Metro Gnome

Anything with a propeller
User avatar
Sundancer
Posts: 94
Joined: 07 Jun 2018, 09:28
Location: Limousin, France

Metro Gnome

Post by Sundancer » 24 Jun 2018, 20:04

IMG_4290.JPG
Thanks to Mike for unearthing the plan for this one. I was planning on building a couple more rudder only models to further astonish my French flying friends who view my flying of the Frog Witch with the 2.4 gig converted MacGregor button Tx as some kind of magic, they have christened it "flying by morse code". Of course they are all far too young to have had experience of any kind of radio before digital proportional, my tales of rubber band powered actuators and Galloping Ghost being met with scepticism if not outright disbelief. So once I saw the Metro Gnome plan, and having all the required hardware for an electric powered version in the drawer, I was seized by an urge to build one.

Thanks to the weather here (S-W France) blighting flying outings for the last few days with pretty well constant 20 knot winds which don't drop, as they usually do, in the evenings (well, not until it is too dark to fly) I have managed some workshop time and after a very few hours of effort have a complete set of flying surfaces. As is my habit I couldn't resist a few mods. I didn't fancy 1/16" tail surfaces and used instead some lovely light and stiff "C" grain 3/32" which was in the last delivery from Balsa Cabin, adding anti-warp tips. Apart from the ribs having flat bottoms to enable more secure pinning down the only mod to the wing was to pre-join the leading edge (1/8" x 3/16" spruce in my case not beech) with a 1/16" ply brace at the correct dihedral angle, although I did fudge this to 1.1/4 inches a side instead of 1 inch, which seemed a trifle minimalist to me. The wings were then built one panel at a time onto the leading edge. Oh yes, and I also used a centre rib as an aid to getting a better join.

Fuselage tomorrow. The only significant change I am planning here is to move the tailplane to the top of the fuselage and slot the fin into it. I dislike low mounted tailplanes, probably because I keep knocking the one on my Sharkface off, despite having RET control on this, it just lands too fast. I'll be using a vertically mounted servo with a torque rod, Orange 6 Rx and a 50 watt motor and 450 2S lipo. The wing and tail weigh nothing, so the finished thing should end up pretty light.
Attachments
IMG_4294.JPG
IMG_4292.JPG
IMG_4291.JPG

User avatar
Sundancer
Posts: 94
Joined: 07 Jun 2018, 09:28
Location: Limousin, France

Re: Metro Gnome

Post by Sundancer » 28 Jun 2018, 13:02

Further progress has seen the fuselage assembled and, yesterday, all the gear and the torque rod assembly installed. At this point three things became apparent;

First, after carefully testing and installing the servo, this promptly went u/s. $$**£$**& and similar expletives, as it was glued in. Nothing for it, some surgery later a replacement was in and this did work.

Secondly, on running up the little motor on a 2S 450 lipo and 5 x 3 GWS prop the amount of power at full throttle was clearly WAY in excess of that which a TD 010 or Pee-Wee would produce, if it weighed the expected 5 ounces we were looking at something north of 150 watts per pound, at which level it would almost certainly be an uncontrollable missile. I don't have a smaller prop to tame this, no problem when flying on the propo Tx, just keep off the loud pedal, but careful adjustment of the mid throttle position on the single button Tx would be needed, probably around the 40% mark and care will be required to make sure, when flying, that the next throttle command is "off" not "full"!

Thirdly, on roughly assembling the bits it became apparent that, even substituting a 320 lipo (the only space for the battery is in front of the wing), it was horrendously nose heavy. This is contrary to the usual case when building an IC model for electric. I had thought the nose looked a bit long but stuck to the plan, but with only the rear top and bottom sheeting to add behind the wing the CG is actually IN FRONT of the leading edge! So, short of adding a lump of lead to the back, which I definitely do not want to do, drastic action is required and when I go back in the workshop this afternoon the plan is to remove the firewall and hack at least an inch off the nose, leaving just enough space to get the ESC and 320 lipo in the nose bay.

So, to anyone deciding to build one for electric; either shorten the nose or rearrange things so the lipo can go under the wing, preferably behind the CG position. And use a motor you can prop down to 20 watts or so.
Attachments
IMG_4297.JPG
IMG_4296.JPG

Tobe
Posts: 173
Joined: 16 Feb 2018, 06:19
Location: Varberg or Stockholm, Sweden

Re: Metro Gnome

Post by Tobe » 28 Jun 2018, 14:36

...mm
I would rather elongate the tail then shorten the nose.

