RANS S7 Short Tail
WHAT’S GOING ON WITH THE CG LIMITS
AND
DID YOU KNOW THAT RANS CHANGED THE ARM
MEASUREMENTS?
If you fly a short tail Rans S7, you may want to struggle through the following
discussion. You may find some things mentioned here just a little curious and
they may prompt you to give Rans a call. The net of it is that the early models
of the S7 used a much further aft CG limit than the new ones and the arm
measurements for pilot, passenger and baggage have changed by as much as 5”
when there was no
physical dimension changes in
the cabin section of the airframe. Perhaps there were some issues with
previous measurements that have been corrected in the new models and maybe those
of us who fly the older models should be using the new numbers.
Maybe we should even be using
a more forward aft CG limit! Perhaps
Rans should weigh in on this.
Link to:
Summary Conclusions
and Recommendations
BACKGROUND:
The S7 short tail has been
in production for about 15 years. The Weight and Balance calculations used data
provided by Rans in a diagram included in the manuals. The builder weighs his
completed kit and plugs in his weights beside the moment arms provided by Rans.
The Datum from which the arm measurements are taken was the front face of the
prop flange. The chart says that the allowable CG range is 74 to 81” from the
prop flange datum.
Here is the diagram:
THE BIG CHANGE:
In about 1999 Rans came out
with a lengthened version of the S7 called the “Long Tail”. This was
essentially the same fuselage except with 22” added to its length. Also about
that time, they made several more changes from the firewall forward (as well as
some wing changes) and called this the S7S model. With these new models Rans
changed the arbitrary datum used for calculations of the CG from the prop
flange to the firewall (just makes for a different arithmetical calculation and
does make more sense) but the other, more significant change was to specify a new, more forward CG range. Curiously, because of
the datum change, unless you specifically wanted to compare the CG ranges of
the two models, you would not detect that the actual range of CG as a
percentage of the MAC (mean aerodynamic chord, the relative position of the CG
on the wing chord) had been moved forward. And the change was not
insignificant: the S model’s aft CG limit appears
to be over 4” forward of the short tail’s. This is big.
The short tail range above is
74 to 81” which converts to 48 to 55” from firewall. The new long tail/S model
range is 46 to 51”, so the change looks like 4”. I’ll show later that actually
it is more like 48
to 53” so a 2” shift but still a significant
difference.
The reason why the shift is
not actually that apparent 4” is that Rans, along with the datum change, made
some significant changes in moment arm measurements even though the actual
fuselage was the same!
OK, so how does a short tail,
long tail and S fuselage compare from front to back? The answer is quite simple:
ALL THREE ARE IDENTICAL LONGITUDINALLY
FROM THE FIREWALL THROUGH THE CABIN AND BAGGAGE AREA RIGHT TO THE
I’d known this for some time but
before writing this I checked a 97 short tail to 2004 S model to be sure. ALL
station positions are identical. The extra 22” in the long tail/S is aft of the
battery and all measurements from firewall to gear, seat tubes, baggage station
and battery are IDENTICAL.
As with the CG range change
that wasn’t immediately obvious, Rans has slipped in changes of arm lengths for
pilot, passenger and baggage even though the fuselage measurements are
identical.
To sort this all out, you
will have to compare the arm numbers in the short tail weight and balance sheet
above to the ones below on a long tail/S:
Since the long tail/S models
use a different Datum, we will have to convert from the prop flange to the firewall.
On the 97 I measured it is 26.5 inches (although for years I’d been using 26”).
Here is the short tail
diagram with converted arms:

Because the main wheel arm is
the same when we use 26” from prop flange to firewall, let’s use 26” as the
conversion factor. Thus if we subtract 26” from the short tail arms we have to
end up with exactly the arms used in the long tail/S because the fuselages ARE
THE SAME in these positions. So, compare the column on the far right above
(converted arms) to the column beside it (S7S arms) and note that the pilot arm
differs by 5”, passenger by 2” and baggage by 2.5”.
Let me confuse things even
more. Those S numbers written on the right are what several guys are using as
arms for pilot, passenger and fuel in their spread sheets yet they all differ
from what is in the Rans manual for the S7S just above. Where did they come
from? Who is right? In any case whether you use what other guys are using or
what is in the manual both are quite a bit different from what Rans called out
for years on the short tails. We can only assume that Rans has improved the
accuracy of these numbers and thus we should use them.
A Partial Explanation
The table below the following
spreadsheet sets out the various arm measurements. The new arms place all the
variable weights (except fuel) much further forward. Thus for the same loading the CG calcs on a long tail/S will give a more forward CG than you get
by using the early Short tail numbers. This accounts for about one half of
the CG range change. The other half of the change is a result of Rans selecting
a more forward aft CG limit.
Here is a spread sheet that
illustrates (focus on the coloured blocks):
What can we learn from the
above? The yellow block is an aft CG
loading using the arm measurements provided in the Short tail manuals but with 26” subtracted from each to give us a firewall
datum.
The original numbers are
shown to the right of the yellow block. The loading was designed to give the
maximum aft CG number of 81” from prop hub or 55 from firewall.
For most aircraft, the
allowable CG range is between 25 and 30 % of the length of the wing chord. Here the position is much further back at
37.4%, quite a big difference from most aircraft and perhaps a little to far
aft to be safe.
But, maybe it is not actually
that far back. Rans now says that the arm measurements for pilot, passenger and
baggage have changed. Since all fuselage are identical in the battery to firewall
stations, we should be using these new arms for all fuselages.
The middle, green block uses the new arm numbers and the result is that the same loading gives an actual CG of about 2” further forward.
The blue block is a loading
that illustrates the current maximum aft CG for long tail and S airframes and
even it is a little further back than 30%.
If you want to play with the
actual Excel spreadsheet click here
Here are the current and old
arm measurements to the nearest inch and using the aft seat position numbers:
Pilot Fuel Passenger Baggage
37 52 65 93 pilot and passenger numbers are in the aft
of the ranges in the manual to be conservative
Compared to 40 52 70 95 From Rans Short tail sheet.
Others (like Gordon in the
If you stay with the older
weight and bal calculations and arms, as a minimum, adopt a more forward, aft
CG limit of 79” (53” with Fwall datum)
If you choose to use the new
arm measurments, you
MUST adopt a more forward aft limit and 77” (51” with Fwall
datum) would be a safe choice.
I encourage all short tail
owners to rework their weight and balance calculations using not only the
firewall datum and newest arm measurements, but also also adopt a new aft CG
limit of 51”
If you see any errors in this
please tell me. If you get any input from Rans, I’d like to hear that too.
Thanks
petercatpipcomdotcom