A CRITICAL
ANALYSIS OF THE KR2
By Neil Bingham
Introduction
What is written here is from one
man’s viewpoint, although the author has discussed these things with a number
of KR builder/ pilots, in fact, as many as will give him some time. The result is
a combination of many inputs, but biased somewhat toward his own opinions,
judgments, experience (with N81NB) and critical engineering analysis.
For additional information, see
the author’s articles, Engine Installation In a Sport plane, March 1986 Sport
Aviation, and Light is Better, December 1986 Sport Aviation.
The reason for doing this
treatise is simple. The author has come to the conclusion after reading and/or
listening to many flight reports that all too many times the real facts of test
flights never come out. How can a man expose himself to criticism after just
coming off two plus years of hard work, itchy hands and a defunct wallet? It is
infinitely easier to thank his wife (if he still has one), Aunt Mildred and
Uncle Fred for their help, and conclude by saying,
“What an airplane!”
“It flies great!”
Even
those who want to be object and helpful and share their innermost thoughts find
it so difficult, they seldom do. How does one explain that he almost lost it on
the first landing because his skills at the stick in a slick little tail
dragger were not razor-sharp? It is just not easy to admit that!
Many years ago in a university
physics lab, a student instructor taught the author a valuable truth. He said,
“There are no liars in the lab. Take down the data the way it happened. You may
not be able to Interpret the data, nor your professor. But if it’s valid data,
someone, sometime is going to thank you for it.”
In an ad that appeared on
page 81 of the February 1976 issue of Sport Aviation, Ken Rand advertised this
about the KR-2:
EMPTY WEIGHT |
420 lbs. |
GROSS WEIGHT |
800 lbs |
ENGINE |
VW 1600 |
FUEL CAPACITY |
12 gals |
TOP SPEED |
150 mph |
CRUISE SPEED |
140 mph |
SEATS |
2, side
by side |
If
every KR-2 builder had stayed with those numbers and built his plane correctly,
there would be no bad flying KR-2’s! Further, there would be a lot less
low-time KR’s for sale. But, no, we can’t do that. We take this little jewel of
a design, change it here and there, and add everything but the kitchen
autopilot and wind up with 640 lbs empty!
Then, since it has two seats, we
stuff another 170 lbs. of humanity in the right seat and with 24 gallons of
fuel go out and attempt to bore a hole in the sky! All kinds of things happen. First, by now the loaded weight is
1124 lbs., 39% over gross. It seems a little shifty and doesn’t fly all that
well, had a little trouble getting her down right. So we say, “Got to have more
power.” We add another 25 or 30 lbs. getting to a bigger engine, when all along
it is not raw power that makes an airplane fly well, it is the entire
aerodynamic design.
There are very few faults with
Ken’s original design. What few there are, tweaks and adjustments to the
design, some of which are subjective instead of objective, i.e., personal
wants.
Let’s start with what an airplane
is supposed to do – fly. If it doesn’t
do that well, it is something less than an airplane, maybe a rock, rocket or a
barn door. All will fly, with enough power, if one is willing to accept the
risks and flight management tasks.
Admittedly, it is very difficult,
if not impossible, to build a 420 lb. KR-2. But it is possible to build a 450 lb
KR-2, and that should be the goal.
When it was inferred in
the introduction that the KR-2 aerodynamic design would not accept more than an
800 lb. Gross, it must be understood that this is a design parameter, and as
with most design parameters there are tolerances. But the limit of these
tolerances is never infinite and seldom more than 10%.
If we were to apply the
10% rule to some of Ken Rand’s numbers, we get these for upper limits:
Empty
weight: 420 x 1.1 = 462 lbs.
Gross
weight: 800 x 1.1 = 880 lbs.
Now, let’s take a
real-life scenario using these numbers. The author’s KR-2 (N81NB) weighed in at
620 lbs. when certified in July 1985. After a drastic surgical procedure during
the winter of ’85 - ’86, he was able to get her empty weight down to 539 lbs as
she stands today.
She is still 90 lbs
overweight but flies, oh, so much better.
In fact, at the old 620 lbs, with a full 24.5 gallons of gas and another
person, the airplane was so over gross that it did not fly well at all. Not only
did it not fly well, it was just plain unstable and unsafe even though the CG
was within the design limits.
A few rides were given,
but it was not encouraged. The passenger usually did not sense that anything
was wrong, but the pilot did. Enough so that he’ll guarantee that if you build
too heavy you’ll be disappointed, for it will not be the fun airplane it can
be.
