The Murphy Builders Site

Yes, everything you add moves the CG aft therefore it would be nice to have
the empty CG closer to the front of the range.

Mike
044SR

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Tim
Hickey
Sent: Sunday, March 08, 2009 5:22 PM
To: rebel-builders@dcsol.com
Subject: [rebel-builders] What I learned this week!

First a public "Thank You" to those who responded to my email about direct
communications between builders.

And thanks to Wayne who showed me how to find the address from the incoming
email.

And I agree with Bob P that it is best to ask the questions here on the list

so that everyone has a chance to see the discussion. But there are still
times when a direct question from me to you might be best off line, if only
to conceal my own "cloudlessness" while I dig for understanding.

The choice of engine for a Rebel has many facets. Since we are building to a

1320 LSA gross, the empty weight of the completed machine is very important.

Considering that the Rebel was designed to a 1650 GW, we are in a bit of a
hole to start with. In other words, the airframe is a lot beefier for a 1320

gross that it might need to be. That being said, there are a few recognized
areas that can stand some additional "strengthen" to avoid problems in the
future.

The airframe will weigh what it weighs when we are done. We probably don't
really have too much influence on the empty weight of the airframe. The best

thing we can do is not go overboard on interior trim, and we should keep the

panel light and simple. After all, this is a day, vfr machine only.

The engine choice is another matter. I really prefer to use a Lyc O-235 with

118 hp if I can stand the weight. If not, then I am moved to used the Rotax.

My questions about Jim Coles C of G, and the 3 inch extension of his motor
mount we based on wondering if the extension should be more than the 3
inch. Just off the top of my head, it would seem that everything added to
the aircraft in the way of payload serves to move the C of G towards the
rear. If that is true, and I have not yet investigated that, then would it
not be best to have the C of G of the empty aircraft plus the pilot located
well towards the front of the permissible range?

We are months away from having to make that choice, but it is an important
one, and it is never to soon to start gathering data.

One more question:
Does anyone have hard data on the weight of a paint job for a Rebel?

Thanks.

Tim Hickey
R808




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This is true Mike, except that, when limited by the LSA gross, "everything"
you can add in live weight may not be enough to keep the CG aft enough for
best performance. I've been struggling with similar issues to those Tim
relates. The lower gross really complicates optimizing balance issues.

Tim, I presume you've already ruled out the two engines with weights in
between the O-235 and 912ULS (quite a spread there), namely the Jabiru 3300
and almost-shipping full FADEC UL Power 360. They both bring the high
RPM/tip speed concerns, but should at least be in the initial decision
matrix.


Ron

On Sun, Mar 8, 2009 at 6:02 PM, Mike Kimball <mkimball@gci.net> wrote:

Yes, everything you add moves the CG aft therefore it would be nice to have
the empty CG closer to the front of the range.

Mike
044SR

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Tim,

*Calculating Weight of Dry Paint

*

*Q: I want to paint an aircraft I am building, but weight is a concern. I
know I can weigh the paint by the gallon, but does the weight when it dries
differ from the weight when it is applied wet? How can I figure this out?*

A: Refer to the Material Safety Data Sheet (MSDS) where it should tell you
the percent weight solids of the paint. Suppose the % Wt solids is 60% and
the weight of the paint is 9.0 lbs/gal, then the weight of the solid paint
will be 5.4 lbs/gal. If you intend to spray apply the coating to the
aircraft using an HVLP spray gun, you can guesstimate that your transfer
efficiency will be about 50%. In other words for every gallon of paint that
you use, only 2.7 lbs of solid paint is deposited on the aircraft. The
remainder falls to the floor or goes into the filters of a spray booth.
Hence, if you know approximately how many gallons of paint you will need to
purchase, you can estimate the weight of the solid paint that will be
deposited.
Web Link -- www.paintcenter.org/rj/oct07n.cfm


On Sun, Mar 8, 2009 at 6:21 PM, Tim Hickey <tjhickey@iowatelecom.net> wrote:

First a public "Thank You" to those who responded to my email about direct
communications between builders.

