Kolb Twinstar
Richard Carlisle
I bought an older twinstar the other day and had a few questions. Kolb was
no help as the "New" Kolb wont answer any questions on the older planes.
Here are my questions....
(1) What is the usful load of this plane? The new Mark 3's are 550lbs and I
know that the older Twinstars were less. Im guessing 450lbs. Am I close?
(2) Im used to flying a 65HP Phantom. This Twinstar feels "Brocken" in
flight. What I mean is that it is very un-responsive. I can go stop to
stop on the ailerons with very little response from the plane. The same
move in the Phantom would put you inverted. The Twinstar just feels very
sloppy.
Is this just the way they fly? Is this something I'll need to get used to
or is there something wrong with this plane? I suspect that this plane is
just radically different than what Im used to and I'll need to adjust to it.
Also...I installed a new 503 with Powerfin prop yesterday.
The guy I bought it from took it up and did a loop, split S and some wing
overs before I bought it. He looked like he was having fun.
(3) I had one incident where I got into enough turbulence (classic mountain
flying) where the plane started to roll and there was not enough control
response to arrest the roll. I ended up rolling inverted and doing a split
S. Can I increase the aileron throws without worring about getting into an
aileron stall situation? The rudder feels OK but it could use more elevator
and aileron.
(4) There is no tactile feel to the controls. IE: No resistance when
pulling back. Its very hard to fly this plane "Seat of the pants". With no
control pressures, the plane is not telling you what it's doing or what it
wants. With the Phantom, if I'm holding back pressure, I know I need to
trim up. The Phantom also talks to you as you approach stall. I can tell
the plane is going to stall before it does. You can tell the Twinstar is
stalling after it stalls and breaks into a left hand spin...which it
recovers from quickly, but still...
With no control feedback, you really have to fly the instruments. It climbs
flat so the only indication that you are climbing is the VSI. There is no
sense of speed or control feedback indicating speed so you have to stay
glued to the airspeed indicator.
I was flying along at 5000RPM today looked at the airpeed...55 MPH...about a
minute later the plane stalled and spun to the right. My airspeed evidently
went from 55 to below 35 with no real indication from the plane.
(5) Today was a bit windy 10-15 with our usual mountain rotors. Is this a
problem for the Twinstar? Is this a strictly flat calm airplane?
Sorry about the length but as I was typing and thinking about todays flying,
more questions came up. I'd really love to talk to someone with some
experience with ths plane or something similar. This is also my first
"pusher" plane. I expected some difference between this and my Phantom, but
I didnt expect this much difference. The Phantom flies like an airplane.
The Twinstar flies like it is alway on the verge of being out of control.
Thanks very much for any comments...Good or bad...
Ross
Rc
Mark-lll that was damaged in a bad landing. Both of these planes gave me
exactly the same control problems that you mention. I had never flown an
ultralight type plane with such terrible aileron & elevator response. They
felt like the controls were spring dampened.
This kind of poor control is not because they were pusher types, because I
have owned several pushers including a Rans S-14, a Thunder Gull-J, a
couple Quicksilvers, and a Pterodactyl. Apparently the Kolb is just a poor
performer, especially after flying performance planes like the Phantom, the
S-14 and the Thunder Gull-J.
I tried all of the things I could think of to improve performance (no help
from Kolb), but never improved flight, that's why I sold them. The only
fix that really works is to get a different plane, or just get used to the
sloppy and possibly dangerous flying.
Rc
"Richard Carlisle" <rrc...@nhvt.net> wrote in message
news:B7F8DAF4.81B%rrc...@nhvt.net...
Richard Carlisle
na...@company.com wrote on 10/21/01 10:05 PM:
> I tried all of the things I could think of to improve performance (no help
> from Kolb), but never improved flight, that's why I sold them. The only
> fix that really works is to get a different plane, or just get used to the
> sloppy and possibly dangerous flying.
This is what I was thinking. I hate to do that. This is the only plane
thats ever fit in my trailer. Plus, it's only 15 minutes from engine shut
down to closing the trailer doors. Its a real convenient plane to trailer.