Back to the real-life
scenario, here’s is how it works out now that we’re lighter:
Empty weight |
539 lbs |
Full fuel (14 gals.) |
84 lbs |
Pilot |
175 lbs |
Total |
798 lbs. |
The
empty weight, per se, really means nothing when talking about “Flyability”. It
is the loaded weight in relation to the designer’s specified gross weight that
is of concern (in this case 798 lbs vs. 800 lbs).
To continue the scenario,
on September 6,1986 at a local fly-in, the author flew the show line and
recorded these data:
þ Air temp -
85 degrees F
þ Field
elevation - 4330 ft msl
þ A/C load -
798 lbs.
þ Engine -
80 hp Limbach, L2000EO1
þ Prop -
53x52 Warnke Constant Speed
þ Wind -
Light and variable
þ Cruise/2950
rpm - 166 mph. True
þ Top
speed/3300 rpm - 186 mph, true
In a high speed low pass, 20 feet above the deck, from
1500 ft. AGL: Speed at 3500 rpm – 245 mph,
true.
Now, let’s examine these flights
and compare the data with those from Ken’s ad. He had a VW 1600, which was good
for about 55 hp. In the author’s case, he was flying 80 hp, which yielded a
cruise speed 26 mph greater than Ken’s.
Note, that the author was
flying within two lbs. of Ken’s original recommended gross for the airplane.
The author’s flights were
within the limits of the airplane and were very manageable. Even at 245 mph
(which is really smokin’) the KR was manageable, which serves to introduce the
next subject.
It is interesting to
examine the wing loading on different popular aircraft:
Boeing 737 80 + lbs./ft 2
Beech King Air 30 + lbs/ft 2
Glasair RG 22 lbs/ft 2
Cessna 152 12
lbs/ft 2
KR-2 (at 800 lbs. gross) 9.2 lbs/ft 2
Where we make a big mistake is
expecting a KR to ride like a 737, and it won’t!
Time for an opinion: One has to
examine his desire for speed in these little craft and balance that against the
number of times he’ll fly at those speeds. Most of the times he’ll find himself
coming back on the throttle to 140-145 mph cruise to settle the comfort level
of the ride, even though he has the capability of cruising at 165 mph. This is
due to the very substantial effect of even mild turbulence.
The
author has tried low passes at over 200 mph in what could be considered mild
turbulence and it is worrisome. At those speeds, he was all over the sky and
didn’t get very close to the deck.
The opinion is 60 hp is enough for
a properly built KR-2 and anything beyond 80 hp is not only foolish but
downright dangerous in the wrong hands. If the builder wants an aircraft that
will carry more than 300 lbs. of humanity at speeds above 140 mph for long
trips, he should choose another design. It is that simple.
On the other hand, if he wants a sweet
little utility aircraft that will thrill him to his very toes every time he
takes her out, and do a lot of affordable “sport flying” for a long time, he
will do well to decide on the KR design. For the buck, it’s still the best deal
around, even 10 plus years after she was born!
The KR-2 is quick! The author has
claimed repeatedly that the average Cessna or Piper driver is guaranteed to get
a wing tip, prop strike, unscheduled trip into the boonies, or all three (if
not worse), if he tries to fly the KR the first time without some concentrated
tail dragger instruction. The most
sensitive control of all is pitch. Many, if not most, KR pilots get into early
difficulty with this, and way too much porpoising takes place. It’s many times
as sensitive as a C-152 or Cherokee.
To ease this problem,
four things can be done.
1.
Get the tail feathers
back some. The author added 13.5 inches, 24 would be about right. This
lengthens the tail moment arm about 17% and really helps.
2. If using a
center stick, per plans, a good armrest needs to be put in so that the airplane
can be flown by wrist action. If dual sticks are installed, they should be as
long as possible and shaped so that the forearm rests comfortably on the
pilot’s upper thigh. The KR is definitely not a Cub or Champ where “inches” are
required. It is estimated that 95% of maneuvering of the KR is done with less
than an inch of total stick travel (1/2 inch radius of movement). Full stall,
three point landings being one exception.
3. Narrow the
fuselage. Yes, you heard right. If there were a “next time”, the author’s KR-2
would be a KR, period. It would have one seat, a center stick and the fuselage
width would be held at the firewall width back to the baggage compartment, then
tapered to the tail. That’s about 30 inches, plenty of room for even a 200 lb
pilot. This would be a happy pilot because he’d never have to worry about over
grossing, banging his head on the stock canopy in rough air or flying a
squirrel (he’d always be under Ken’s 800 lbs if he built it right).
4. An
additional mod Ken talked about was balancing the elevator and rudder like the
ailerons. In fact, he stated that the redline speed of the KR-2 was 200 mph
if the tail feathers were not balanced.