User avatar
ozrs
Posts: 73
Joined: 16 Feb 2018, 15:21
Location: West Australia

Re: Metro Gnome

Post by ozrs » 28 Jun 2018, 15:20

I guess you could mount the servo vertically in the tail (in place of the relay), and have it drive the wire rudder waggler directly?
Richard

User avatar
Sundancer
Posts: 94
Joined: 07 Jun 2018, 09:28
Location: Limousin, France

Re: Metro Gnome

Post by Sundancer » 28 Jun 2018, 18:13

Both worthy ideas guys, and I agree that if I had been designing it the tail moment would have been a bit longer, but too late - the deed is done and the nose is now 1 inch shorter, the CG looks fine with just the undercarriage to add (very lightweight balsa/ply wheels) the weight is just on 4.5 ounces. The wing and tail have had four coats of dope so with just the tissue covering on the fuselage and minimal tissue trim on the wings to add it is going to be just about spot on 5 ounces. One thing for sure - lack of power is not going to be an issue! Photos when I have finished the trim.

User avatar
iflylilplanes
Posts: 53
Joined: 13 Mar 2018, 03:26
Location: Sydney Australia
Contact:

Re: Metro Gnome

Post by iflylilplanes » 29 Jun 2018, 07:02

Where would we be able to find the plans? they don't seem to be in Outerzone. Would love to see a video of it in flight.
Cheers,

Dave

User avatar
Sundancer
Posts: 94
Joined: 07 Jun 2018, 09:28
Location: Limousin, France

Re: Metro Gnome

Post by Sundancer » 29 Jun 2018, 08:24

You will find them on this forum Dave in the "Theme for 2019" thread, here http://www.mode-zero.uk/viewtopic.php?f ... 2&start=20 sixth post down the page - Mike_K posted them. They print on two sheets of A4 if you select "poster". I pinched Mike's PDF and passed it to Steve at OZ, so it will be on there when he has had time to upload it.

User avatar
Sundancer
Posts: 94
Joined: 07 Jun 2018, 09:28
Location: Limousin, France

Re: Metro Gnome

Post by Sundancer » 29 Jun 2018, 22:28

IMG_4301.JPG
IMG_4298.JPG
Completed the "nose job" and changed the 320 lipo for 220 30C type and the CG is now around 25% - still a bit forward for my taste but as the model, complete except for light balsa wheels, some tissue trim and a fin strake and tailplane fairing comes in at 122 grams - less than 4.1/2 ounces - it won't hurt to add a few grams of lead at the back if necessary.

The second photo shows the long nose prior to the "nosectomy" the others are of the model as it is tonight prior to the little finishing jobs
Attachments
IMG_4300.JPG
IMG_4299.JPG

User avatar
iflylilplanes
Posts: 53
Joined: 13 Mar 2018, 03:26
Location: Sydney Australia
Contact:

Re: Metro Gnome

Post by iflylilplanes » 30 Jun 2018, 12:45

Thanks for the directions, found the plans and article. I would love to see the original actuator in action, an interesting idea. Thanks again. Looking forward to your maiden flight video and or report.
Cheers,

Dave

User avatar
Sundancer
Posts: 94
Joined: 07 Jun 2018, 09:28
Location: Limousin, France

Re: Metro Gnome

Post by Sundancer » 06 Jul 2018, 13:00

Well, the Metro Gnome was finished almost ten days ago and taken for it's first outing. I had to resort to a lighter 220 lipo as, even with the shortened nose, it was still nose heavy with the 450. All up weight was thus only 120 grams, so n worries about lack of power!

I used the proportional Tx for the first attempts as I needed to establish the correct power level to set on the three position single channel Tx, and also to establish where to set the rudder yoke for bang-bang operation. We had some longish grass in one area of the field so I tried a few test glides which told me nothing except that it didn't want to glide. So, off with half power and it motored into the grass four feet in front of me, a real case of under elevation. Added 1/16" packing under the leading edge and tried again. This time it made 8 feet! Added another 1/16" and it almost got away, but the prop flew off and I lost the "O" ring from the prop saver. Not having a spare that concluded matters and since then we have had mostly windy weather. I have re-rigged it with a total of 3/16" positive on the wing and hopefully this should be close when I get to try it again.

So all I can say at present is that it is a cute little thing, but with the incidences and downthrust shown on the plan is grossly under elevated. My advice at present would be, as well as doing something about the nose length (or increasing the tail moment) it either needs less downthrust or more incidence, but I'll report back once I have it sorted. It does seem to bounce very well though!!
Attachments
IMG_4303.JPG
IMG_4302.JPG

Post Reply