Rigging Geometry of Floats
(Homebuilt 170/180hp on
2009/11/22
This information is relevant and accurate for light wing loading and low stall speed aircraft such as the 1200lb gross Rans S6 and 7, however, the same considerations were used in rigging the homebuilt 170 on 2425 Edo’s shown above.
Click to skip to Lotus 1450 issues
To position a set of floats
on an airframe we need to decide on four measurements:
1.
The distance between the
floats;
2.
The height of the airframe
above the float
3.
The fore/aft position of
the airframe on the floats and with Lotus floats, where to position
the spreader bars/tubes;
4.
The angle between the
centerline of the airframe and the bottom of the float.
First, Centre to Centre width.
If you look at successful
float installations it is reasonable to conclude that the first two are based
on personal preference and what look the builder wants. There is a range of float
center to center distance of from 40 to 50% of float length. My Murphy 1500
floats (174”) came with a width of 48%; I cut them back to 80” or 46%.
Many 1260 Full Lotus are
mounted at 72” which is 44%; My 1350 LAS floats came with a width of 41% but I’ve
lengthened the spreaders to 77” or 46%.
A wide stance will improve
cross wind stability but will make standing near the side of a tandem fuselage
more difficult.
This S6 on Czech amphibs has
a 75” width (and 20” height). Because it is a side by side plane with a wider
fuselage the owner feels the struts look too vertical but it still works just
fine. Also the attachment points to the floats are on the spreader bars so that
further narrows the apparent stance of the struts.
The 1450 Lotus floats
described further down in this write up have a different looking geometry and a
centre to centre distance of only 66”
or 40% of the length on this
Rans S7.
Whereas these 1260 are 73” centre to centre:
Height of fuselage above floats.
This measurement also seems
to be highly related to personal preference. Yes you need to keep the prop away
from the water but most installations result in the prop tip being much further
off the water than the minimum 12”,
My first Rans S7 was mounted
at about 16” above the floats like this and it swung a 72” prop:
It flew well (except for some
typical Lotus porpoising) and was very easy to get
into without the need for a step on the struts but certainly lacks “dock
appeal”.
One of the highest mountings is
on Murphy Rebels where the company rigging shows 31.5” for the fuselage above
the floats. One experienced Murphy guy uses 29”.
This Rans S7 on Czech amphibs
is at 20”
So, with all this in mind, I
went with 80” width and 24” height on these Murphy 1500 on my Rans S7:
There is also some
correlation between height above the floats and step position when you adopt
one approach to positioning the step which will be discussed later.
Angle between datum and floats.
An interneter from
Frey points out that for best
take off performance we want the wing at the angle of attack for maximum lift
while the floats are riding in the water at an angle for minimum drag. He
suggests that a flapped wing needs 14 degrees (this is not true for all
airfoils but close) so the geometry has to provide this. So, how do we achieve that 14* angle of attack?
First we need to use the
horizontal datum line of the aircraft as the reference line for rigging the
floats. Next, most designers have built in a positive angle of incidence of the
wing center line to the datum line of 2 to 3 degrees. Let’s use three for now.
Frey points out that early
studies showed floats need to ride at 8 degrees for minimum resistance while
planing. Thus, if we mounted the floats
parallel to the datum line we would have an angle of attack of 8 + 3 or 11
degrees when the aircraft is on the step. So, we need to mount the floats at 3
degrees negative to the datum line to get our 14 degrees.
A knowledgeable friend
determined from studies of similar airfoils that my Rans S7 actually achieves
max lift at 18 degrees. This would require not 3 degrees between float and
datum but 7. I have mounted the floats a little more than 3 degrees and
takeoff, cruise and landing performance is excellent; the best performance I’ve
had over four different S7 float planes.
I would predict that cruise
speed and landing characteristics would suffer with more angle between float
and datum. The compromise here is that we don’t want the nose of the floats too
low while in level flight to increase drag or to make it difficult to achieve a
slight nose up position of the floats on landing. While I will experiment with this in the
future, for now, 6 degrees between float and wing CL works well.
