Up scaling a KR2

[Bennie Vorster]

What is the possibility and the consequences of up scaling a KR2?

Build it scale 1:5. How will this impact on the flight cararisticks of the plane and will to be able to take more useful kg's.

[Henni]

Hi Bennie,

Biggest problem is that the power requirements goes up with the squared of the weight which means a bigger engine = more weight = stronger airframe = more weight = requires yet a bigger engine.

Most KRII's in this country is already way over weight for the 2 place only design and most of them will fly with one up only for most of the time. The VW has a hard time coping with this, let alone if everything would be scaled up.

This does not mean that it cannot be done. A good place to start looking for a optimum 4 place on low horsepower design is the new MCR - carries 4 adults (not 2+2) on a Rotax 912/914 giving very good climb & cruise performance. I'm sure this design will soon be copied and we'll see many more economical 4 seaters on the market.

Now if only someone can produce them for less than the cost of a Jabiru 4 (which is already very good value for money as is), a winner will be born.

Just my 2 cents on this subject.

Henni

[Bennie Vorster]

Ok henni so how about building the KR with composite materials or something lighter and fitting a Rottax 912/914 and lengthening the tail to make it more stable?

[Morph]

Ok henni so how about building the KR with composite materials or something lighter and fitting a Rottax 912/914 and lengthening the tail to make it more stable?

Now you are talking about building a completely new model. I would not recommend this. Would you be the test pilot?

[Henni]

Hi Bennie,

I have just the opposite ideal - The prototype KRII weighed only 450 lbs empty using a VW engine. Nowadays everyone wants to go heavier.

The KRII is an extremely good design. I am convinced, that if built strictly to plans, no cheaper, smaller or easier/simpler to construct two place aerie can be had - period! So how can it be improved?

"Ok henni so how about building the KR with composite materials or something lighter and fitting a Rottax 912/914 and lengthening the tail to make it more stable?" - A composite structure will not weigh less than the KRII wood stringers & ply structure, it would only be easier to build. Fitting all those gusset blocks is a lot of precision work - ask me, I've been there a few times.

An installed 2L Volla weighs 160 lbs upwards - most of them over 200 lbs fully installed & producing realistically only 60 - 70 hp at the rpm used regardless of who claims what.

Many newer and much lighter engines can be had producing even more power mostly because they run at higher rpm. Thus, a KRII can be had at an even lower empty weight.

I've owned the very light MXs & MAC CDLs using the Rotax 503 and now the much heavier Mizer using the Rotax 582. The lighter aircraft could climb much better with two up and stayed in the air using much less fuel. They had single surface wings, so a lot of drag and could not really go anywhere.

But take now the Skyfox, using the same light weight structure with a Rotax 503 but with dual surface wings - I'm convinced that it will out climb the Mizer with two up and cruise the same on only 53 horses. Thus, it would cost a lot less to fly.

So, bigger is not better, but lighter is, especially in our kind of environment. Hence the term: "Micro" Light or "Ultra" Light!!!

In the late 60's Ken Rand and Stuart Robinson were working as flight system engineers for Douglas Avionics. Ken was working as an electrical engineer, having previously worked as an autopilots project engineer, while Stu's degree was in aeronautical engineering from Northrop University. They were two of the guys at the end of the DC-8, 9, and 10 assembly lines responsible for correcting some of the nits and picks in various systems before delivery to the customer.

Stu had decided to build an airplane, and began looking for a partner to share in the effort. Ken seemed a likely candidate. He had grown up with the airplane bug, sketching aircraft designs since childhood. He had been into R/C models for years, and had already built a glider. He was a bit of a daredevil as well, having done some time racing motorcycles in cages in Atlantic City, among other things. No doubt who the test pilot would be.

