Would a scale model have a different CG position than the fullsized original?

[tim hooper]

I'm building a Hild Marshonet from Ian Eastons plans - there's a build thread over in Scale;

http://www.rcgroups.com/forums/showthread.php?t=234417

I'm puzzled by the model's CG position in that it lies forward of the wheel centres. Assuming the undercarriage to be accurately placed, I assume that the full-sized version had the CG further aft in order to taxy without burying its nose in the dirt. I believe that Ian has to handlaunch his prototype, so the issue has not come to light before now.

I realise that all the incidences ard wing-sections are different - could that affect the CG?

Here's a 3-view of the original 1919 design; the upper wing is sweptback, whilst the lower wing is swept forward.............

I'm not sure whether to re-locate the undercarriage to bring the wheels further forward.

tim :confused:

[Discharger]

Could it be in this instance the actual CG would be averaged for both wings? If this were so, from my casual glance of the plan it should be just aft of the wheel axle?
An appealing little craft. Looking forward to seeing the photos!

[Sparky Paul]

I get this for the c.g. using...
http://perso.wanadoo.fr/scherrer/matthieu/english/mce.html
.
Wheels properly in front of the c.g.

[tim hooper]

Gents,

Thanks for taking the time to reply! Here's an extract from Ian's plan which shows the CG at 20% MAC (ahead of the wheels).

I wonder if the problem lies with me assuming that Ian is using exactly the same planform as the original aeroplane.

Paul,

Bearing in mind that I'm a simpleton, could you tell me how you arrived at your answer? I've seen a calculation method that involved adding the root chords to the tip chord (and vice versa), and then drawing a big cross to arrive at the mean chord. I've also read of methods that a) treat each wing as separate entities and then use a weighted average (to account for their different areas) to arrive at a result and b) methods that simply discard any overlap of the wings in planview, and treat the wings as a single area.

Is there a bozo-proof calculator out there?

tim

[vintage1]

Tim. Not an exact answer, but some observations.

First of all regard the notation "20 hp engine" This is a grossly underpowerd plane.

Secondly regard the history of the aircraft. Was it a massively suucessful plane?

Thirdly look at the decalage. Massive.

This is a scaled up rubber powered plane! Hugely forward CG, lots of decalage to make it extremely stable, and not enough power to make it unpleasant to fly under power, if indeed it was capable of flying at all.

All that is required wheels wise is that the aircraft doesn't tip over...which it won't with its arse on the ground, as the CG will then fall behind the wheels. 20bhp won't be enough to tip it over either, but landing it will be tricky - its got to be stalled onto the ground.

The model plan that you have shows far less decalage than the full size 3 view..I would have thought it would fly at more or less where Sparky puts it.

Without knowing the exact history of the plan, is it possible that it started life with more decalage, needed a very forward CG to fly, that ws removed at a later re-redraw, and the CG left where it was?

[tim hooper]

Vintage,

Thanks for the thoughts! :)

I've downloaded the calculator in Pauls post and I'll fiddle with it later, although at first glance I can't see provision for multiwinges models.

tim

[Sparky Paul]

For "multi-wing" planes, treat the topview as one surface.
Yours looks like this..
and at Jean-Claude's site...

[Ian Easton]

Interestingly, I came up with the same CG position as Paul and is where I put it for the first flight which came out to be tail heavy. I remember it because I noticed that it lined up fairly well with the upper wing tip line.

Vintage,
Here's the history of the plan - I drew it up in CAD based on the three view you see here, making a few changes to make it more model friendly. I built the original model, made lots of scratchy notes, calculations and scribbles on the plan, then tidied up the CAD plan that Tim has now. The CG is where I flew the model, which flew fine.
My first flight was almost a catastrophe because when it took off it became apparent quite quickly that it was tail heavy. I moved the battery pack as far forward as I could and away she went. I have ROG'd it since and no problems showed up on take-off, but the area that I fly from prohibits any decent landings so in reality I haven't experienced how it handles after it touches down. I take off from a running track and land in a rather "tuffty" grass infield.
The thing about taking off with it is that it's in the air within 10 ft so there's not a lot of time spent on the ground. It was throttle up, tail up almost immediately, a touch of up elevator an up she goes.
The CG is set at 20% MAC, which may err a little on the nose heavy side, but I like my models a little nose heavy. Perhaps Tim could compromise and set it at 25% for the inital flights, which would help bring it closer to the axle.

Also when you look at this plane and consider the era that it came from it looks like everyting on it was an experiment, as apparently none of it's features ever lingered on in aviation history!

Ian

[tim hooper]

Thanks all, Gents! :)

I think I'll take a stab at 25% for the CG as Ian tentatively suggests, and if I lengthen the rear undercarriage legs a little it'll push the wheels forward without spoiling the oh-so-racey looks of the model!

As Vintage suggests, the design seems to bristle with innovations - most of which were never to be seen again............ :rolleyes:

tim