When I was installing 2425
Finally, where to position the airframe on the floats
(Usually looked at as the fore/aft location of the step)?
Based on what
While most float mounting
instructions have the airframe positioned with the datum line level (an
in-flight attitude), the
The discussion of angles
above deals with the transition to flight, in cruise and landing attitude, none
of which has much bearing on the position of the step. The loading of the
floats by setting an airplane on them could be compared to loading a boat. The
small outboard sitting at the dock rests at a specific, more or less level,
probably a bit nose up, attitude. If we are loading several people into the
boat, we position them not all at the front or all at the back but more or less
evenly distributed to retain that level attitude. I suggest we are loading the
boat by distributing the weight equally around the C of B.
Most floats sit in the water
with about a 3 degree nose high attitude (although the two sets I’ve installed
are at 4.5* to water). The centre of buoyancy is close to the C of G of the
float and both are a few inches ahead of the step. Frey says we draw a VERTICAL
line up from this point ahead of the step while the float is in this floating
attitude (not level, not with the 3 degree nose down to datum that we know we
need). It is on this vertical line that we position the cg of the bare aircraft minus landing gear.
You need to make a drawing to
see the impact of this:
This drawing illustrates
several points.
First, it shows that the
horizontal distance of the step from the cg depends on how high the plane is
above the floats when we do the rigging with the datum horizontal.
Further it illustrates that
if we go back to the more traditional set of rigging instructions which do
everything with the datum horizontal (try rotating the picture counter
clockwise to get the datum level), we see that the cg is well ahead of the
step. But what cg is this?
What this all boils down to
is that to simplify rigging instructions in an environment where minor
variations are not very critical, people have adopted methods that are easy to
use and generally work. It is simpler for us to deal with a level datum and put
the step at or aft of some cg point.
My S7 above is a good
example. The bare cg – gear is 74” (aft of prop hub); the max rearward is 81” a
range of 7”. In fact, I put the step at 1”
aft of the aft cg. (datum level) or 8” aft of the forward cg. Using the above diagram and the height of the S7 cg above the approximate
C of B of the floats, that 6 degree angle line puts the C of B about 4.5“ aft of the forward cg. Let’s say the C of B
is 3” ahead of the step making the step 7.5” aft of the forward cg. This means that I’m within ½” of the
So, it is easy to see that
the
Perhaps a better translation
of the Edo method is: With the aircraft
datum level drop a vertical line from the CG of the airframe without landing
gear; run another string from the CG angled rearward by the number of degrees
between the float datum and the centerline of the fuselage + the float at rest
nose up angle to the water (3 to 5 degrees). This sum will frequently be 3 + 4
=7 degrees. Locate the
Here is another hypothesis:
the center of buoyancy is probably pretty close to the C of G which is easily
determined by weighing the floats using two scales under the spreader bars. The
CG of the Murphy’s is 4.5” ahead of the step; the CG of the 1350’s is 8” ahead
of step.
Here is another curve. After posting this site to the Matronics
Seaplane list, Hagen Heckel from
With this in mind, I moved the
step on the 1350 floats to 5” aft of CG. This also seems to work fine. With a
220lb person in the back seat, the heal of the float submerges slightly when I
also stand on the float beside the rear seat, so I am going to move the
fuselage another inch forward. Why not
if fluid dynamics is the only issue? Yes overall CG is still fine.
The symptom of the aircraft
placed too far forward on the floats is a strong nose over tendency on landing
in a level attitude, something I’m not seeing yet.
These floats have a unique M
shaped bottom forward of step. They appear to accelerate more quickly as they
get on the step but ride noticeably harder on waves than a straight V bottom.
Lotus floats have less of a
rise from the step aft so the S-7S below is mounted at 4.5 degrees to the float
top so that little rotation is required at lift off. The wide angle is
noticeable but they are still faster in cruise than a set of Murphy 1500’s that
were on the plane previously.
