link to comment
Share on other sites

Absolutely loving this. We don't even have a short list to vote on for the Mass Build yet, and we've already got a positive debate going about the merits of various ways of doing things smiley

Shows that the Kwik Fli options are certainly popular!

Link to comment
Share on other sites

Ian. I think we are just kicking around a few ideas before Martyn draws up his 2 plans .

I suppose we can only consider a proven design to recommend as a Mass Build so I guess Martyn will have his design ready before anybody makes a decision on which model. As the Kwik Fli is a proven design in several sizes there are no worries about how it will fly, it's just getting a plan together in a form that will show the construction that is certain to work. Frankly I would like to see a prototype built from Martyn's plan BEFORE we suggest it as a Mass Build for newcomers to balsa construction. Otherwise it would have to be for experienced builders who have the knowledge to modify as necessary. Having a clear idea whether the CG comes out OK with the rear mounted servos is one aspect.

Link to comment
Share on other sites

Posted by kc on 14/10/2016 15:45:30:

Ian. I think we are just kicking around a few ideas before Martyn draws up his 2 plans .

I suppose we can only consider a proven design to recommend as a Mass Build so I guess Martyn will have his design ready before anybody makes a decision on which model. As the Kwik Fli is a proven design in several sizes there are no worries about how it will fly, it's just getting a plan together in a form that will show the construction that is certain to work. Frankly I would like to see a prototype built from Martyn's plan BEFORE we suggest it as a Mass Build for newcomers to balsa construction. Otherwise it would have to be for experienced builders who have the knowledge to modify as necessary. Having a clear idea whether the CG comes out OK with the rear mounted servos is one aspect.

Sure, I get all that but

- there are plenty of plans and kits already out there with things wrong with them wink

- Martyn knows what he's doing

- probably needs someone else other than the designer (i.e. someone who doesn't necessarily know what was intended by the designer) to do a test build and unearth any wrinkles

- this is based on an existing proven design so highly likely to be well "in the ballpark". I agree that wouldn't apply if it was a completely new project

TBH the point about the cg/rear mounted servos isn't really a factor imo. Sure, it's good to know that you're in the ballpark "as drawn" but there are plenty of RCM&E plans where the gear appears to have been drawn in just to give a rough idea, and is miles away from the actual placement needed for the cg - ask me how I know frown. That's part of the learning process during the build

Link to comment
Share on other sites

Re: the undercarriage bracing. I had a similar dilemma when I built my Kingpin. I did vary from the plan slightly, and so far, it has proven robust enough.

Firstly, I made the u/c "block" out of 3 pieces of plywood - the main load bearing bit, with two thinner sections glued: to it, with a gap just wide enough for the u/c wire to fit in:

I put thin ply bracing strip either side of the end ribs supporting the u/c block, and then glued a shaped piece of maple engine bearer wood to the outside of the inner rib. Its just about visible in the picture, if you look carefully. Then all I had to do was drill down through the gap into the bearer to locate the inner ends of the torque rod.

This makes it very easy to remove and replace bent undercarriage legs, and has so far proven strong enough for the job.

I found this a much easier approach than carving a groove in hardwood bearers for a thickish u/c wire! The wire is held in place by a couple of thin metal strips - they don't need to be that strong, as they never carry any load. The ply has so far proven strong enough to withstand my landings (!), and there is no sign of the fixed end of the u/c shifting in the maple blocks either.

Just a thought!

--

Pete

Link to comment
Share on other sites

Correction. I said "or the u/c can be bent to the dihedral angle instead of 90 degreees." but of course that is wrong! The wire won't install flat on the bearer unless the stub is bent at exactly 90 degrees. Nobody else noticed......

Link to comment
Share on other sites

Thanks for all the suggestions so far. I think that this is what I have decided on:

wing-root2.jpg

12x6 Beech block (turquoise) spanning centre 2 bays and bonded to the ply dihedral brace

W! is 6mm balsa sandwiched between 2 3mm lite ply ribs Yellow/red

W2 is now a full length lite ply rib. (red)

This W1/W2 combination also sorts out another problem. Securing the wing bolt plate,

The U/C plate is still 3mm birch ply

W3 and W4 still have lite ply doublers.

I have also added the webs and the centre brace now and generally tidied up the wing. It looks like the ailerons will work now

That should be strong and light enough.. Remember its only a sub 2kg 40 sized model

I am away for a few days (back Wednesday Evening) - may only get updates from the phone. I'll update the PDF plans when I get back

Now I am working on the front fus for the E version. The ic version is much simpler

BTW, I really enjoy these consultative design exercises. They stop me making silly mistakes so please keep up the ideas and feedback

Thanks

Martyn

Link to comment
Share on other sites

I still cannot see why a horizontal stub end to u/c wire is chosen! The force from a hard landing goes in a different direction - downwards rather than forwards. In a hard landing the screws pull out. Maybe that is the reason - a weak link? But if they hold then the wing spar & doubler might get ripped. One hard landing might wreck the wing spar. Not easy to replace the eventually loose screws with bolts and nuts as the wing sheeting is over top.

A drilled vertical wood block on W2 would be lighter, easier, cheaper & if it did rip out it probably only damages the rib not the spars. It is also aerodynamicaly a bit cleaner ( no spanwise saddle ) Also the u/c wire can be formed in a flat plane which is easier - doesn't need to be handed unless it is also raked forward.