I bought it as a 2 place but I dont think it is safe to carry passengers in.
Did you try increasing aileron and elevator throw? Or maybe increasing the
size of the elevator? I had planned on recovering it this winter so
rebuilding is an option. I also thought that making wider, shorter ailerons
might help. Semi span ailerons are more effective than full span
ailerons...and if I did that, Id add flaps.
I also thought that maybe the plane is tail heavy. It flys like a plane
with a severe aft CG problem (always on the verge of going into a spin). I
am 240LBS and the previous owner was 170, so Id have to think that since a
heavier pilot would shift the CG forward, the problem is not aft CG.
Im trying to figure out why a plane with such piss poor flying qualities
seems to have such a following. Lots of people seem to love them.
Ross
Marc McNaughton
I don't know specifically about the Kolb (I'm thinking about purchasing a
used Firestar myself), but there are some ways you can improve aileron
effectivness if you're willing to do some mods as you indicate.
Rather than messing with the span and chord of the ailerons, you might try
making them a little thicker instead. If the cause of your problem is that
the airflow is detaching at the rear spar (so your ailerons are working in
very turbulent flow), then making the aileron a little thicker than the spar
(say 0.125"-0.25" each on top and bottom), can help reattach the flow.
Fortunately, this also makes for an easy mod as well. It could even be
'scabbed' onto the existing control surface structure if you're careful
about it. You'll want to be careful about maintaining the same balance
around the hinge line - flutter can be a problem at any speed.
This will also work for the elevator and - assuming flow detachment was the
problem in the first place - should improve your control feel for any
surface to which you apply it.
--
Best Wishes,
Marc McNaughton
(remove the 'e' before "home" to reply via e-mail)
"Richard Carlisle" <rrc...@nhvt.net> wrote in message
news:B7F90D42.10C6%rrc...@nhvt.net...
Ed Snyder
dangerously wrong with that plane. Any plane that goes into a stall spin
without any indication will kill you. I would not fly in it for anything. The
first thing I would do is a good weight and balance calculation and find out
where the true c.g. is empty and then with you and fuel in it and determine if
the c.g. is within specifications. New Kolb, old Kolb what's the difference, it
is still a Kolb. Kolb's attitude towards you and your plane sucks. See if they
will provide you with weight and balance calculation data for that model. If
they won't, then they really suck.
AL Mills
You mean you didn't do weight and balance before you flew it? You're not
talking about certificated aircraft here. Ultralights need to be weight and
balance checked for every pilot/passenger combination. Two of my friends
died last summer over this very argument. Weight and balance! If you chart
it, you can easily substitute weights just like you do (you do, don't you?)
in a GA aircraft using the POH. You might also put a protractor on the
elevator and ailerons to make sure you're getting equal throw in both
positions. My 2 cents...
AL
Harry Burns
flutter, classic instability, or adverse yaw? Just wondering, since all were
prevalant in the 1902 glider. I'm most concerned with how weight and balance
effect control and stall. Please keep us posted on the effects of your mods.
Harry
Richard Carlisle
If anything, there would be a tendancy to be nose heavy, not tail heavy.
Nose heavy isnt a problem...tail heavy is.
Im not sure what good being anal about the W&B would do in this plane. The
pilot and passenger sit so far ahead of the wing CG that if I gained 5 lbs
it would change the CG. Im sure not going to do a W&B every time I fly this
thing. I know that with a 170LB pilot it is at the aft end of the CG
envelope and at full gross it is at the forward end. This is based on the
1/3-2/3 rule (wing CG is 1/3 the chord length from the leading edge) and the
fact that the long tail moment gives this plane a wide allowable CG
envelope.
Of course, Kolb will not give up any data on the older planes, so some of
this is guess work.
In talking with Twinstar pilots, I now realize that the plane is flying as
it should and I just need to adjust to it.
Ross
in article
7989B27C093A5F9B.8DE8DB62...@lp.airnews.net, AL Mills
at alm...@crown.net wrote on 10/22/01 12:15 AM:
Richard Carlisle
pwrs...@ix.netcom.com wrote on 10/22/01 3:29 AM:
They wont give up any data on the older planes. I couldnt even get them to
give me a ball park on the gross weight. Its really not a big deal, Ive
built and flown enough planes to figure it out on my own and after talking
to some other owners, I realize that these planes just fly like crap
compared to what Im used to.
