What size Floats?
2009/02/23
94 S7 Short Tail on Aerocet 1100
amphibs
While
your choice of floats for 1300 lb aircraft is somewhat limited, you do need to
consider some important issues before you buy. While my experience is primarily
with Rans S7 and S7S aircraft, these considerations
apply to all light aircraft.
Two of
the issues are:
1.
What
does adding floats do to my useful load both practically and legally?
2.
How
does the size of the float affect performance?
Except
for some smaller ultralight floats which can be categorized by length, usually
floats are assigned a number which is the maximum gross weight that the floats
will support. Thus a pair of 1260 floats
will nominally support 1260lbs maximum all up weight of loaded aircraft on
floats.
GROSS WEIGHT CONSIDERATIONS
The first
issue is what happens to useable load when you go to floats.
With most
S7 and especially S7S aircraft, if you put full fuel and two 180lb people in
the plane with the floats installed you will be well over the manufacturer’s
1232 gross weight number. Using typical
weights, straight, non amphib floats can add about 150lbs to the empty weight
of the airframe AFTER removing the wheeled landing gear. You can easily
calculate what this means to your useful load. It would be rare to find a set
of amphibs of the right size that could match this 150LB added weight; amphibs are typically 75 to
100lbs heavier than straight floats.
With many
certified aircraft, float manufacturers successfully argued for an increase in
gross weight when on floats. Sometimes you hear the (unsubstantiated) comment
that “the floats carry their own weight”. Unless the airframe manufacturer
spells this out, legally, it does not happen.
So, the
net of this is that you will not be able to legally load your plane on
floats the way you did on wheels.
PERFORMANCE CONSIDERATIONS
What is
legal and what flies just fine are two different pieces of fruit. Both the S7
and the S7S will perform better than a lot of float aircraft even when they are
100lbs over legal gross weight. What does affect performance is the size of the
float and here, bigger may be a lot better. In this context, “performance”
means how well it gets off the water. Yes, bigger floats are likely to reduce
top speed but there will not be major differences in speed between different
sizes of floats. With any floats your top speed drops dramatically anyway by
maybe 20%.
Float
displacement vs aircraft weight is a very crucial comparison. As a general
rule, if you have a choice and there is not a huge weight penalty go with a
displacement a lot larger than your gross weight; not the same and never
smaller.
On the
other hand, there may be some variation from manufacturer to manufacturer
on the significance of their displacement number so, in other words, not all
1150's across different manufacturers are the same. I suspect that few
manufacturers have better than only an approximate displacement number for
their floats.
I have
owned and flown Aerocet 1100 amphibs on a 94 S7 short tail (weight was likely
650 on wheels); 1260 Full Lotus on short tails and on a 750lb S7S; 1300 Quebec
floats on a short tail, and 1500 Murphy on several shorts and the S7S.
The 1100
Aerocets were a very good looking float and they would support the S7 with two
on board but you had to be very careful because it was quite easy to actually
submerge the bow of the floats under certain conditions (like turning in a
wind) and, if you have never experienced that, the first time it happens is
quite unnerving. The conclusion is that 1100 floats on a 650lb empty plane are
too small. (I don’t have the complete weight and balance for that installation
but can provide it for all the others mentioned here).
On the
other hand, when I sold that plane, the buyer wanted to fly off both land and
water so he met me at a land strip on a hot calm day. We took off there, flew
to a nearby lake, did a few landings and takeoffs and flew back to the land
strip. He was quite satisfied and bought the plane.
1260
Lotus work quite well on a short tail but they are certainly not too large. You
will be able to sink the heels of the floats when you are helping the rear seat
passenger in. At gross they are quite acceptable on the short tails but you
will find situations on a 750lb S7S (like the one above) where you would prefer
a larger float.
The
1260’s above were on the plane when the buyer test flew it. He agreed they were
useable but knew he would want larger. He has now purchased a set of Lotus 1450
floats. The 1450 is a curious design.
See the bottom of the page on Float Geometry for
some thoughts on Full lotus floats and some concerns about the 1450 in
particular.
All
things considered, the 1650 may be a better choice for a heavier S7S.
The 1300
floats above were perfect on this 670 lb S7 but these were light floats at
167lbs so after removing 42lbs of landing gear they only added 125lbs.
The
floats that give the best all around performance and handling are the Murphy
1500 shown above. At about 180 lbs complete, these floats weigh about the same as
Lotus 1260’s but have 240 lbs more buoyancy. They were perfect for the fully
loaded S7S but also worked just as well on the lighter short tails. You would
never submerge the nose (or the tails) of these floats. You can see from the videos on my home page that off the water
time is excellent (typically around 12 seconds) depending on the plane and at
lighter loading.
Here are
the same floats on a 625 lb 80 hp 94 S7. It gets off the water in about the
same time as the 100hp S did.
Another
float that may be a good size is the 1400 Aerocomp. I
have seen these in both straight and amphib but have no experience with them. A
local dealer likes his straight set on an S6. One thing I don’t like about them
(and some early Czech floats were similar) is the narrow, 4” backbone for walking.
Conclusion
In
general, you will be happier with floats that have at least 100 lbs more
flotation than the total weight of the loaded, float equipped aircraft.
Anything less is a compromise that you might regret. The best plan would be to
fly before you buy and do it in wind and load conditions that would show up any
under floating.
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