This month’s question comes from a long time
member and glider guider Bob Film. This type
of thing is near and dear to my heart!
Since there are 100’s of airfoils, and each one is
usually efficient at an air speed, I would assume
that the thicker the airfoil & under camber airfoils,
would be for slower air speed, so is there a simpler
formula to determine each one’s angle of
incidence (not dihedral). So, for instance, if you
put different wings on a plane; how would you set
its incidence for max performance. I assume that
angle is always in reference to elevator being
zero degrees.
Since I am a sailplane “NUT”, this becomes more
critical.
Thanks – Bob Film
Thanks for the question Bob. You have hit the
nail on the head here. Airfoil selection and
setup (incidence, planform, washout ect.) is
the heart of how the plane flies. A simple
incidence change sometimes can turn a dog
into a thoroughbred. Unfortunately there is no
simple formula to figure this out, but there are
some guidelines that I like to use. There is
much, much more, that can be added to this
discussion, so I’ll try to keep it in the context of
your question. Please feel free to contact me
for further discussion if you wish.
Up front I’d like to state that I generally am not
a big fan of using incidence and/or decalage
to trim an airplane. These are compromises to
make a plane behave a certain way at a
certain airspeed, and my feeling is airfoil
selection plays a big part here and if done
correctly, you won’t need the incidence bandaid.
More often than not, my designs will be
setup with zero incidence. They certainly have
their place, Trainers and Old Timers for
instance, but since you asked about max
performance, I’m going to assume you equate
that to efficiency. It’s all about trim drag. If you
need up or down elevator trim at certain
airspeeds, it means your incidence, CG or
both are wrong and there is unnecessary drag
from trim detracting from performance.
Climbing under full power is a classic
symptom of this.
First, you must define the purpose of your
airfoil selection and its use. An Old Timer type
aircraft with an undercambered section will
likely be flown more slowly and benefit from
some positive incidence. All airfoils have a
natural pitch down tendency, but UC sections
react more than most. This positive incidence
coupled with a necessary more forward CG
makes for a very sedate flying aircraft.
For a high performance TD (Thermal
Duration) sailplane, Racer or even an
aerobatic plane you’ll find that incidence
needs to be at or a fraction of a degree of
within zero.
So how do you measure it? You are correct
that on a sailplane the reference line, “0”
degree or datum is often the elevator, and the
wing is measured in degrees positive from
there. Power planes are a different story, with
their thrust angle, positive incidence, decalage
(tail incidence), they often use a line through
the center of the fuselage for reference.
For most airfoils you can find a published set
of ordinates that will include useful information
such as chord line, camber line, zero lift line
and other fun things like lift vs. drag polars
and laminar bucket information. In most
instances you can use the chord line to
measure from which is often simply a line from
the leading edge to the trailing edge. For
highly cambered sections, the chord and
camber line can actually exit the bottom of the
airfoil making measurement a bit trickier.
There are many online and printed references
out there. For printed references I like Airfoils
at Low Speeds (Soartech series) available
from Carstens Publications SoarTech, and online there is the
UIUC’s webpage from Professor Michael
Selig. With this reference material it will tell
you specifically at what angle of incidence it
will likely perform best.
Bottom line for me is I will set up a plane with
the least amount of incidence possible (often
zero) and move the CG aft incrementally until
the trim change with speed goes away and I
still have acceptable stability.
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Red's Corner, July 2010
Written by Red Jensen
Monday, 14 June 2010
All about Lithium Polymer Batteries
Hi Red,
I have a question about LiPo batteries. I don't think I'm
the only one either because nobody I've asked seems
to know. Perhaps you could talk about how they are
rated. For example I know what 3S means (3 in series = 11.1
volts) but I don't know what 3S1P means, could you
explain the basics?