Richard Carlisle
bigger tail surfaces. Ive also noticed that the tail losses effectiveness
before reaching stall. In order to recover from a near stall you have to
bump the throttle which pushes the nose over. If you let it stall, the nose
drops and breaks left. Right rudder has little effect because the tail
stopped working 5 MPH ago. Forward stick has no effect as the elevator is
not working at this point. Tail surface effectiveness starts to fade at
about 40 and true stall is 34.
This is definitely the worse flying plane Ive ever flown, and Ive flown some
real junk over the years (back when I was young and stupid). Fortunately
they all exibit the same poor flying qualities to a degree, so the problems
should be easy to fix.
Ive always thought just by looking at it that the tail feathers on Kolb
aircraft were not large enough. Now I know for sure.
As far as W&B and stall...an aft CG problem will make a plane stall easier
and recover harder. If the CG is too far back, it won't recover. It will
flat spin into the ground.
I used to load lead into the tail of a Pitts S1 years ago so it would flat
spin easier during airshows. To recover you had to go to full throttle and
forward stick. The propwash over the tail was enough to kick the nose down.
A plane with a forward CG problem will tend to be hard to stall and require
alot of back stick/trim to fly straight, but other than that forward CG
doesnt effect the flying characteristics as much as aft CG.
If there is ever any doubt, always err on the side of forward CG. The few
planes Ive scratch built were always tested nose heavy. We would remove
weight until the plane became "too easy" to stall. That dictated the CG
envelope. With that in mind, we put the plane on the scales, weighed it in
the nose heavy configuration and then in the tail heavy configuration.
The Wright Flyer is a canard configuration of sorts, is it not? You would
want to balance it so that the canards pick up some of the gross weight. Im
not sure, but I think the rule for modern Rutan canards is that the canard
should pick up 1/3 and the wing 2/3.
Of course real airplane manufacturers now do all this in the computer before
the plane flys.
In many ways this Twinstar is similar to a scratch build. I cant find any
data on the plane and I know it has (and always has had) aerodynamic
problems. I figure at some point you gotta put on the test pilot hat...and
a current BRS...which this plane will have before I fly it again.
Ross
in article 20011022105401...@mb-bk.aol.com, Harry Burns at
harr...@aol.com wrote on 10/22/01 10:54 AM:
Richard Carlisle
small to be effective at the speeds that this plane flys. I think its a
size issue and not so much a separation issue with the tail. I think
increasing the aileron throws will help. Total aileron movement from stop
to stop is only about 5 inches at the trailing edge. Not much at all
compared to other planes. In light winds the ailerons wouldnt be an issue,
but in turbulence there is not enough aileron throw to keep the plane level.
Im also thinking that you could apply some aileron differential. IE: More
up aileron than down. This might avoid the possibility of stalling one wing
in a slow turn due to too much down aileron.
Anyway...I think fixing the tail will make it a safe and much easier to
handle plane. Right now, I feel that it is unsafe in anything but calm
conditions.
Marc McNaughton
> small to be effective at the speeds that this plane flys.
Possible, certianly, but Kolb's have had a pretty good reputation over the
years and I wouldn't be too quick to look beyond possible deficiencies in
your own particular aircraft. Unless the original builder modified it from
the plans (or simply built it wrong), the general stability and control
should be fairly good. You want to be VERY careful about changing things
like the size of stabilizers and throws of control surfaces. You're getting
into some serious engineering there and unless you have the appropriate
background (perhaps you do?) you can get to be a statistic very quickly that
way.
Despite what might have gone before in terms of the previous owner's weight
vs. yours, I'd still do a careful weight and balance as others have
suggested. No, you don't have to W&B for every flight. Just a good solid
check to establish the basic parameters. From your description there's
obviously something very wrong with your plane and W&B is a good place to
check first, since it's so easy to do. BTW, going out of the front end of
the approved CG range can be just as dangerous as going out the back end.