The step is also further aft to
provide more rearward flotation when loading because these floats tend to have
minimal rear end flotation.
Some
thoughts on Lotus Floats. Skip to June 17
update
First, I should point out that, overall, I have been a proponent of Full Lotus floats
for years ever since I bought my first Rans S7 on 1260’s in 2003. In fact, the
company has used my testimonial on their site:
(Full Lotus floats are
terrific. They can take a lot of abuse from rocks or shallow water and handle
really well.)
and Aircraft Spruce has a
picture of one of my ex planes on their Full Lotus page. What follows is not
meant to be a condemnation of all Full Lotus floats nor even of the 1450’s but
rather an attempt to resolve some reasonable questions which arise from the
unusual shape of the 1450 float.
General comments:
These “air bag” floats
perform quite well and have advantages over other materials which include:
Less easily damaged when beaching,
Provide some shock absorbing on a hard landing,
Quite useable in the winter and more maneuverable than skiis,
A puncture may be easily repaired temporarily and will
affect only one bladder of the 8,
No pump out required.
The disadvantages are that
they do take on a small amount of water inside the bladders which have to be
drained at least annually and the air pressure must be monitored frequently due
to temperature changes. While they tend to be inexpensive, they do have a
limited life.
Here is a link to a video on
draining the floats: http://www.youtube.com/user/kitfoxflyer
For more thoughts on the pro’s and more con’s
of these floats see Dave Loveman’s site:
http://www.ultralightnews.com/lotus1/lotus.html
While Dave makes some good
points, I would disagree with a couple of things he says. For example he feels
that: “1260 floats do not have enough
floatation in the front section of the float for most two place, tractor
aircraft.”. It is not reasonable to make this blanket statement
without specifying the gross weight of the aircraft. From my experience aircraft like the early
Rans S7 at 1200lbs gross and 625 lbs empty, work fine on the 1260 floats.
Dave also suggests that the
configuration of the aircraft and the position of the significant weights such
as engine, pilot and passenger have a bearing on float performance. He says:
“In most pusher
configuration aircraft the weight put on the craft is distributed over the full
length of the float.” And: “On a
tractor aircraft the full weight of the engine sits on the front section, with
two pilots and full fuel normally located near or on the middle area of the
float.”
My understanding of the
physics of this is that the only crucial issue is where the C of G of the
aircraft is positioned on the floats. The floats only see this CG weight and
they “know” nothing about how it is distributed in the airframe. Thrust lines
could make a difference but not whether or not the engine is up front.
Position
of spreader bars/tubes.
Most rigid floats have the
spreader bars positioned more or less equally ahead and behind the step.
This also seems to work fine
with Lotus floats but occasionally you see variations. These 1260 floats have
the spreaders much further forward, perhaps to suit the location of hard points
on the airframe but this setup does result in some additional flexing of the
stiffener tubes. Perhaps a third, partial stiffener should be added.
The 1450 installation below
also has the spreaders further forward but they do come with pockets for the
third stiffener and the tail section is shorter than the forward section (and
shorter than the aft section of 1260’s) so has inherently more stiffness than
with the 1260’s above.
Here are 1260’s with a 3rd stiffener:
Why focus on the
1450?
Until recently there were
three sizes of Lotus floats in the light aircraft range: 1220, 1260 and 1650.
Clearly there was a large gap between the 1260 and 1650. The 1260 are a
satisfactory size for 1200lb gross weight aircraft like the earlier Rans S7
which had an empty weight of 600 to 650 lbs. but as mentioned above, the 1260
floats could use a little more flotation in the heels and are a little small
for 1300lb gross aircraft. Now that it
is common to see the S7S at 750 lbs empty, the 1260 is a marginal choice yet I
suspect many people would feel that the 1650 was too big a float (although it may
not be).
Now Lotus is selling the 1450
model. It turns out that the 1450 is not an enlarged 1260 with proportionate
increase in all dimensions. The company was quite creative and expedient in the
way they came up with this higher displacement float with an unorthodox shape
and as a result have generated some questions which they are more or less
unwilling to acknowledge let alone discuss informative answers.