They both wanted to build a fast, inexpensive airplane which was also economical to maintain. Several designs were considered, and plans were bought first for the Jeanie's Teenie and then the Taylor Monoplane. The Monoplane was more to their liking, but would require some modification to fit their needs. A cooperative redesign effort ensued, with virtually no dimensions left untouched. Only the basic fuselage structure, airfoil, and powerplant were retained. The tail shape was Stu's, and came directly from the big DC-8s parked on the ramp outside his office window. The landing gear was designed by Ken, after seeing similar gear on a Dewey Bird at Santa Paula airport.

Construction began in 1968, with both Ken and Stu building an airplane each. Stu was progressing faster, until he met a girlfriend who would later become his wife. This development allowed Ken to complete his airplane first, and it was registered N1436. First flights were in the Spring of '72. Ken was a low time pilot with only about 80 hours at the time, and had borrowed Stu's Curtiss to get some taildragger (tailskid, actually) time. On the first flight of the KR, Stu and three others flew chase in a Stinson, but didn't get to see much of the KR1. It was much faster than either of them had anticipated! That first landing was a real greaser. It looked as though he had done it a thousand times.

Ken also flew to Oshkosh in '72, where his KR1 was awarded Best Aircraft Application of Materials because of its composite wing construction. Ken's and other early KR's used Dynel fabric rather than fiberglass as the covering material. The composite wing had been Ken's idea. Stu, being the more conservative of the two, would have preferred a more traditional fabric covered wooden wing.

Plans were offered for the KR1, priced at $15 a set. Popular demand soon dictated a two place version, and the KR2 was quickly designed. It featured a wider cockpit, almost two more feet of length, and about four more feet of wingspan to handle the extra load. The KR2's first flight was in April of 74.

Stu sold his interest in Rand Robinson Engineering in 1979, and moved to northern California. He already had five acres of land about 60 miles north of Yosemite, and felt it a more desirable place to raise his children than L.A. He and his wife built a new home, then he went on to build several more in the next few years. He now works as an electrical engineer for a large limestone and dolomite mining operation, and is quite content with his move

Ken was killed in his KR2 a short time later while flying over Cajon Pass in what was apparently a bad weather / low fuel accident. Ken's wife Jeannette became owner of RR overnight, and stepped up to keep the plans and parts coming. Much of the engineering needs are handled by Bill Marcy of Denver, who's been helping out since early '79.

To date, almost 6000 KR1, 9200 KR2, and 760 KR2S plan sets have been sold. 1200 KR2s are estimated to be flying, with 10 KR2Ss now in the air worldwide. Much of the development work done on KR's is now done by the builders themselves. KR builders tend to be innovative, which leads to some interesting modifications. Some of the mods that work eventually creep into the plans. The KR2S is a case in point. Many builders who'd heard of the pitch sensitivity and tight cabin of the KR2 began to build an enlarged version, with the length determined by the most commonly available longeron material. The result is a KR2 that is stretched 2" between firewall and main spar, and 14" behind the main spar. Higher gross weights dictated more wing area, with the new standard becoming the Diehl wing skin. Those who plan to carry passengers commonly stretch the cabin width a few inches, although 1.5 inches is the limit if you still want to use RR's premolded parts.

Asked it he would change anything in the KR2 design now that several hundred are flying, Stu mentioned stretching the fuselage length and height, as well as adding a little more wing area. Interesting enough, that is exactly what knowledgeable builders have been doing, and as the KR2S plans now reflect. He also thinks that for planes flown close to sea level, that 2.5 degrees of wing incidence is plenty. The prototype KR1 also exhibited a somewhat abrupt stall, with the left wing dropping quickly. He would add stall strips to the leading edge to soften the stall. He is still amazed at how well the prototypes flew, and mentioned that no changes were made to the original plans after the initial flights.

Henni

[Quentin Ferreira]

Bennie wat nou???
Ek sien jy is lus vir bou........
Hoekom n KR ??
Waar is jou planne vir die ander a/c wat jy wou he?

