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.”

 

 

 

 

 

 

 

Conclusions/Recommendations

 

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. 

 

 


Discussion / Analysis

Flyability

 

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:

 

ž     Air temp - 85 degrees F

ž     Field elevation - 4330 ft msl

ž     A/C load - 798 lbs.

ž     Engine - 80 hp Limbach, L2000EO1

ž     Prop - 53x52 Warnke Constant Speed

ž     Wind - Light and variable

ž     Cruise/2950 rpm - 166 mph. True

ž     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.


Wing Loading

 

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!

 

Thinkable Modifications

 

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.

 

 

Weight and Balance

 

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.

 

Stability on the Ground

 

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.

 

 

Engines

           

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.

           

Electrics

           

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