You can run out of nose-up control power in such a situation and you'll end
up a lawn dart.
Since apparently The New Kolb Company won't give you appropriate
information, a good place to look would be the Kolb list - an e-mail forum
just for Kolb aircraft. You can sign up here:
http://www.matronics.com/kolb-list/index.htm. Since this list is only
populated by Kolb owners, they might have some very helpful information to
share. You might even be able to get copies of plans, rigging information
and a pilot's manual.
If you can't find out the right information to check out your plane, then
you'd probably do better to write the thing off as unsafe and move on to a
different aircraft with which you're more comfortable. I know that's a tough
call to make, but whatever it costs it's a lot less than the cost of your
life.
--
Best Wishes,
Marc McNaughton
(remove the 'e' before "home" to reply via e-mail)
"Richard Carlisle" <rrc...@concentric.net> wrote in message
news:B7FA0A57.97%rrc...@concentric.net...
Christopher
in it.. i have flowing it a bit.. ihavne't flown anythign else so i cant
tell you any differences between others, but it flys about like yours does
too.. there have been times inturbulence where i have needed quite a bit of
deflection.. but the plane has always responded, sometimes there is a quick
delay, but we dont fly in anything over 10 mph winds and morning and evening
flyers,, so we dont push the airplane to the limits, even this one.. but we
have been close on one afternoon flight.. i know what you are talking
about.. about the spinning thou,not so sure,, when we practiced stalls for
my training i made sure to get off the rudder and neutralize that becuase i
had heard they spin easy.. but there was a definate sink right before the
break in the stall, and very little right wing drop, but the stall was very
easily recovered with just a little stick forward, then some power and back
flying.. i have never even seen the plane wnat to think about spinning,,
maybe because i dont get near putting it there..??? i cant say it has been a
bad flying plane, definately not a hot rod.. it is interesting to read your
report thou because you have much more experience then me and its nice to
see someone have some of the same thoughts about control.. we just dont fly
in very turbulent conditions fort he plane and us..
chris
"Richard Carlisle" <rrc...@nhvt.net> wrote in message
news:B7F8DAF4.81B%rrc...@nhvt.net...
Richard Carlisle
just has sloppy handling characteristics. Most of our flying is also in
winds of 15MPH or greater. Sunday was probably a little over 20 and
turbulent...But thats an average day here.
in article Wa2B7.28279$%B6.10...@typhoon.southeast.rr.com, Christopher at
smo...@sc.rr.com wrote on 10/22/01 7:43 PM:
Richard Carlisle
at mar...@ehome.com wrote on 10/22/01 7:11 PM:
> From your description there's
> obviously something very wrong with your plane and W&B is a good place to
> check first, since it's so easy to do.
acually, everyone else seems to have the same issues. The difference seems
to be that Im the only one flying it in gusting mountain winds. I agree
that a weight and balance is in order. I dont think there is a problem
there, but it cant hurt. The plane stalls and recovers fine, its just very
sloppy. Hopefully I can find the data on this plane.
I am going to at least make new tail feathers. I know from flying this
plane from VNE down through a stall that there is not enough control
authority at speeds approaching stall.
I ran into the same thing on a scratch built biplane a few years ago.
Adding some area to the horizontal stab and elevator solved the problem.
Part of the "out of control" feeling is the fact that the plane yaws all
over the place in the wind. Other Twinstar owners have commented on the
same thing.
Hopefully we will get a nice flat calm day between now and next summer where
I can go up and practice without worrying about getting blown into the side
of a mountain.
I dont want to give up on this plane. For all of its faults, it is still
one of the only truly trailerable 2 place planes...which was the reason for
choosing this plane. this plane has all of the potential to be a nice
flying plane. It just needs some refinement. Maybe the new Kolb has
addressed some of these issues on the Mark 3. My Twinstar is probably one
of the first to come out of the factory. It was built in 1984.
Ross
Aaron Smith
in the wing. When you deflect the aileron, the back of the wing lifts up and
the AOA decreases, hence undoing the action of the aileron. Sometimes this same
condition even leads to control reversal.