And thanks to Wayne who showed me how to find the address from the incoming
email.

And I agree with Bob P that it is best to ask the questions here on the
list
so that everyone has a chance to see the discussion. But there are still
times when a direct question from me to you might be best off line, if only
to conceal my own "cloudlessness" while I dig for understanding.

The choice of engine for a Rebel has many facets. Since we are building to
a
1320 LSA gross, the empty weight of the completed machine is very
important.
Considering that the Rebel was designed to a 1650 GW, we are in a bit of a
hole to start with. In other words, the airframe is a lot beefier for a
1320
gross that it might need to be. That being said, there are a few recognized
areas that can stand some additional "strengthen" to avoid problems in the
future.

The airframe will weigh what it weighs when we are done. We probably don't
really have too much influence on the empty weight of the airframe. The
best
thing we can do is not go overboard on interior trim, and we should keep
the
panel light and simple. After all, this is a day, vfr machine only.

The engine choice is another matter. I really prefer to use a Lyc O-235
with
118 hp if I can stand the weight. If not, then I am moved to used the
Rotax.

My questions about Jim Coles C of G, and the 3 inch extension of his motor
mount we based on wondering if the extension should be more than the 3
inch. Just off the top of my head, it would seem that everything added to
the aircraft in the way of payload serves to move the C of G towards the
rear. If that is true, and I have not yet investigated that, then would it
not be best to have the C of G of the empty aircraft plus the pilot located
well towards the front of the permissible range?

We are months away from having to make that choice, but it is an important
one, and it is never to soon to start gathering data.

One more question:
Does anyone have hard data on the weight of a paint job for a Rebel?

Thanks.

Tim Hickey
R808




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I have not ruled anything in, or out as far as engines go. Actually, I fell
in love with the Lyc IO-233 that was displayed at OSH last year. But it
seems that they may not bring it to the market. I traded some emails this
week with one Mel Asberry, who writes a column for Kitplanes under the title
of "Ask the DAR", and with Marc Cook, who is the editor of Kitplanes. I will
post some of my exchanges with Asberry in another email.
Marc Cook says about the IO-233...
___________________________________________________________________________
Don't hold your breath on the IO-233. As far as I know, no OEM has expressed
an interest in the engine, and it's a LONG way from done. Lycoming will not
develop it just for the kit market, so it'll take a fairly large LSA OE to
launch that engine. My gut says it's about 90% the engine will never arrive.
But I've been called cynical!


Marc
____________________________________________________________________________
I respect his knowledge of the industry, so that is probably the way it will
end up.
He also said.. in response to my question

_____________________________________________________________________________
On Feb 23, 2009, at 6:17 PM, Tim Hickey wrote:


One last question about the engine. I had been lead to believe that since
both Cessna and Lycoming were owned by the same outfit, that there would be
corporate pressure to put the Lyc in the new Cessna light sport.
But maybe that has all changed. I think I recently heard somewhere that
Textron might sell Cessna.
I don't think that argument is as strong as it once was. There are people
inside Cessna who remember, fondly, when aircraft like the 182 were powered
by Continental engines. Also, I think the fact that Continental already has
a certified, lightweight version of the O-200, and is willing to (let's just
say) price it aggressively, seals the deal.
__________________________________________________________________________

That IO-233 was a nice looking engine, 200 lbs, 118 hp, and based on the
O-235.

The new O-200 Continental is about the same weight, 100 hp, but it has been
displayed for 2 years, with no intro date to the market.

A long way to say I still don't know what we will do. Options open.

Which leads me to a another question. How does CG location affect
performance?
I thing that a forward CG makes for a more stable machine, but how does
moving the CG back improve "performance"?

Tim
R808



----- Original Message -----
From: "Ron Shannon" <rshannon@cruzcom.com>
To: <rebel-builders@dcsol.com>
Sent: Sunday, March 08, 2009 9:13 PM
Subject: Re: [rebel-builders] What I learned this week!

