C of G, etc.
Posted: Fri Feb 17, 2012 8:39 pm
Hi Bruce !
Give me a week or so to dig out some numbers ....
'Way back in the early days, the C of G range WAS quoted at
7.5 inches to 22 inches !! I believe that they now recommend a
range of 12" to 17.5" (but I've been wrong before ! ;-) )
(These are only recommendations, in any case - you could check
the documentation on the NACA 4415 airfoil for the theoretical
limits ...) This was an excellent choice - it's a big, fat,
high-lift wing, with gentle, predictable stall characteristics,
no matter how it's loaded, and it's not particulary sensitive to
C of G position !
I have flown Rebels at 7.8" and at 12.5" empty - there's
not a GREAT difference. The controls are so powerful, that it's not
a problem of really high forces, or skittish handling. Not NEARLY as
sensitive to loading as my old Aeronca Super Chief !!
The only 2 Rebels that I've had a hard time getting a
decent flair with both had Lycoming O-235's, with the standard
firewall position. Both did NOT have even the spec'd 25 degrees
of up elevator - going to 27 to 30 degrees, (and carrying a bit
larger toolkit ;-) ) made them both pleasant to land.
Fully loaded Rebels are really not much different either - like
any heavier aircraft, they stall a little more sharply, at a higher
speed, and sink faster. The sink is definitely a Rebel characteristic
that can bite you !! If you slow below about 65 mph on final,
it can develop a pretty impressive sink rate (say - 1,500 fpm !).
This is deceptive, because, unlike most other aircraft, the Rebel
still has lots of elevator authority in this regime. Unless you
lower the nose, or add a TON of power, before you get down to a
hundred feet or so, you will get to the runway, flare, and hit
like a ton of bricks ! Even though the nose comes up, the sink
goes on ! That's why I prefer to approach with full flap at
75+ mph, aim short, break the glide at 20 feet or so, and bleed
off the speed and altitude gently, to give me time to get to the
right attitude. This works for me, <perhaps> because I owned a
Schreder HP-11A glider for 23 years - it had HUGE, 90 degree (!!)
flaps for glide control, instead of spoilers like most gliders
of its vintage ! You could point it almost straight down, and
barely maintain 55 KT on approach !!
The 912 Rebel has very long engine mounts (almost enough
room behind the engine for a spare !), so the lighter weight has
a longer arm, and the C of G is still well forward. (Although we
DID fly with a 10 lb. block of lead on the top of the engine for
several months ! ) Some heavy hose covers were added, and the lead
removed - and it worked just fine.
My feeling is that it is slightly easier to get the tail
down for a good 3-point with the latest Rebel with the O-320, than
it is with the old 912 Rebel. There are SO many differences between
them, though - No. 001 even has one degree higher angle of incidence
on the wing than ALL the other Rebels, AND totally different bellcranks,
control travel, control forces, etc...... I can fly it around
at 30 mph, with full control, but it gets very skittish at 110 !!
Between the extra weight and all the other differences, the
newer Rebel is more inclined to stay down if you are close to the
right attitude on touchdown - the 912 is more inclined to 'skip' !
I find that one of the best ways to get close to the right
attitude and altitude for a good landing is to try and fix in my
memory, the view I see over the nose and out to the sides just before
takeoff, and try to put the Rebel into that same position after
flaring. Carrying a bit of extra speed all the way down final helps
to get through wind shear, and gives a bit of 'floating' time to
allow me to play with the elevator and get the nose up to the
proper position, while bleeding off speed. Don't worry about landing
too far down the runway, just move your 'aiming point' back to
a few hundred feet before the threshold.
I've rambled on again .... Hope it was useful !
Sorry if I sound like I'm preaching - I've been a glider instructor
for over 26 years, and sometimes I lock in "patter" mode !! :-)
......bobp
I'd be happy to fly with 'most any Rebel builder who wanted
me along - I've done several test flights, checkouts, and ferry flights.
(From the north-east corner of Newfoundland to Bellingham, Washington,
and as far north as Kuujuaq, PQ (near the Bering Strait), and south to
Lakeland, Florida .... )
--------------------------------orig.------------------------------------
At 12:02 AM 1/6/01 EST, you wrote:
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Give me a week or so to dig out some numbers ....