Thanks, Marty
Thank you very much for the great question
Marty! I am sure you are not the only one
either, as the electric industry as a whole is
really not that well defined when it comes to
explaining its various ratings and acronyms. It
can be very difficult, for instance when
converting an IC (internal combustion)
powered model to E power, to spec an equal
performing power system. More about this
later, but that should be really how you think of
this type of thing, as system. (motor, ESC,
battery and prop all have a great deal with the
“power” your system will produce) Thankfully
the battery nomenclature is pretty well defined
and easy to follow once you understand the
system.
On to your question! By definition all batteries
are made up of individual cells, whether they
are NiCad, Nickel Metal, Lipo or any other
chemistry. A battery is a cluster of cells wired
together either in Series and/or in Parallel.
Let’s take a simple 2100 maH 11.1v pack as
an example and look how it is defined. The
2100 is the total capacity of energy the pack is
able to deliver effectively rated in
milliamp/hours. If you have a system that
draws 30 amps you can expect to fly for about
7 minutes. (Provided the 30 amp draw falls
under the packs “C” rating, more later!)
The 11.1v rating as you noted comes from the
fact that this pack is wired in Series as the 3
individual cells are wired in such a way (pos &
neg soldered together) that their individual
voltages (3.7v) are added together make to
the 11.1v rating.
Parallel (pos & pos/neg & neg soldered
together) wiring is used when a larger capacity
is required. The largest typical single cell size
is around 2700 maH (there are other larger
single cells, but they are rare and don’t really
apply in this example). Parallel wiring sums
the capacities together. So if you buy a 4200
maH pack it likely is two 11.1v 2100 packs
wired in Series/Parallel or 3S2P configuration.
That means there are 6 individual cells wired
into two 3S packs to get 11.1v, and those two
11.1v packs wired together to get in parallel to
get a capacity of 4200 maH. So for any battery
the first number indicates how many cells are
needed to get the desired voltage and the
second number is how many packs of cells at
the desired voltage are added together to get
the desired capacity. You can stack any
number of cells together to get the desired
voltage and capacity. The only rule is all cells
must be of the same starting capacity. (i.e all
2100 cells) It is not uncommon to see up to
10S4P setups in larger aircraft.
I suppose some of the confusion is that a
3S1P battery really only is referred to as a 3S
battery leaving out the 1P part of the
designation. By default it is a 1P battery.
Which is fine as 1P batteries are all most
people fly with anyway. The 2P (or 3,4,5 or
more) is not really seen very much at all.
Earlier I mentioned C rating. The C refers to
capacity, and the rating is used to tell how
quickly the pack can safely discharge current.
Using the same 2100 maH pack as an
example, one might have a 15C rating while
another may have a 35C rating or more. The
reason it is important to pay attention to this is
because it is possible to permanently damage
your packs if your system exceeds the C
rating of the pack. How it works is simple. You
take the C rating and multiply it by the
capacity to get the maximum available current
the pack can deliver safely. Let’s say our 2100
pack is rated at 15C, you multiply 2100 x 15 =
31.5 maximum amps continuous. So for our
example 30 amp setup mentioned above, this
battery would suffice but I would consider it a
bit too close for comfort. You can see that the
35C battery with its 73+ amps continuous
available is a much better solution. Most
batteries also have a burst C rating, most of
the time it doubles the continuous rating. The
burst rating lets you know that if you have a
hotter system and your battery is on the edge
of being adequate, that you can exceed the
normal C rating for short periods of time (i.e.
take-off, hovering etc.) but you should not
exceed more than 10 seconds or so at this
higher current draw.
In normal operation, LiPo batteries should
never be brought down below 3v per cell (9v
for a 3S pack). Doing so can permanently
damage the cell and can lead to poor
performance and shorter run times. In extreme
cases they can even swell and catch fire. Most
ESC’s have this 3v limit set at the factory, but
it is possible to disable it on some controllers
like the Castle series for instance. At any rate,
flying until you hit the low voltage cut off each
time is not the best way to preserve your
packs. It’s best to land when you notice the
power getting soft.
Balancing is always a good idea, you cannot
over balance. I balance every time I charge.