This is true Mike, except that, when limited by the LSA gross,
"everything"
you can add in live weight may not be enough to keep the CG aft enough for
best performance. I've been struggling with similar issues to those Tim
relates. The lower gross really complicates optimizing balance issues.

Tim, I presume you've already ruled out the two engines with weights in
between the O-235 and 912ULS (quite a spread there), namely the Jabiru
3300
and almost-shipping full FADEC UL Power 360. They both bring the high
RPM/tip speed concerns, but should at least be in the initial decision
matrix.

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Hi Tim !

I agree there are some questions that are better offline - at least
to start. Anything that looks like general interest can then come
here to be shared. You never know who might find the answers
useful - and didn't even know the questions ! ;-)

The 3" extention is about as much as I'd feel comfortable with.
(Considering the factory suggests 3" rearward for heavy engines,
it is certainly within reason ...)

That 3" came about as a result of test flying with standard Rebels,
with standard mounts, radiators, etc., and standard tailwheels.
The fuel tanks were reduced to 2 bays/side - strongly recommended !
We found that the reason the factory suggests a 1,450 gross for the
912 in that configuration is that you can not put more weight in
the baggage area without going outside the aft C of G limit !
We had bags of water softener salt tied to the backs of the seats,
to keep the weight as far forward as possible..... and in that
configuration, you stop at 1,450 lb.

We know that the Rebel will easily pass the climb test requirements,
with only the 80 hp. Rotax 912, at 1,550 gross, and even 1,650 gross,
but you have to use very dense weights, FORWARD of the seats -
not a realistic loading scenario for everyday flying ! We did do it,
flying on warm days, and still had satisfactory performance.

The answer then, was to move the engine forward. Based on
Tom's experiences, Jim spoke with Daniel about possibilities, and
they came up with a 3" longer mount - a very nice ring mount,
which leaves a lot of space behind the engine, and gives a nice
lean look to the front end of the Rebel. Even though Jim used the
heavy Scott 3200 tailwheel, the C of G came out around 12.5" ...
very comfortably allowing 200 lb in the baggage area !

I will certainly do the same on my NEXT Rebel, although I will
likely use the standard Murphy tailwheel (it's a LOT lighter),
likely with a substitute pneumatic wheel & tire (Princess Auto
$4.95 cheapy !) !

It is POSSIBLE that you could come in under the 872 lb. ?? (890 ??)
max. allowed for LSA ..... JUST... with the Lyc. O-235-L2C.
If you use a Lyc. O-235-C2C or similar, the 80 hp. Rotax will
outperform you every way ! I owned BOTH configurations for
about 10 year ... The O-235-C2C Rebel was VERY light, with
.020 corner wraps, etc., and weighed 835 lb. - unpainted, no interior.
The 80 hp. Rotax Rebel weighed 716 lb., with leather interior,
full carpet, and painted.... Now, current Rebels, with the
Ontario Mod. upgrades, will be heavier - I would expect
about 750 - 790 lb. approx....

Paint jobs - if you do a full 2 colour paint job (not just a vinyl
stripe - like some have done !) the minimum weight gain will be 25 lb.
-- 35lb. is typical, with a skilled painter. I have seen the "deluxe,
3-coat, 4 colour automotive quality" paint job that added over 60 lb !!
Watch out for those ! :-)

To keep the weight down, and give good sound & heat insulation,
I would use the rolled foil-coated bubble pack material used to insulate
chicken coops. It's VERY light, very cheap, and works GREAT !
You could do 2 layers (it comes in 1/4" and 1/2" thicknesses)
in the doors, front fuse panels, roof, and as a curtain for the
aft of the baggage area. Just glue lightweight automotive
headliner fabric, or Naugahyde... with a few spots of spray
contact cement - and hold in place with Velcro, or contact cement.

