From: nwaero@northwest-aero.com
Subject: Re: Testing and refining designs with models (long)
Date: 13 Jul 1998
Newsgroups: rec.aviation.homebuilt

Howdy, Coming from models, some almost big enough to ride in, I think it can
be a pretty economical way to cut through some of the more major potential
problems of a design, especially if it's something non-conventional. For the
testing of non-conventional ideas, you can start with something small and
simplified, even a glider, and then once you get the thing somewhat stable,
you can add power and make it bigger and more to scale.

You can't scale air, so things like exact airfoil and loading might be
somewhat unrealistic, but you can still determine CG, moments, control
surface area, and get a good idea of what the handling and stability is going
to be like on a full size version.

It's been my experience that a given design will display the same fairly
detailed set of flight characteristics when it gets scaled up, or down for
that matter. The best example that I personally built was probably my 1/3
scale Pitts. With the incidences setup the same as the full size S1S (going
by the plans), it displayed very similar flight characteristics to the full
size unit, as well as the 2 hole version that I have flown. In spectating
from the ground, manuvers like spin entry, inverted spins, snaps, the amount
of hang time over the top in a hammerhead, etc., are so convincing with the
model that I would almost forget I wasn't actually in it when I was flying
it.

The differences between the model you build and the full size end result will
probably be:
power to weight (model has more)
power to area (model has more)
wing loading (model has less)
airfoil (because of the above)

The bigger the model gets, the less difference there is. These differences
will affect the performance (stall speed, top speed, speed range, climb rate,
etc.), but not necessarily the other flight characteristics that are
sometimes harder to determine on paper like stability, handling, and possibly
quirky bad habits.

You could build the model to the same power and loading as the full size bird
if you want to, but it becomes a matter of being able to fly the thing from
the ground with no intrumentation and from a fixed vantage point. It becomes
unpractical, and most of what you want to learn with the model can be learned
with the lighter loading and less mishaps. You can still duplicate the
'style' of airfoil as well, but the reynolds will be way different (usually).
For example, on my Pitts, I used symetrical airfoils with all of the flight
surfaces setup to 'plans' incidences, and the CG to plans too. It flew and
landed just like it's full size mentor. With the smoke on and doing large 'in
the box' show type manuvers, the thing was very real. So real, that more than
once people that lived near the field drove over to see why there was a full
size aircraft flying out of there. ;)

As for size limitations, the only limits I know of are imposed by the various
santioning groups (like AMA) and that's just a function of their insurance.
Their limit is 55 pounds. Most all of the flying fields out there are run by
clubs that belong to one of the sanctioning bodies, and get their insurance
that way. If you can get out in the middle of the dessert somewhere, who's
going to be bothered by your 300 pounder?

Biggest thing I have ever flown RC was 14' WS, 115 pounds, 22hp.

I have a proof of concept design on paper that I would like to eventually
build a model of. I'll probably never get to it, but if I do it will be a
small glider first, then a larger simplified powered unit (maybe even control
line), then a larger RC version, and then a huge RC version which would be a
collective of all of the mods and improvements that were made along the way
to it's predicessors. On the final model version I could even work out some
of the more detailed problems like landing gear configuration and other
systems that will be the same on the full scale unit. Final aerodynamic mods
could be made to the large model until it flew the way I wanted it to.

At that point, I would have quite a bit to go on when looking at the
feasability of going full scale, and I wouldn't be in it for too much dough
either, especially since I have all sorts of engines and radios stashed away
from my modeling days.

Speaking of radios, with all of the limits, differential, and mixing modes
you can program in on modern RC transmitters, you can fine tune what will end
up as rigging ratios on the full size ship, in just a couple seconds with the
push of a couple buttons. You could get a lot closer on the first try with
the control hardware on the full size ship than you might otherwise.
Especially if there is a mixer or a lot of differential involved, or some
other "non-standard" control scheme.

-j- (I say just go for it!)


In article <35a8774e.0@stnws02.atl.mediaone.net>,
  chull@POBoxes.com wrote:
> Since no one laughed at or got offended by my first questions I'll
> try another. If someone who lacks the training and resources of an
> aeronautical engineer wants to try their hand at designing a plane,
> is it practical to start with a small radio controlled model and, as
> they think they are getting it right, work up through several steps
> to a full sized plane? Ignoring for the time being difficulties with
> the R/C, and with the amount of work involved, are there any big
> gotchas with this approach? In particular, what are the legal
> restrictions on larger models/unmanned aircraft?
>
> BTW From what I've seen so far this seems a particularly well mannered
> group. It's interesting that most of the contentious, non-flying
> related posts I initially ran into were those about needing a moderated
> group to control off topic posts. Since I have no wish to start a fight
> I won't ask how much of this is my timing and how much is indicative of
> something else. :)