Instead of a wood block it might be better to use laminations of birch ply to hold the torque rod stub end.. Lots of fuselage designs use a 3mm ply with slot for wire and a covering ply, then glued to fus doubler ( often !/32 ply) Seems strong enough on all my models. The glueing area is ( or can be) greater on ply than a small beech block. if the u/c needs to be raked forward then it's easier to saw a slot at an angle than drill at an exact angle. Personally I fret the slot out slighly undersize and after glueing I run an exact size drill thro to enlarge.

Peter Christy's method looks best and neater/ cleaner too when the wire is sunk into the ply. His idea of metal instead of saddle clamps is good but don't be tempted to solder them to the wire,at least on the outer end, because the wire needs to be able to rotate as it's a torque rod.

If designing for other people then their landing strips might be worse than yours......

Link to comment
Share on other sites

Hi kc

Apologies for the delay. Tethering off my mobile phone and only limited MB left

Ultimately, the construction of an airframe is what we feel comfortable with.. Personally, I think we tend to over engineer our aeroplanes.

This model is 90% full size. The revised wing area is therefore .9 x .9 = 81% of the original. The target AUW is actually about 66% of the original so we are operating on a much lower wing loading ( about 81.481481481481481481481481481481%), The original wasn't a heavyweight either.. It would not have flown on the power plants available at the time if it was. I think I'll do some calculations on the force on the leg if I get chance later - just to satisfy my own curiosity.

I guess that if anyone doesn't like the way I am doing things then it is a relatively minor mod to add extra strength if deemed necessary or change the components as they see fit.

AWF - Very nice!

Ian

The rake on the undercarriage was lifted straight off the original. The only slight change was to adjust the wheel sizes to take standard diameter wheels - scaling downwards in diameter. The rake need to be maintained to get the wheel centre line behind the CG. The Nose leg on both variants has been moved forward very slightly as I am advocating a fixed not steerable nosewheel.

Link to comment
Share on other sites

Posted by Martyn K on 16/10/2016 21:43:04:

The rake on the undercarriage was lifted straight off the original. The only slight change was to adjust the wheel sizes to take standard diameter wheels - scaling downwards in diameter. The rake need to be maintained to get the wheel centre line behind the CG. The Nose leg on both variants has been moved forward very slightly as I am advocating a fixed not steerable nosewheel.

Ah yes, now I see what you mean. I was looking at the KF3 plan from the kit (Graupner?)

**LINK**

which doesn't have the rake, whereas this one does

**LINK**

My mistake, I hadn't noticed that before. And thumbs up to "I am advocating a fixed not steerable nosewheel." Although I do have one of the original Graupner twin legged items wink

Link to comment
Share on other sites

Hi Ian

There are quite a few errors on the Graupner kit plan - probably generate to simplify kit production. The tail plane is the biggest culprit...

Here are the latest versions of the plans

Kwik Fli III - 40

kwik-fli-3-40-Fus-ic

kwik-fli-3-40-fus-E

kwik-fli-3-40-wing-tail

Basically a tidy up and annotated version of the original draft versions. For the E version, there in now space for a 60A ESC located under the LiPo with a screw down hatch. I guess that the proof in the pudding will be to build one or two..

I have also done a similar exercise on the Kwik Fli IV. This was a tapered wing version which won the US Nationals in 1969. This has a 20% thick root chord tapering to 15%. As the Root chord is wider this means that there are some minor changes to F2 and F3 to accommodate this. I don't think it affects the placement of the LiPo although the space for the fuel tank has reduced.

Drawings for the Kwik Fli IV - 40

kwik-fli-4-40-Fus-ic

kwik-fli-4-40-fus-E

kwik-fli-4-40-wing-tail

Comments please..

Martyn

Link to comment
Share on other sites

Martyn, thanks for the clarification re the Graupner plan. I won't be rushing to use that, then smiley

I wondered if you would be doing the IV aswell at this size. I know this started off with the Kwik-Fli III (actually, the Flea Fli suggestion) but if enough people are interested in the Kwik-Fli for the MB we now have six variants available, most if not all as free downloads

- Flea Fli

- Flea-Fli +10%

- Kwik-Kli III 40

- Kwik-Fli IV 40

- Kwik-Fli III 60

- Kwik-Fli IV 60

Sizes available to suit all tastes and pockets - whatever spare kit peeps have lying around - and the IV brings the tapered wings option to those who prefer that. Nice looking model, good club aerobat, no real complications in the build, and 2017 is the 50th anniversary of the world champs win. What's not to like?

Link to comment
Share on other sites

There is still a bit of work to do. Especially around the cowl. I have trouble visualising exactly what needs to be done without actually having the hardware in my hand - and better still seeing it assembled.

When I built the KF4, i made a total loss foam cowm but I probably wouldn't do it again. It was rather heavy and brittle but it is still in use.

I will probably adopt kc suggestion extending the fus sides but tapering them at the intersection with the nose ring them building up the cowl from there using hollowed out balsa sheet. I have found that a dremel and drum sander and lots of care is a very quick and easy way if making hollowed out structured.

I am at the stage now where I need to get the plans printed and start building.

Martyn

Link to comment
Share on other sites

since I currently have 40 and 61 TSs and a 70FS looking for homes, but I don't have anything larger, then personally I wouldn't look at anything bigger than the standard KF3 for now. But that's just down to the spare kit I have around, and other people may well have a different view. I do like this idea of marking the 50th anniversary; as Ian says, what's not to like!

Simon

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...