To create the 1450 they used
the longer front end from a 1650 mated to the shorter heel of a 1220. In other words, compared to the 1260,
we have a bigger front end with a SMALLER rear end with the result that the
step is far aft of the mid point of the float.
Just how big is a
1450? (The following paragraphs
reflect the status of the company site as of April 2009. They have since updated
their specs sheets and do have info on the 1450 but
have a new format which does not highlight the step position measurement nor
are there cross section measurements.)
It is difficult to come up with
the actual size of the floats because there is no diagram of the 1450’s per se
on the FLM web site. When you ask for specs, you are told to figure out the
dimensions using the front half of the 1650 and the back half of the 1220.
However, even this is complicated by the fact that Jeff Holomis at Lotus quotes
even different dimensions and suggests that my scale side views shown below are
“way out of whack” as a result.
1.
The company web
site has only one measurement for the 1450: the length, which it shows as 13’
9” or 165”.
2.
To get the rest
of the dimensions Jeff tells you to go to the 1220 specs and take the
measurements for the aft section (5” x 20” and 66” long); then go to the 1650
specs and take the front measurements (16 x 27.5 x 97” ). Thus the overall
length from this method is 97 + 66= 163” (not 165).
3.
However, Jeff in
his email communication quoted 168” as the length in one note, then 14’6 or
174” in another and states that the size at the tail is not 5 x 20 but 7 x
24.
Thus, there are inconsistent
measurements on the company site which also differ from what is quoted in
emails.
Here is a chart
showing the dimensions of the floats taken from measurements on the company web
site where the 1450 numbers are derived from the 1220 and 1650 diagrams:
|
FLOAT |
LENGTH |
FORWARD |
AFT |
FWD/AFT X SECTION, STEP % |
|
1220 |
148 |
82 |
66 |
16x28 /
5x20 55% |
|
1260 |
166 |
82 |
84 |
16x27.5 /
4x18.5 49% |
|
1450 |
163 |
97 |
66 |
16x27.5 /
5x20 59.5% * |
|
1650 |
181.5 |
97 |
84.5 |
16x27.5 /
5.5x22 53% |
Even the depths of the floats
vary between the diagrams and Jeff’s numbers. He says the depth of the 1260 is 14”
while the depth of the 1450 is 17”. Taking numbers from the diagrams, however,
gives a depth of 16” for both floats. What do we believe? To clear up this multiplicity of numbers it
would be great if the company would simply post the specs for the float they
are manufacturing.
On
You can see from the above
that the cross sectional area of all of the forward sections of these floats is
approximately the same so overall bulk does not change just the lengths.
I’ve cut out some scale side views of these
floats based on the above dimensions to illustrate the differences between the
floats (top -1450, bottom - 1260):
These cutouts show how the
added length at the front contributes significantly to the increased flotation
(1260 to 1450 = 190lbs) but also that the heal of the float aft of the step is
much smaller (a rough calculation yields maybe 50 lbs but based on Lotus
numbers it is closer to 35).
This superimposed view shows
the heel volume difference with the 1450 having the smaller volume:
The patterns taper to a
sharper point than the actual dimensions would suggest because there is also a
narrowing of the float from side to side and the objective is to represent the
comparative volume. Since the actual measurements mentioned above show a
slightly thicker and wider tail end on the 1450’s, the pattern above should be
just a little larger at the tail end. The length difference, however, is
correct so that the decreased aft volume does still exist.
What this means is that if
1450 floats are replacing 1260’s and if they are mounted with the step at the
same position (since most people use the step as the significant reference
point) then there will be LESS flotation at the aft end even with these larger
floats. Clearly, mounted this way, they will make the aft flotation issue worse.
When asked about mounting these floats the company’s response is: “they are
mounted the same as the 1260” yet clearly this will result in too little aft
flotation.