I used it in a very light U/L Rebel - weighed 598 lb. with
the 80 hp. Rotax 912, no radio, no seat cushions, BARE ! We flew
it from Chilliwack to Northern Ontario in September 1992 ...
after adding foam for the seats, the foil bubble pack, and spraying
some flat black on the top of the cowling, plus a handheld radio
& intercom... Loved landing & stopping BEFORE the numbers ! :-)

--
......bobp
bobp@prosumers.ca
http://www.prosumers.ca/Ramble09

http://bpatterson.qhealthbeauty.com
http://apatterson2.qhealthzone.com
http://apatterson2.ordermygift.com

-------------------------------orig.-------------------------
On Monday 09 March 2009 01:21, Tim Hickey wrote:

First a public "Thank You" to those who responded to my email about direct
communications between builders.

And thanks to Wayne who showed me how to find the address from the
incoming email.

And I agree with Bob P that it is best to ask the questions here on the
list so that everyone has a chance to see the discussion. But there are
still times when a direct question from me to you might be best off line,
if only to conceal my own "cloudlessness" while I dig for understanding.

The choice of engine for a Rebel has many facets. Since we are building to
a 1320 LSA gross, the empty weight of the completed machine is very
important. Considering that the Rebel was designed to a 1650 GW, we are in
a bit of a hole to start with. In other words, the airframe is a lot
beefier for a 1320 gross that it might need to be. That being said, there
are a few recognized areas that can stand some additional "strengthen" to
avoid problems in the future.

The airframe will weigh what it weighs when we are done. We probably don't
really have too much influence on the empty weight of the airframe. The
best thing we can do is not go overboard on interior trim, and we should
keep the panel light and simple. After all, this is a day, vfr machine
only.

The engine choice is another matter. I really prefer to use a Lyc O-235
with 118 hp if I can stand the weight. If not, then I am moved to used the
Rotax.

My questions about Jim Coles C of G, and the 3 inch extension of his motor
mount we based on wondering if the extension should be more than the 3
inch. Just off the top of my head, it would seem that everything added to
the aircraft in the way of payload serves to move the C of G towards the
rear. If that is true, and I have not yet investigated that, then would it
not be best to have the C of G of the empty aircraft plus the pilot
located well towards the front of the permissible range?

We are months away from having to make that choice, but it is an important
one, and it is never to soon to start gathering data.

One more question:
Does anyone have hard data on the weight of a paint job for a Rebel?

Thanks.

Tim Hickey
R808

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Tim,

My statement was meant to be that to achieve maximum speed, all else being
equal, a more aft CG is desired, at least in a conventional single engine,
single wing design. It is a little counter-intuitive, but I believe the
analysis goes something like this.

First of all, in flight (as opposed to on the ground) the center of lift is
the pitch control pivot point, and is (should be) always aft the CG,
otherwise the plane will be highly unstable at stall, where you need CG
(weight) forward of the center of lift to bring the nose down for stall
recovery. In addition, we know that all lift, whether upward or downward
acting, produces drag. The more lift, wherever and whatever its vector, the
more drag.

When in trim, straight and level, with the CG forward of the pivot point
(center of lift), tail down force (negative lift) is required to maintain
level attitude (and low angle of attack, least drag.) Here's the first key,
that we often forget: in flight, as opposed to on the ground, the pitch
rotation occurs around the center of lift, not the CG. The second and more
important key is: because tail down force produces unproductive drag from
unproductive downward or negative lift, minimizing it is "a good thing."

To increase speed, thrust must be increased to overcome increasing drag AND
the angle of attack must also be minimized to reduce drag. Increasing thrust
initially results in a nose up motion (from increased lift) which in turn
requires greater inefficient, drag producing tail down force (trim) to
maintain a low angle of attack. Consequently, the more aft the CG (the
closer CG is to the center of lift pitch pivot point) the less additional
tail down force is required to offset the nose up effect of increased
thrust, *because there is less _moment_ to that weight which is forward of
the center of lift* (pivot point). In other words, the CG is closer to the
pivot point (center of lift) against which any weight/moment supplied by the
tail down force must work. Or to put it slightly differently, with aft CG
the tail has better leverage due to the minimization of the moment between
the CG and the center of lift pivot point.