Quentin Ferreira
ZU-ATP
Newcastle

[Rudix]

Hi everyone,

I agree that the KR2 is a great plane, like Henni said a major part od the problem is that most samples these days are a lot heavier than the original. Builders also tend to install fancier seats than the original canvas sling seats and that results in the pilot sitting higher and thus having less space due to the canopy.

I think if you want to scale it up it should only be a bit, maybe 1.15 with a more powerfull motor but still making it a 2 seater. Bennie, the tail on the KR2S is longer than the original and that does help a bit.

Henni is correct in saying there are a few engines today that produce more power than the VW for less weight but it is not that easy. The original KR2 (and S model) is very prone to an aft CofG so saving weight on the engine does not help, you will just have to add it in the form of lead again to get the CofG correct, you will gain power and that will help. I am going the other way, replacing the VW with a 3300 Jabiru, the CofG should be fine and the additional power will be nice, but it does require a new cowl

There are KR2's flying with Jabiru's, I know of one that was flown from the UK to Australia (2 up and loaded) on a Jabiru 2200A.

Fly safe,
Rudi

[Henni]

Henni is correct in saying there are a few engines today that produce more power than the VW for less weight but it is not that easy. The original KR2 (and S model) is very prone to an aft CofG so saving weight on the engine does not help, you will just have to add it in the form of lead again to get the CofG correct, you will gain power and that will help.

Hi Rudi,

This is how it is done in the above case:

[Rudix]

Henni is correct in saying there are a few engines today that produce more power than the VW for less weight but it is not that easy. The original KR2 (and S model) is very prone to an aft CofG so saving weight on the engine does not help, you will just have to add it in the form of lead again to get the CofG correct, you will gain power and that will help.

Hi Rudi,

This is how it is done in the above case:

Hi Henni,

Yes, that will work ! It will make the stability "problem" worse though since you will now have more side area in front of the CofG than before, to maintain stability (yaw/spin) you will have to add vertical fin area or lengthen the tail a bit. There was a KR2 at the KR fly-in (in the USA) a few years ago with the lengthened cowl (Might have been the same one) where it was noted that stability became marginal.

Nice looking engine :D

Regards,
Rudi

PS, vlieg jy die naweek ?

[Tailspin]

Hi All

If you want to check the mods etc for the KRII try to have a look at Steve Boshoff's SBII it has the mods that Bennie is talking about now a different aircraft. Steve has kits available and he is still flying the original one he made from the KRII he modified

[Henni]

Hi Gavin,

Link please?

Henni

[Rudix]

Hi All

If you want to check the mods etc for the KRII try to have a look at Steve Boshoff's SBII it has the mods that Bennie is talking about now a different aircraft. Steve has kits available and he is still flying the original one he made from the KRII he modified

Hi Gavin,

Yes, the SBII is a great looking bird !

Here is the link: http://www.eaa.org.za/projects/steveb

Thanks for the information,
Regards,
Rudi

[Boet]

Bennie, as jy saam met die Stings en die RV`s wil vlieg, bou vir jou n KR2S. Bietjie groter as die KR2. n Baie mooi errie, en sit n Rotax 912 enjintjie in. Dit behoort n uitstekende kombinasie te wees. En n moerse ent goedkoper ook :D

[Bennie Vorster]

Bennie, as jy saam met die Stings en die RV`s wil vlieg, bou vir jou n KR2S. Bietjie groter as die KR2. n Baie mooi errie, en sit n Rotax 912 enjintjie in. Dit behoort n uitstekende kombinasie te wees. En n moerse ent goedkoper ook :D

Yip ek stem saam so waar kry ek 'n kit en die kostes vir so 'n projekie en kom die KR2s ook as 'n neus wiel en retractable under carriage? En hoeveel ure om dit te bou , hoe groot is die KR2S se envelope

[Boet]

Morph het n rukkie terug n KR 2 S kit adverteer. Vra bietjie??