Note:
While we are here, let’s
insert something on a subject we’ll talk about in more detail later weight and
balance. Increasing the length of the fuselage puts the weight of the tail
feathers (plus the weight of the added materials) further back from the MAC
(Mean Aerodynamic Center) and shifts the final center of gravity aft. It has
quite a pronounced effect because the moment arm is large. Some builders might
need to add weight to the tail to bring their CG in, and this mod will help
them. But if they don’t then the engine will need to move forward some to bring
the final CG to where it should be.
Some have flown their
KR’s over 200 mph with unbalanced elevator and rudder, but that is flirting
with disaster. Flutter is a dynamic phenomenon, triggered by complex forces.
It’s possible that no one alive knows exactly where his KR control surfaces
will go into flutter (resonance). What is known, however, is that one only has
about three seconds to disintegration of the control surface and that accurate
balancing pushes the resonance points up out of any reasonable flight envelope.
Over the years, the
author has seen some bad-flying airplanes, some modified “Wichita Spam Cans”
and some homebuilts. Of these, none have shown more “squirrelly” characteristics
than the ones where the rules of weight and balance have been ignored or
broken.
Simply stated, there is an
“envelope” or range under all configurations of loading into which the final
(loaded) CG must fall. In the case of the KR-2, it is from a point 4 inches
in front to the rear surface of the main spar, to a point 4 inches to the rear
of this surface, or a total envelope length of 8 inches.
As one approaches the
forward limit of the envelope, his aircraft will become more and more pitch-stable.
It requires more and more pitch trim to achieve hands-off level flight. As the
aft end of the envelope is approached, the aircraft becomes less pitch-stable
and more pitch input sensitive. Lesser amounts of elevator trim are required to
achieve hands-off level flight. Pilots like to favor the aft portion of the
envelope saying that they can fly faster at a given power setting because the
aircraft is fighting less trim (drag).
The author would like to
take issue with an 8-inch CG envelope for the KR-2. He believes that going to
the aft end of this envelope will guarantee an unstable KR. Worse, in a
departure or approach stall situation, a flat spin is likely to quickly develop
and recovery would not be easy if even possible.
The author would never purposely
spin his KR, but will, by the same token, never load the plane such that the
final loaded CG is further aft than 2 inches to the rear of the main spar aft
surface. It has been tried beyond there and the results are not pleasant.
Wallowing, undulating and general instability show up back there.
The answer? Consider defining the
CG envelope as being only 6 inches long, and drop off the last two inches of
the advertised envelope.
A couple more suggestions in
relation to weight and balance:
As discussed before, the gross
weight limit of the KR-2 should be set, in the author’s opinion, at 800 lbs. If
one can build to the 450 lb empty weight goal, limit his fuel to 12 gallons
then he may have a two-place airplane. In no case should he carry more than 300
lbs in the cockpit.
Ken Rand was a small guy,
weighing, it is said, 135 lbs. His KR, it is also said, weighed only about 450
lbs, and had a 12 gal. Fuel tank. He gave a lot of rides.
Follow this scenario; let’s say
he had 9 gallons of fuel, which would be enough to do a lot of running around
the countryside at 3 gallons per hour. Let’s also say that he flew the
“standard” passenger of 170 lbs. (no baggage).
Aircraft weight |
450 lbs |
Fuel |
54 lbs |
Pilot |
135 lbs |
Passenger |
170 lbs |
Total |
809 lbs |
But why did the gross weight
finally wind up at 900 lbs? Apparently, in a near sea level situation where the
air is dense, the 900 lbs posed no real problem. That became the published number.
In my opinion, however, 800 lbs is a good number for my altitude. My field
elevation is 4222 and my typical operational envelope is from 5800’ msl to
10,500’ msl. Occasionally we’ll go to 12,000’ but we won’t stay there except to
get over a mountain.
A final comment on weight
and balance: For heavens sake have the upper fuselage longerons level with the
world when the CG measurements are taken. If you don’t, you ain’t got a good
number! Both have a significant effect on where the CG falls, and after all, it
is the flying attitude that we want to simulate.
Also, when you balance
your KR, do it with the engine. That is, build the entire aircraft, including
painting, before you hang your engine. Then when you know what that weighs and
where it’s CG is, you can determine the weight and CG of the engine,
accessories, prop, prop extension, spinner assembly and everything that hangs
on the engine, and hang that where it needs to be to bring the final empty CG
where you want it. The controlling CG position is the one with your lightest
possible pilot, no passenger or baggage and a full tank of fuel. That CG should
fall exactly on the front end of your CG envelope, i.e., 4 inches in front of
the main spar aft surface. Assuming that your empty weight is close to 450 lbs.
and you never exceed the 800 lb. max. gross, all other things being right, you
will have one sweet flying airplane.
