My KFM Wing Build

[jross]

Take the CG back towards the original position that you were using and do the balance test as set out above in post #29.

Roger that. I moved the CG forward to 15 cm, not quite as far as the full 93% scale of my plane which is 14.7 cms, as you pointed out earlier. Glad I made a reference mark at 16 cms as soon as the build was done. Nose is a bit flatter now! Hard to guess exactly where it is/was.

I noticed a couple of other things I need to address. One of the FT linkage stoppers is binding up and flexing the control rod. I love the idea of linkage stoppers but half the time, they bind up on me and don't float with the control rod angle. Not sure if there's some trick I've missed. The identical stopper on the other side installed the same way works fine. One of the elevons isn't rising as high as the other. Gotta check that out. Probably why it was pulling left.

When I flew today, it felt like it was flying better in terms of not being so unpredictable and erratic so I think sliding things back a bit should help. I don't think it will take a lot. Learning a lot about CG. It's funny how someone can describe something to you but until you see it for the first time, their advice doesn't register and you have to go back and read it with knowledge of your new experience. I'm happiest when I'm learning so even when the plane doesn't fly, I still enjoy scratching my head.

 

[Hai-Lee]

Once you learn about setting up wings and how they fly, )how easily), you wonder why you struggled. I know I did but when the penny drops it is so very clear and simple!

Those who see them as being too difficult should build one and try it for themselves.

You are getting on top of it all obviously.

As for the banged up nose if it gets real bad there is nothing to stop you from giving it as nose transplant! Just add a skewer or the like for a little extra grip or strength!!

Have fun!

 

[jross]

Confession time.

Was getting pretty frustrated with the KFM. Other than the first flights where it was tail heavy, I haven't gotten it in the air again. Tried today but it was pretty windy.

Last week, I decided to build an FT Arrow to see how it flew and if it gave me the same issues as the KFM. Because the CG is more or less set in stone and the build video references putting a 900 as far forward as you can get it in the pod, I was much closer to a proper CG and had less trouble making it fly. The reflex gauge in the plans helps too. After @Headbang suggested moving the battery back slightly, it flew really well.

Bugs the hell out of me I can't make that KFM fly well. Need a nice calm day to try again. Starting to realize the neighbour's field is in a little wind hole where the valley turns and constricts. Makes for some weird air flows.

The KFM wins the crashibility test by a mile. The pod on the Arrow is already mush at the front and the motor pod slid forward and sliced up the trailing edges. Still flies but doesn't look mint like the KFM which has hit the ground/trees/garage/etc way more. Wish a wing expert lived close by.

 

[Hai-Lee]

Confession time.

Was getting pretty frustrated with the KFM. Other than the first flights where it was tail heavy, I haven't gotten it in the air again. Tried today but it was pretty windy.

Last week, I decided to build an FT Arrow to see how it flew and if it gave me the same issues as the KFM. Because the CG is more or less set in stone and the build video references putting a 900 as far forward as you can get it in the pod, I was much closer to a proper CG and had less trouble making it fly. The reflex gauge in the plans helps too. After @Headbang suggested moving the battery back slightly, it flew really well.

Bugs the hell out of me I can't make that KFM fly well. Need a nice calm day to try again. Starting to realize the neighbour's field is in a little wind hole where the valley turns and constricts. Makes for some weird air flows.

The KFM wins the crashibility test by a mile. The pod on the Arrow is already mush at the front and the motor pod slid forward and sliced up the trailing edges. Still flies but doesn't look mint like the KFM which has hit the ground/trees/garage/etc way more. Wish a wing expert lived close by.

We all have those that give use trouble! I had a replacement order where the client wanted to add LED lighting to improve visibility, more weight), and bigger winglets, (again more weight way behind the CG point), as he kept losing orientation at distance and crashing. Firstly examination found that he was losing radio signal not orientation. OK, so I agreed to fit the LEDs, and bigger wing tips, as requested but that put a lot of wire and other weight behind the CG/balance point. I managed to get it to fly with a 2200 3S even after bringing the motor a further 25mm forward. I also had to add some form of cover to tidy up the wiring and so I ended up with something that reminded me of a B2, (well sort of)!

Here is the pic;


I am about to push the design in 2 different directions. First version; bring, (curve), the wing tips a little further forward and broaden the nose area for more contact area to load share in the case of a severe crash as well as add more mass to the area where weight is needed. Second version; I am adding an extension of the "Cover" shown in the pic as well as adding a fixed angle canard so that the balance can be more easily achieved with lighter batteries.

Should you wish to get involved in the testing just let me know!

Have fun!

 

[FoamyDM]

On my mini-version of this, (V2) shrunk down to 480mm, I have about 5° of reflex. CG is just at the frontmost part of the top step. but to get a smoother front, I taped in a Wendy's drinking straw (nearly full length) This did two things.
1. better aerodynamics without shaping foam, and fast.
2. Better Crash proofing against wing hits.

@jross is right, this wing is very durable. with a straw, now even better. I glid it into into the corner of my driveway headwall, nothing. no visible damage. The straw was great, and elastic, for energy dissipation. I couldn't feel any ding in the foam.