On most floats the step is
positioned at close to the mid length point of the floats with the Centre of
Buoyancy typically a few inches ahead of the step (like it is on the 1260).
Earlier I mentioned the importance of the C of B in rigging the floats. While we tend to use the step as a reference
point, it is really the C of B position relative to the aircraft CG that is
critical (based on the material from Edo Corporation, see details earlier in
this page).
Now suppose we line the
floats up along a line joining the estimated C of B of the floats:
By using the C of B as the
primary guide rather than the step, the problem of the reduced heal flotation
would be addressed automatically but we would need to have the step at least
12” further aft and the question is would this affect rotation and lift off?
Clearly then, these floats
have different proportions to other Lotus floats and to floats from other
manufacturers so one would expect the manufacturer to provide some additional
guidance for rigging them on an airframe. The initial response from the
company, however, is that they should be mounted just like their other floats
with the step between 0 and 6” aft of the aircraft Cg. Given the smaller aft volume this can simply
not be the case. They must be moved further aft by some amount to compensate
for the reduced aft volume and prevent modest aft loads from sinking the float.
It is also not very precise
to use the term “aircraft CG” without specifying what configuration and loading
you are talking about.
After repeated attempts to
get the company to deal with this issue they suggested that instead of the
mounting “window” being 0 to 6”, it could be 0 to 8”. There is no data available from the company
on the positioning of these floats because they claim that they do not need to
be repositioned compared to the 1260. Based on the comparison of their shapes,
it is doubtful this can be the case.
On the other hand, with the
added flotation that these floats have over the 1260, maybe moving them aft a
modest amount may still allow them get off just fine. To confirm this, I wanted
to talk to people who have actually installed and flown this float after using
a 1260 to see how they have dealt with the aft flotation issue and the step
position. The owner of Full Lotus, Jeff Holomis, refuses to provide such
references nor will he comment on any research they have done on this issue
except to point to some YouTube videos which show airplanes taking off and
landing. I suspect they may not have
even thought about it and certainly not done any real, substantial testing. All
Jeff will say is that there are many happy customers.
One final possible issue is
that if the floats are mounted with the step in the same position as it was on
1260’s, the C of G of the float will be further forward and may complicate
weight and balance issues as well. With the early S7, the aircraft tends to
have a forward CG and mounting floats whose cg is more forward could be a
concern.
As of 2009/03/24 I have been able
to locate the following information:
Ken
Smith has installed a set of 1450’s on a Rans S7 but it has not yet flown. He
says he moved the step back maybe 5”. He did say a set is working OK on an S6.
A
reputable S7 builder on the east coast will not install 1450 floats and has
other “comments” about the company.
A
dealer in eastern
Finally, you see from the
above dimensions that the cross sections of the forward float barrels on the
different models are all basically the same. This means that the drag of the
floats is NOT going to increase significantly with increasing float size
because drag is proportional to “flat plate area”. Thus there won’t be much of
a drag penalty by going to the 1650 compared to the 1260 or 1450. Going back to
the table of dimensions above, it appears that moving from a 1260 to a 1650 may
be a more sensible alternative.
Conclusions:
My sincere attempt
to get the company to respond to reasonable technical questions has been
totally unsuccessful.
If you are
considering installation of Full Lotus 1450 floats you would be wise to locate
an installed set for your own evaluation before ordering rather than accepting
the company’s claims at face value.
This past weekend
I was able to take measurements and fly the 1450 installation that Ken Smith
made up for a Rans S7 long tail with a 100hp Rotax.
Frankly I was pleasantly surprised.
Turns out Ken had
not actually moved the floats further back but used his 51” step position as he
does for most of his installations which makes it easy to evaluate the company
suggestion of not changing the rigging from what 1260’s used. By comparison,
the 1260’s on the yellow S7S shown above are mounted almost 3” aft of the 51”
position. Here is a picture of Ken’s setup:
The first test was
to put a person in the rear seat and stand on the float beside him. As
expected, the heels of the floats submerged illustrating that there was not
enough aft flotation..