'Way back in the early days, the C of G range WAS quoted at
7.5 inches to 22 inches !! I believe that they now recommend a
range of 12" to 17.5" (but I've been wrong before ! ;-) )
(These are only recommendations, in any case - you could check
the documentation on the NACA 4415 airfoil for the theoretical
limits ...) This was an excellent choice - it's a big, fat,
high-lift wing, with gentle, predictable stall characteristics,
no matter how it's loaded, and it's not particulary sensitive to
C of G position !
I have flown Rebels at 7.8" and at 12.5" empty - there's
not a GREAT difference. The controls are so powerful, that it's not
a problem of really high forces, or skittish handling. Not NEARLY as
sensitive to loading as my old Aeronca Super Chief !!
The only 2 Rebels that I've had a hard time getting a
decent flair with both had Lycoming O-235's, with the standard
firewall position. Both did NOT have even the spec'd 25 degrees
of up elevator - going to 27 to 30 degrees, (and carrying a bit
larger toolkit ;-) ) made them both pleasant to land.
Fully loaded Rebels are really not much different either - like
any heavier aircraft, they stall a little more sharply, at a higher
speed, and sink faster. The sink is definitely a Rebel characteristic
that can bite you !! If you slow below about 65 mph on final,
it can develop a pretty impressive sink rate (say - 1,500 fpm !).
This is deceptive, because, unlike most other aircraft, the Rebel
still has lots of elevator authority in this regime. Unless you
lower the nose, or add a TON of power, before you get down to a
hundred feet or so, you will get to the runway, flare, and hit
like a ton of bricks ! Even though the nose comes up, the sink
goes on ! That's why I prefer to approach with full flap at
75+ mph, aim short, break the glide at 20 feet or so, and bleed
off the speed and altitude gently, to give me time to get to the
right attitude. This works for me, <perhaps> because I owned a
Schreder HP-11A glider for 23 years - it had HUGE, 90 degree (!!)
flaps for glide control, instead of spoilers like most gliders
of its vintage ! You could point it almost straight down, and
barely maintain 55 KT on approach !!
The 912 Rebel has very long engine mounts (almost enough
room behind the engine for a spare !), so the lighter weight has
a longer arm, and the C of G is still well forward. (Although we
DID fly with a 10 lb. block of lead on the top of the engine for
several months ! ) Some heavy hose covers were added, and the lead
removed - and it worked just fine.
My feeling is that it is slightly easier to get the tail
down for a good 3-point with the latest Rebel with the O-320, than
it is with the old 912 Rebel. There are SO many differences between
them, though - No. 001 even has one degree higher angle of incidence
on the wing than ALL the other Rebels, AND totally different bellcranks,
control travel, control forces, etc...... I can fly it around
at 30 mph, with full control, but it gets very skittish at 110 !!
Between the extra weight and all the other differences, the
newer Rebel is more inclined to stay down if you are close to the
right attitude on touchdown - the 912 is more inclined to 'skip' !
I find that one of the best ways to get close to the right
attitude and altitude for a good landing is to try and fix in my
memory, the view I see over the nose and out to the sides just before
takeoff, and try to put the Rebel into that same position after
flaring. Carrying a bit of extra speed all the way down final helps
to get through wind shear, and gives a bit of 'floating' time to
allow me to play with the elevator and get the nose up to the
proper position, while bleeding off speed. Don't worry about landing
too far down the runway, just move your 'aiming point' back to
a few hundred feet before the threshold.
I've rambled on again .... Hope it was useful !
Sorry if I sound like I'm preaching - I've been a glider instructor
for over 26 years, and sometimes I lock in "patter" mode !! :-)
......bobp
I'd be happy to fly with 'most any Rebel builder who wanted
me along - I've done several test flights, checkouts, and ferry flights.
(From the north-east corner of Newfoundland to Bellingham, Washington,
and as far north as Kuujuaq, PQ (near the Bering Strait), and south to
Lakeland, Florida .... )
--------------------------------orig.------------------------------------
At 12:02 AM 1/6/01 EST, you wrote:
---------*Bob,
You say your 0320 C of G without firewall mod is 8.1. The MAM recommended
forward C of G is 10.85. Is this MAM recommended forward limit for an empty
weight or a loaded flying weight? What is the C of G for your 0320 when it is
loaded with you and about 20 gal. of fuel?
The 912 Rebel had to have a C of G far aft of that of the 0320. You must have
noticed a difference in the're landing characteristics, didn't you? Please
share more of you personal experience, as part of my decision to not remodel
my firewall was listening to you're and Wayne's comments on this subject
several months ago.
Bruce G 357
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