In the mini-version Glider, I found 10g in the nose (4 pennies) was enough to balance it. That is precisely the weight of 2-5g servos.... did anyone try this as a DLG?

 

[jross]

Finally got around to putting this KFM in the air today, @Hai-Lee. Blew up my arrow so it was time. Flew like a dream. Took a 1500 mah 3s as far forward as I could get it and still be on the plane. My time on the arrow taught me a lot. The KFM flew out of my hands and hardly took any trim at all. Stoked! What a sweet little plane. Probably had 30 failed launches last time I tried and gave up in frustration. Left the motor, ESC and rx on it as I wasn't giving up, just learning some skills.

Also successfully maidened my second spear today. Destroyed the first one after 30 seconds due to disorientation. It flew well too. With a 2600 4s, it stayed in the air for 25 minutes with amps to spare. Time to get the FPV setup dialed in for the spear. Can go a fair distance in 30 minutes.

 

[mayan]

Probably had 30 failed launches last time I tried and gave up in frustration.

I realized that what worked for me in launches is to go pretty high on throttle, ~75% and then just let it go. I tried hard tosses but my hand always flicked which turned the wing inverted and then up is down and down is expensive.

 

[Hai-Lee]

I realized that what worked for me in launches is to go pretty high on throttle, ~75% and then just let it go. I tried hard tosses but my hand always flicked which turned the wing inverted and then up is down and down is expensive.

I test flew my latest version yesterday, (With a HK flight stabilizer), and it flew quite well, (Didn't even need to be trimmed). I handed it over to its owner and it was promptly flown flat out into a 50 metre galvanized steel light pole. I have repaired the wing as it ripped an outer wing panel off, ripped the motor off which sliced through the wing, and damaged a servo by stripping a servo gear.

The servo repaired, (stripped gear replaced), and the wing glued back together. The repaired wing will be used and abused by the newbie owner to fly solo and sharpen his skills on. This will allow him to transfer the electrics to the new one when he is competent enough to fly without crashing!

After analyzing the failure a few strengthening modes are to be added on the next 4 I build, (an order batch), to test the effectiveness of the mods. Will post the mod details and they will be quite easy to add to an existing model. You will need a few FB scraps, some glue, and a couple of Skewers! The mods will not overly effect the flight performance but WILL enhance its "POLE" resistance.

I found that the current FB I am using has 2 layers in it and the FB split along the Foam to Foam joint/layering, (in the centre of the FB sheet). As you can imagine the stop was extremely violent and sudden. The motor itself traveled 30 metres from the pole and the battery, (1000mA 3S), made it 25 metres away!

More later as the pics of the repaired wing and the new recommended modes will be on the original KFM wing thread later.

Have fun!

 

[Piotrsko]

Have you ever modified the angle the wings make from the current 60 degrees to a flatter one of say maybe 30 to 45 degrees? Generally makes them less CG sensitive.

Among a zillion other things, I am trying to build one using my table saw to cut wing parts. I can't see why that wont work. I figure maybe 5- 10 minutes to cut the wings including bevels. I will keep you posted on results.

 

[Hai-Lee]

Have you ever modified the angle the wings make from the current 60 degrees to a flatter one of say maybe 30 to 45 degrees? Generally makes them less CG sensitive.

Among a zillion other things, I am trying to build one using my table saw to cut wing parts. I can't see why that wont work. I figure maybe 5- 10 minutes to cut the wings including bevels. I will keep you posted on results.

I have and do build the retro style retro wings where the Sweep angle is much less and in a lot of cases zero! Rather than being less CG sensitive they are generally much more CG sensitive and whereas the swept wing rolls well and has a slight loss of effectiveness in pitch the older style are extremely sensitive in pitch and somewhat lethargic in roll unless you keep the wing span down!

My retro plank flying wings can outperform the KFM and other sweep wing designs, of similar span, easily in all aspects, (Climb, roll, dive, and speed for applied thrust), but the reason I love the KFM is its robust nature, simple nature and real resistance to hangar rash! I often see them cartwheel on landing or do a vertical landing and then they are just given a quick check, and new battery then relaunched immediately. I have no other planes that are that solid.

The one I split on Sunday was diving under full power when the plane was pulled up to level and then it slammed into a steel pole half way along the LE of one side. The LE was crushed totally to a depth of about 25mm and the prop cut a slice in the TE on the same side at about the same time. This slice was in line with the impact point and so the wing tore from the cut to the impact point.

Not a normal sort of crash but a weakness identified and hopefully rectified. The wing is repaired and ready for its next flight already, (it was ready about 24 hours ago as it is now 5:30 am on Tuesday here)!

If I want a somewhat relaxing flight, (or even combat), I just chuck up the KFM but if I want speed and fantastic maneuverability I fly one of the retro plank designs.

Have fun!

 

[Piotrsko]

Thanks for the quick and atypical correct @Hai-Lee reply. Table saw has stops for 30 degrees so 60 it is then.