In short (?), with more aft CG, e.g., closer to but still forward of the
center of lift, the less tail down force is required to counteract the nose
up pitching moment resulting from increasing thrust. Because less downward
lift force is required to maintain a low angle of attack, the tailplane
produces less drag in doing its job to compensate for the increase in nose
up, caused by the increase in speed from the increasing thrust.

One more time, to review, left alone with lift at the wing only and neutral
force at the tail, the nose will drop because the CG is forward of the
center of lift. (Or at least, it better be!) Increasing speed increases
drag, which must be minimized by maintaining low angle of attack, and
minimizing unproductive drag produced by the balancing, downward force
(negative lift) at the tail. Less "downward lift" is required to be produced
by the tail when the moment of the CG, always forward of the pivot, is
least.

I hope that makes sense. It's more or less as my CFI explained it to me, I
think. No doubt, he used about 20% of the words! :-) But then again,
repetition can be beneficial.

At the Reno Air Races, you'll find all the contestants striving to maintain
the greatest aft CG they possibly can.

Ron

PS - Conversely, the main advantage of a canard design is both lift surfaces
generate desirable, productive, upward lift. No drag is wasted to prop up a
CG forward of the center of lift pivot.


On Sun, Mar 8, 2009 at 6:39 PM, Tim Hickey <tjhickey@iowatelecom.net> wrote:

...
Which leads me to a another question. How does CG location affect
performance?
I thing that a forward CG makes for a more stable machine, but how does
moving the CG back improve "performance"?

Tim
R808

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Well, as usual, darn it, there's at least one mistake in my long winded
explanation. I wrote:

"Consequently, the more aft the CG (the closer CG is to the center of lift
pitch pivot point) the less additional tail down force is required to offset
the nose up effect of increased thrust..." which is not what I meant to say.
It should be (as it is elsewhere):

"Consequently, the more aft the CG (the closer CG is to the center of lift
pitch pivot point) the less additional tail down force is required to offset
weight pulling the nose down forward of the center of lift..."

If I keep at this, there's a remote chance I'll write it right. :-(
Hopefully, you get the drift.



On Sun, Mar 8, 2009 at 8:35 PM, Ron Shannon <rshannon@cruzcom.com> wrote:

Tim,

My statement was meant to be that to achieve maximum speed, all else being
equal, a more aft CG is desired, at least in a conventional single engine,
single wing design. It is a little counter-intuitive, but I believe the
analysis goes something like this.

First of all, in flight (as opposed to on the ground) the center of lift is
the pitch control pivot point, and is (should be) always aft the CG,
otherwise the plane will be highly unstable at stall, where you need CG
(weight) forward of the center of lift to bring the nose down for stall
recovery. In addition, we know that all lift, whether upward or downward
acting, produces drag. The more lift, wherever and whatever its vector, the
more drag.

When in trim, straight and level, with the CG forward of the pivot point
(center of lift), tail down force (negative lift) is required to maintain
level attitude (and low angle of attack, least drag.) Here's the first key,
that we often forget: in flight, as opposed to on the ground, the pitch
rotation occurs around the center of lift, not the CG. The second and more
important key is: because tail down force produces unproductive drag from
unproductive downward or negative lift, minimizing it is "a good thing."

To increase speed, thrust must be increased to overcome increasing drag AND
the angle of attack must also be minimized to reduce drag. Increasing thrust
initially results in a nose up motion (from increased lift) which in turn
requires greater inefficient, drag producing tail down force (trim) to
maintain a low angle of attack. Consequently, the more aft the CG (the
closer CG is to the center of lift pitch pivot point) the less additional
tail down force is required to offset the nose up effect of increased
thrust, *because there is less _moment_ to that weight which is forward of
the center of lift* (pivot point). In other words, the CG is closer to the
pivot point (center of lift) against which any weight/moment supplied by the
tail down force must work. Or to put it slightly differently, with aft CG
the tail has better leverage due to the minimization of the moment between
the CG and the center of lift pivot point.