This will maintain your packs to the highest
possible standard. I have packs that are going
on 5 years old and still perform as new.
Balancing becomes even more appealing
when you are running multiple parallel packs
together as the different performance
characteristics of each individual cell will
unbalance them rather quickly.
Well Marty I hope this answers your question,
and thanks for sending it in.
Email Red at
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Float Fly, September 6th
Written by Patrick O'Halloran
Tuesday, 24 August 2010
Please join us on September 6th from 10am for "Day on the pond II" at the Sal Lake float fly site. Take the Lytton Springs exit just north of Healdsburg, enter through the CDF parking lot.
cold drinks and restrooms available
2.4 GHZ preferred, call 584-4428 for FM channel arrangements
2010 AMA memebrship is required to fly
Email Sid "the Airman" Maxwell at
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or just give him a call for all details at 707-584-4428.
Red's Corner, June 2010
Written by Red Jensen
Monday, 14 June 2010
I’d like to write a monthly column that
addresses the more technical side of our
hobby. I am probably more addicted than
most, but I really enjoy the how’s and why’s of
our toys and would like to share as much as I
can with you. I take great pleasure in tweaking
whatever project I am working on to get the
most performance out of it whether it be radio
setup, or getting an engine to run perfect or
dialing in C.G and control throws. What I
would really like is for people to submit a
question or topic that they may be interested
in, and I could write about it here. Some
columns may go hand in hand with a live
demonstration at our monthly meeting,
perhaps something like an introduction to
composite part making or other relevant
topics.
To get the ball rolling I thought I would touch
on some radio basics that there still seems to
be some confusion about, and some
reluctance to use. There really is no excuse
for not using whatever features your radio has
to offer. Even the most basic aircraft can
benefit from some of the advanced features
that even the mid level radios seem to be
equipped with nowadays. My goal here is to
hopefully explain some of those features a bit
better and inspire some to give it a shot.
Most people don’t realize that the ergonomics
of your transmitter can be changed quite a bit
to suit your flying style fairly easily. For
instance, the control stick length can be
adjusted to a more comfortable position. As a
“pinch” type pilot I prefer my sticks very short.
If you are a “thumber, you might like them a bit
longer. If you tend to be a jumpier flyer, longer
sticks can smooth you out and conversely, 3D
pilots generally like them shorter. The point is
you might not realize what you are missing, so
tweak them and find out. Another stick related
item is tension. Many radios allow you to
tighten or loosen stick tension to suit your
style as well. Jumpy/precision guys like
tighter, 3D looser. I prefer them on the tighter
side. Your switches can also be rotated
slightly in the case to be less cumbersome to
flip depending on whether you pinch or thumb.
The best switch flipping path may not always
be straight up and down.
Another area to look at is exponential, or expo
for short. I know, I know seems mundane but
in reality it seems to be a bit misunderstood
underutilized. Just recently I helped a fellow
member adjust expo values with favorable
results and that got me to thinking it’s not as
widely used as I had assumed. What expo
does is essentially “soften up” the movement
of the control surface in relation to the
movement of the stick around the neutral point
of the controls. This is needed because of the
way our servos act our flight surfaces. The
servo takes rotary motion (the twisting of the
servo arm) and transfers it to linear motion
(back and forth of a pushrod). This inherently
causes some loss of throw as you reach the
ends of the travel as the pushrod moves less
back and forth and more in towards the center
of the servo. In fact if you do not use any
expo, a standard set up will actually have
negative expo and be more jumpy around
neutral! A good rule of thumb is that it takes
20-25% expo to get back to a linear motion. I
never use an expo value of less than 35% on
low rate and as high as 60-70% on some
models. My racer runs about 45% on all
surfaces. Give it a try; you might be in for a
treat. (Ed. Futaba uses the opposite
numbering for this feature, negative values
result in a softening of the controls. If in
doubt, ask someone for help)
I’d love to hear from you. If you guys find any
of this stuff helpful drop me a line. I take
requests.
Email Red at
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