Don't trust me, though. Get a real opinion somewhere :-)
Richard Carlisle
10/23/01 2:48 AM:
> I *think* the lack of aileron response may be due to lack of torsional
> rigidity
> in the wing. When you deflect the aileron, the back of the wing lifts up and
> the AOA decreases, hence undoing the action of the aileron. Sometimes this
> same
> condition even leads to control reversal.
>
> Don't trust me, though. Get a real opinion somewhere :-)
Aaron...Thats acually a very valid point. I was looking at the lack of a
trailing edge strut and thinking the same thing. Maybe a trailing edge
strut would help. Thats a fairly easy thing to try.
Harry Burns
about a year now, tells me that the machine has a good bit of aft CG if you
don't lean forward on it. Having watched Rick Young's 1902 glider fly, its
apparant that the pilot is really working the elevator hard. I'm assuming that
this is a combination of the aft CG problem and the oversized elevator that
Wilbur Wright prefered.
Assuming that I wanted to balance my 1902 correctly (for the sake of not
crashing before Oct. of 2002), where should the CG on a cannard be? The wings
are 32 ft span, 60" chord, with CL at 1/3. Elevator is eliptical, 9 ft span,
42" max chord, with a similar airfoil.
Harry
Harry Burns
glider (provided I have the funds to finish it). Its not only hard to control,
it crashes on every landing! Thank God for sand dunes!
Harry "controlable aircraft #1" Burns
Richard Carlisle
harr...@aol.com wrote on 10/23/01 11:28 AM:
> Its not only hard to control,
> it crashes on every landing!
I have an RC plane just like that. 10 takeoffs...0 landings...
Mark Swihart
#46's Plans and documents is the following specifications for the plane. I
used to own the plane. I kept the web site I had on the net for years about
it still saved on disk. So it was easy to pull up the info you are seeking.
We ever meet; ya owe me beer or a good hangar flying story.....
Let me know if I can help you further via uleip...@hotmail.com delete "UL"
off the address before sending....
Mark Swihart
Bradley, CA
Specifications
Winspan: 30 ft. Wing Area: 160 sq. ft.Overall Length: 22ft 10inches
Height: 5ft 8inches
Power Plant: Rotax 503( 2 cycle, 2cylinder, 497 cc) Engine Rating:
47
Horse power
Speeds for Normal Operation
TAKE OFF:
Nomal Climb Out
50-55 mph/IAS
Short Field Takeoff, Speed at 50 ft
45 mph/IAS
CLIMB:
Normal
50-55 mph/IAS
Best Rate of Climb
50 mph/IAS
Best Angle of Climb
45 mph/IAS
LANDING APPROACH:
Normal Approach
50-55 mph/IAS
Short Approach
45 mph/IAS
BALKED LANDING:
Maximum Power
50 mph/IAS
MAXIMUM RECOMMENDED TURBULENT AIR SPEED:
690lbs
60 mph/IAS
500lbs
65 mph/IAS
MAXIMUM CROSSWIND VELOCITY: 10 mph/IAS
Stall Speeds
Weight Bank Angle
Airspeed
500 lbs 0 degrees
32 mph/IAS
500 lbs 30 degrees
35 mph/IAS
500 lbs 45 degrees
39 mph/IAS
500 lbs 60 degrees
45 mph/IAS
690 lbs 0 degrees
38 mph/IAS
690 lbs 30 degrees
42 mph/IAS
690 lbs 45 degrees
46 mph/IAS
690 lbs 60 degrees
54 mph/IAS
Rate of Climb
Weight Rate of Climb
500lbs 1050rpm
690lbs 750rpm
Take Off Distance
Weight Take Off Speed Ground Roll
Tot. Clear 50 ft.
500lbs 50 mph/IAS 250 ft
400 ft
690lbs 50 mph/IAS 350 ft
525 ft
Conditions:
Full throttle, paved, level dry runway, zero wind.
Note:
Decrease distances 10% for each 10 mph headwind. For operation with
tailwinds up to 7 mph, increase distance by 10% for each 2 mph.