In short (?), with more aft CG, e.g., closer to but still forward of the
center of lift, the less tail down force is required to counteract the nose
up pitching moment resulting from increasing thrust. Because less downward
lift force is required to maintain a low angle of attack, the tailplane
produces less drag in doing its job to compensate for the increase in nose
up, caused by the increase in speed from the increasing thrust.

One more time, to review, left alone with lift at the wing only and neutral
force at the tail, the nose will drop because the CG is forward of the
center of lift. (Or at least, it better be!) Increasing speed increases
drag, which must be minimized by maintaining low angle of attack, and
minimizing unproductive drag produced by the balancing, downward force
(negative lift) at the tail. Less "downward lift" is required to be produced
by the tail when the moment of the CG, always forward of the pivot, is
least.

I hope that makes sense. It's more or less as my CFI explained it to me, I
think. No doubt, he used about 20% of the words! :-) But then again,
repetition can be beneficial.

At the Reno Air Races, you'll find all the contestants striving to maintain
the greatest aft CG they possibly can.

Ron

PS - Conversely, the main advantage of a canard design is both lift
surfaces generate desirable, productive, upward lift. No drag is wasted to
prop up a CG forward of the center of lift pivot.


On Sun, Mar 8, 2009 at 6:39 PM, Tim Hickey <tjhickey@iowatelecom.net>wrote:

...
Which leads me to a another question. How does CG location affect
performance?
I thing that a forward CG makes for a more stable machine, but how does
moving the CG back improve "performance"?

Tim
R808

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Great, now I have a headache! Back to the beer! :)



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Hi Guys!

I seem to remember something fairly recent regarding the fin attachment on
the rebel. I think it was additional reinforcing beyond what the manual
called for. Anyone shed some light? Thanks in advance. Craig




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Sorry about that, Craig, but better get the beer out of the frig right now,
'cause there's more coming soon... on the issue of max. empty weight of a
light sport. Get two. Maybe three. :-)


On Sun, Mar 8, 2009 at 9:23 PM, craig <snowyrvr@mtaonline.net> wrote:

Great, now I have a headache! Back to the beer! :)

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[ALERT: Another long winded epistle from Ron follows. May be headache
inducing. Read only if all issues of Readers' Digest from 1962 are missing
from all libraries with a 50 mile radius, and even then at your own risk,
which may be substantial.]

Bob,

Your mention of a max. empty weight for LSA's caught my attention. I am not
familiar with any regulation in the US of maximum empty weight for an
amateur built experimental airplane (ABE - my acronym only, doesn't appear
in any regs as such), whether flown under light sport regulations or not,
and I don't think there is one.

In the ASTM standards for Special Light Sport Aircraft (SLSA -- factory
built, turnkey aircraft) there _is_ a maximum empty weight standard. If
memory serves, I believe it is stated thusly: max GW (1320 lb.) minus two
170 lb occupants (340 lbs.), minus half the engine's horsepower in fuel. A
100 HP engine would require an allowance for a minimum 50 lbs. of fuel
(approx. 8.3 gal.), so the maximum empty weight of an SLSA so powered would
be 940 lbs. -- if my math is correct.

As you probably know, there is another category of light sport aircraft that
is very confusingly called "Experimental Light Sport Aircraft." The ELSA is
not what most of us are building, which is amateur built experimental (ABE).
The ELSA must first be an SLSA qualified and certified design, though part
of which is built by the owner. Unlike the amateur built experimental (ABE)
type, the owner-built portion of a true ELSA can be as little as say, 5%.
Because the ASTM max. empty weight applies to SLSA's, it would also apply to
ELSA's which in effect have to be approved SLSA's before they can be sold
partially completed as an ELSA.