While overall
take-off and landing characteristics were quite good, the owner does feel that
the fuselage should go further forward which should improve the climb up onto
the step. My feeling is that the fuselage should move forward at least 6” so
that the step is at 57”
aft of the firewall and 6” aft of the most rearward CG of 51”. On the other hand, the overall performance
where it is, is quite acceptable with one or two
people onboard. If you look at the videos you will see that the flotation is
noticeably better than the 1260’s without the look of much bulkier floats (as
you would expect since the forward barrel size is the same cross section).
You can see some
video of this aircraft on my
home page.
Ken’s rigging looks
quite professionally made and his choice of square tube spreaders with some
added streamlining works very well. Ken puts his spreaders closer (45”) than
most people do (55” to 63”) and he mounts the rear spreader closer to the step
than most people do (usually the step is about ½ way between the spreaders).
2008 Rans S7 Long Tail built by Brian Sandercock
in Kenora
With the fuselage
6” further forward, the forward rake of the struts would not be so pronounced
(if the shift were done via the rigging and not just by sliding the floats on
the existing rigging). Ken also has used a narrower float width than I prefer;
his are at 66” whereas 72 to 75 is more typical. All of his rigging was well done including
the water rudder set up and stainless fitting in the floor for the rudder pull
up cable.
Finally, here is
one comment on the choice of angle between the floats and the fuselage where
the typical measurement is 3 degrees. I went to over 4 on the S7S above while
this set of 1450’s is at less than 2 degrees and they fly off and land just
fine. My conclusion is that trying for 3 is still a good approach but a little
deviation won’t likely hurt at all.
Update 2009/06/19
Just
heard that the owner has moved the floats aft 6”. He
reports that handling is much better; climb up onto the step has improved and
flat touch downs require some (normal) back stick rather than forward stick
which is common with Lotus floats to stop porpoising.
The re-positioning was accomplished by sliding the floats back under the
existing rigging. This will put the rear spreader almost right at the step. On
1260’s this would result in considerable flex of the aft portion of the float
but perhaps the third stiffener tube and shorter tail section on the 1450’s
counteracts the flex.
So, my conclusions
are that the 1450 is an acceptable choice but should be mounted at least 6” aft
of 1260’s or other floats with a more typical step position.
Now if Jeff Holomis would just update his web site with complete
specs and mounting suggestions for the 1450, all would be well.
To read all of the notes between Jeff Holomis
at FLM and me, send me an email: playonwater@yahoo.com
Here is another
full shot of Brian’s very pretty S7:
Click for:
More info on
float sizing
Have just spent some time
looking at the setup of this pretty Baby Ace on Zenair floats.
During a recent rebuild, the owner
made several rigging changes to both floats and airframe. The floats are now
sitting at only 4* between float and wing centreline and the step is a full 6”
aft of the aft cg. With these variations it will be interesting to see how it
performs (although it seems to be more in line with the German thinking).
Info on rigging design. Spreadsheet for predicting
take-off time.
C/G anomalies on the Rans S7.
The S7 fuselage was
lengthened in 2001 and called the S7S. There is no change in the airfoil or
forward geometry. As mentioned above, the CG range for the early models was 74
to 81” aft of the prop hub.
For the S model, Rans changed
the datum line for CG calcs to be the firewall. The range for the S model is 46
to 50.25” aft of the firewall. For some
time I assumed that both aircraft had roughly the same CG range and aft limit
but a close look at the numbers shows this is far from true.
First Rans has narrowed the
range from 7” on the short tails to only 4 ¼ on the long tail. Next by
subtracting the 26” distance from hub to firewall, the converted range on the
short tail is 48 to 54”. The S model has had some FAA involvement.
I’m no engineer but maybe it
makes sense that if we have increased elevator authority due to the longer tail
we could tolerate the more forward 46” cg. But why does the later model have a restricted
rear CG position by almost 4”? Should short tail owners learn something from
this???
For years one experienced
float rigger in the