The last thing to build,
using the foregoing procedure, is the motor mount. Do not be afraid to build
your own mount. Take the basic configuration out of the plans, go to a good
“rag and tube” builder with a Heliarc welding machine and get him to weld you
one up to fit your length dimension. Make it from the specified 5/8 dia. x .049
wall, 4130N steel tube and it will be plenty strong even though it may be a
couple or three inches longer than the stock mount. With this tubing, the mount
is well over-designed, but it is used because it is easier to weld than the
smaller, thin stuff.
In some of the flight reports, the
author has read about KR’s nosing over, or being spun around by a blast from
another aircraft’s prop. It is true that the KR has very little weight on the
tail (how else could we get by with such a miniscule tailwheel?). In fact, the
tail on N81NB rises regularly when testing for maximum static rpm. This is true
because of the position of the wheels in down position with respect to the CG.
By design, the wheel position is close to the front of the CG envelope. If the
wheels were positioned farther forward, which would decrease the tendency to
nose over when braking hard, there would be an increased tendency for the tail
to come around and meet the nose (known as ‘ground loop’) during the transition
from taxi to flying and flying to taxi. This is due to the greater
concentration of mass behind the wheel footprint. This would be especially
noticeable in crosswinds. A contributing factor is the lack of rudder authority
at these lower speeds.
As it is, the KR is quite easy to take
off and land in terms of yaw stability. By far the most difficult to manage is
pitch, because it is so sensitive.
Choosing an engine is not
the problem the author once thought it was. That was when he felt that 60 hp
was not enough. Probably the best choice, from a cost and
track record standpoint, would be one of Steve Bennett’s models from Great
Plains Aircraft. The author and his mechanical engineer cohort, Kris Bowers,
plan to install one of the 60 hp units in N81NB.
Leave them off! It’s
possible to build a 450 lb. KR-2 for $5000 if you will! Done right, it will fly
so well and cost so little, you’ll wonder why everybody doesn’t do it. Sure, it
will be a hand prop, day VFR machine, but that is what it is anyway. We only get
into trouble when we try to make it something it was not designed to be.
If you have just “got to
have” electric start, lights, strobes, a big radio stack, etc., here’s about
what you’re in for:
Item |
Weight |
Cost |
Nav light/strobes |
6 |
450 |
Nav/com |
6 |
2600 |
Transponder |
5 |
1300 |
Switches/Breakers |
6 |
75 |
Battery |
9 |
50 |
Alternator |
9 |
150 |
Starter |
15 |
90 |
Grand Total |
56 lbs |
$4715 |
Now
your KR costs twice as much and is probably a single place aircraft, unless the
passenger is very small.
Of course, without an electric
starter, hand propping is a necessity. But is that a big problem? The author
doesn’t think so. Of course, he grew up in a time when inside plumbing and such
were luxuries. Sometimes he wonders how we ever got along before electric start
(darn good, if you want to know!).
Nevertheless, this is how it’s
done on the Limbach:
Step 1 - Place chocks,
(tied together by a piece of rope) in front of the mains.
Step 2 - After
pre-flight, step around in front of the center wing and open the canopy so you
can reach the instrument panel.
Step 3 - With MAG
switch OFF, and throttle in ¾ inch, pull engine through with left hand 3-5
times. If the day is cold, pull choke out and pull through 3 times.
Step 4 - Close
throttle all the way to IDLE, choke all the way in.
Step 5 - Turn mag
switch on.
Step 6 - With right
foot against left wheel chock and right knee against the wing leading edge,
yell ‘clear’ and pull the engine through sharply with left hand, pulling hand
back in one clean motion to clear the prop arc. N81NB starts on first pull,
usually.
Step 7 - If idle is
set at 650 rpm, plane will not move. Pull chocks clear, walk around the left
wing tip and climb in.
A Final Note
Well, that’s it. If you haven’t
guessed by now, the author has more fun with his little KR on a regular basis
than is morally right for a gray-haired guy pushing 60. But, as said before,
done right this little plane is guaranteed to quicken your heart beat and
thrill you to your very toes every time you take her out.
Try this sometime, if you want to
stay young on a warm summer evening when the air is dead calm and all the gang
is standing around the flight office at your little country airport, take off
and fly around a bit to warm up the oil and get things comfortable and under
control. Come back in a couple hundred feet above pattern altitude. While you
are on downwind (make sure you’ve got the space) call Unicom and announce a low
pass to take a good look at the windsock. Roll‘er into a 60 degree abbreviated
base leg and onto final. Open the throttle and aim for the numbers. When you
level her off at 20 feet above the deck, turning out at 245 mph, ask yourself,
“Are we having fun yet?”
Neil D Bingham
2217 N Shannon Way,
Mesa, AZ 85215-2731
(480) 654-6271