On the other hand, when what we are building, a true amateur built
experimental (ABE), is inspected here in the US, the inspector (DAR) is not
concerned with whether it will be flown by a light sport pilot or an
ATP-rated pilot. The pilot's qualifications are irrelevant. Similarly,
within ABE aircraft regs. there is no designation or sub-classification such
as a "light sport ABE" or "ABE limited to light sport pilots", so the light
sport regulations, including ASTM standards for SLSA's, are technically
irrelevant to the inspection and certification of any ABE. There is nothing
on the airworthiness certificate (much less the registration) which says
anything about light sport.

It is the ABE builder who is responsible for establishing the GW of the
airplane he builds, because he is the manufacturer. As you probably know, no
plane, whether experimental or conventionally certificated, can ever be
flown under light sport regluations if it has _ever_ had an airworthiness
certificate issued when its gross weight was more than 1320 lbs. (That's why
a Cessna 140 can never be flown as a light sport, even if the owner somehow
gets a new airworthiness cert., or swears on a stack of whatever never to
fly it heavier than 1320 lbs., even though its original GW is just a little
too heavy. A bummer, but true.) So... in order for a sport pilot to be able
to legally fly a Rebel as a light sport plane, the only requirement
vis-a-vis the airplane paperwork is that the builder must set its max. GW at
1320 lbs. or less at the time of its first inspection, for its first
airworthiness certificate. This is set in two places: on the official first
weight and balance statement, which will probably be inspected, and on the
permanently-affixed metal data plate, which definitely WILL be inspected.

It is certainly possible that some DAR would refuse to issue an
airworthiness certificate for an ABE plane with a GW of 1320 lbs. that
weighed (to push this to an absurd extreme) say, 1300 lbs. empty, and that
DAR may even have in the back of his mind knowledge of the ASTM standard for
SLSA's, even though in fact there is no official FAA regulation for maximum
empty weight of any ABE. The DAR has some discretion when it comes to
something that's clearly absurd, for sure. But if I, as a sport pilot, want
to fly a 1320 GW Rebel that has an empty weight of 1090 lbs, and it can only
carry me, a chart, a toothbrush, scarf, credit card, and an hour's worth of
fuel -- it would be legal. It would be a short flight plan, given reserve
requirements, but....

As the first "from scratch" sport pilot in most of Northern California, I
was forced study this stuff quite a bit. The regs were all new and even more
ambiguous than they are now. Nobody, including the local FSDO and DPE's knew
what some of the regs meant. I bought the Rebel project 3 weeks before I got
my sport pilot ticket and would not have done so if I wasn't sure that there
was no formal max. empty weight involved for me to fly it as a light sport
plane -- because at that time I couldn't know what the empty weight would
end up being. If I'd had the slightest doubt, I wouldn't have bought
something I might not be able to fly.

Now of course my great certainty about all this doesn't make it so, and I
admit I have been wrong at least once before. :-) But if there's a
regulation for max. empty weight of a true amateur built experimental
airplane, whether flown by a light sport pilot or not, I've yet to find it,
and believe me, I'm even more highly motivated to keep a sharp eye out for
it now than I was when I bought 254R! If anyone knows of such a reg, I'd
rather know now than later, while there's still time to take out the DVD
player, air conditioning, and known ice equipment. :-)

Best,
Ron



On Sun, Mar 8, 2009 at 6:50 PM, Bob Patterson <bobp@prosumers.ca> wrote:

...

It is POSSIBLE that you could come in under the 872 lb. ?? (890 ??)
max. allowed for LSA ..... JUST... with the Lyc. O-235-L2C.

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Yup, I have been pleasantly surprised many times how well my Rebel flies
with a bigger load. It does not slow down as much as I would expect. I
attribute this to the more aft c of g when fully loaded. And if that means
less load on my tail, I guess that is good, too, and I can worry less about
it breaking off because of my bigger engine.

I wonder how my VGs affect cruise flight with respect to more or less
downward nose attitude. I have not noticed anything different. But if you
tell me otherwise, then I probably will...

Walter

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Ron
Shannon
Sent: March 8, 2009 9:44 PM
To: rebel-builders@dcsol.com
Subject: Re: [rebel-builders] What I learned this week!

Well, as usual, darn it, there's at least one mistake in my long winded
explanation. I wrote:

"Consequently, the more aft the CG (the closer CG is to the center of lift
pitch pivot point) the less additional tail down force is required to offset
the nose up effect of increased thrust..." which is not what I meant to say.
It should be (as it is elsewhere):

"Consequently, the more aft the CG (the closer CG is to the center of lift
pitch pivot point) the less additional tail down force is required to offset
weight pulling the nose down forward of the center of lift..."

If I keep at this, there's a remote chance I'll write it right. :-(
Hopefully, you get the drift.



On Sun, Mar 8, 2009 at 8:35 PM, Ron Shannon <rshannon@cruzcom.com> wrote:

Tim,

My statement was meant to be that to achieve maximum speed, all else being
equal, a more aft CG is desired, at least in a conventional single engine,
single wing design. It is a little counter-intuitive, but I believe the
analysis goes something like this.

First of all, in flight (as opposed to on the ground) the center of lift

is

the pitch control pivot point, and is (should be) always aft the CG,
otherwise the plane will be highly unstable at stall, where you need CG
(weight) forward of the center of lift to bring the nose down for stall
recovery. In addition, we know that all lift, whether upward or downward
acting, produces drag. The more lift, wherever and whatever its vector,

the

more drag.

When in trim, straight and level, with the CG forward of the pivot point
(center of lift), tail down force (negative lift) is required to maintain
level attitude (and low angle of attack, least drag.) Here's the first

key,

that we often forget: in flight, as opposed to on the ground, the pitch
rotation occurs around the center of lift, not the CG. The second and more
important key is: because tail down force produces unproductive drag from
unproductive downward or negative lift, minimizing it is "a good thing."

To increase speed, thrust must be increased to overcome increasing drag

AND

the angle of attack must also be minimized to reduce drag. Increasing

thrust

initially results in a nose up motion (from increased lift) which in turn
requires greater inefficient, drag producing tail down force (trim) to
maintain a low angle of attack. Consequently, the more aft the CG (the
closer CG is to the center of lift pitch pivot point) the less additional
tail down force is required to offset the nose up effect of increased
thrust, *because there is less _moment_ to that weight which is forward of
the center of lift* (pivot point). In other words, the CG is closer to the
pivot point (center of lift) against which any weight/moment supplied by

the

tail down force must work. Or to put it slightly differently, with aft CG
the tail has better leverage due to the minimization of the moment between
the CG and the center of lift pivot point.

In short (?), with more aft CG, e.g., closer to but still forward of the
center of lift, the less tail down force is required to counteract the

nose

up pitching moment resulting from increasing thrust. Because less downward
lift force is required to maintain a low angle of attack, the tailplane
produces less drag in doing its job to compensate for the increase in nose
up, caused by the increase in speed from the increasing thrust.

One more time, to review, left alone with lift at the wing only and

neutral

force at the tail, the nose will drop because the CG is forward of the
center of lift. (Or at least, it better be!) Increasing speed increases
drag, which must be minimized by maintaining low angle of attack, and
minimizing unproductive drag produced by the balancing, downward force
(negative lift) at the tail. Less "downward lift" is required to be

produced

by the tail when the moment of the CG, always forward of the pivot, is
least.

I hope that makes sense. It's more or less as my CFI explained it to me, I
think. No doubt, he used about 20% of the words! :-) But then again,
repetition can be beneficial.

At the Reno Air Races, you'll find all the contestants striving to

maintain

the greatest aft CG they possibly can.

Ron

PS - Conversely, the main advantage of a canard design is both lift
surfaces generate desirable, productive, upward lift. No drag is wasted to
prop up a CG forward of the center of lift pivot.


On Sun, Mar 8, 2009 at 6:39 PM, Tim Hickey

<tjhickey@iowatelecom.net>wrote:

...
Which leads me to a another question. How does CG location affect
performance?
I thing that a forward CG makes for a more stable machine, but how does
moving the CG back improve "performance"?

Tim
R808

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