Landing Distance
Weight Landing Speed Ground Roll
500lbs 45 mph/IAS 450 ft
500lbs 50 mph/IAS 500 ft
690lbs 45 mph/IAS 500 ft
690lbs 50 mph/IAS 550 ft
Conditions:
Paved, level, dry runway, zero wind.
Note:
Decrease distances 10% for each 10 mph headwind. For operation with
tailwinds up to 7 mph, increase distance by 10% for each 2 mph.
"Richard Carlisle" <rrc...@concentric.net> wrote in message
news:B7F9FF43.95%rrc...@concentric.net...
> in article 3BD3CABE...@ix.netcom.com, Ed Snyder at
> pwrs...@ix.netcom.com wrote on 10/22/01 3:29 AM:
>
CAS CAS
I built and owned Twinstar #25 for eight years and put about 800 hours on it and
do not have near the thoughts on it that you have experienced. I even became a
Kolb dealer for six years before they ceased their dealers and sold directly. I
sold my Twinstar seven years ago to a friend of mine in Minnesota and I bet he
still has all the building plans and info I gave him on it. He just emailed me
telling of his enjoyment of flying it. Twinstars were sold ‘85-89
approximately. The Mark II started to sell in ‘88 approximately, Mark III took
over in about ‘90.
I have flown several Twinstars and I thought they were a good design in the
‘80’s. I am even completing an unfinished kit right now. Unfortunately, I
didn’t get any plans or info with this unfinished kit. I felt my Twinstar 503
Single Carb could fly 425 lbs of people safely. Another friend of mine could
fly nearly 500 lbs of people, still with a Rotax 503 but I believe his was dual
carb. I believe the reason is that I built the trim tab into the elevator which
essentially decreased it. His trim tab was on the outside of the elevator. The
roll rate was deceit compare to slower planes such as T-Bird. Stalls were
normal and predictable and I never stalled it unexpectedly. I got to the
experience level to fly in 30 mph gusty surface winds and thought it was fun.
For a while, I flew it with the two foot center wing gap seal off and that also
diminished the performance and smoothness and carrying capacity.
Concerning roll rate, does you Twinstar have the aileron gap seals doped on?
My only complaint about Kolbs is that if you damage one, you got a big problem
verses the newer designs with removable fabric, insertable wing ribs, tri tail
and not single tail boom tubes, and replaceable bolt-on tube type airframes.
T-Bird uses all these designs but still has a slow roll rate.
There isn’t such thing as a perfect plane. They all have their pros and cons.
Kolbs fold up great. T-Birds are easy to repair. Most two seaters fly like a
stuck pig with two big men. But if you want transportability, Kolbs, Kitfox,
and Avid are the best.
Fly Carefully ……Tom.
CAS
I corrected my email address and this one is correct. I just set up a new
computer and your email was the first one and I saw that it had my original
address and not my personal one (alias).
I email my friend in Minnesota who has my old Twinstar and asked him about
paperwork I gave him. I will see if he can scan them and email you a copy of
the ‘Flying’ page. If my memory serves me correctly, Gross weight is explained
rather than by pounds but by ‘if you attempt to take off and you find your
Twinstar will not climb out of ground effect,…… LAND. You are probably
Over-Grossed’. I can see why ultralights don’t get a lot of respect sometimes
from Certified Aviation pilots.
I have NEVER had an air current push me over in 27 years of flying. And I love
to fly on windy days. You must have some strong winds over there. Fly Careful.
I think it is fine to get the most travel you can out of the ailerons but don't
get the rod end bearing too close to the end of the threads. I believe five
threads would be the minimum. Also, make sure there isn't much play in the
control system because this is usually the reason for minor anoying flutter of
ailerons. Also, once, years ago, I put my ailerons in a higher adjustment
beyond neutral experimenting for better speed and made the tail flutter which
was very scary.
I will ask my friend if it is OK for you to contact him and will get you his
email address. I am sure he would like to know a fellow Twinstar pilot.
Tom.
Thanks for the input Tom. Its nice to hear from someone with extensive
experience with this plane. Part of the problem may the fact that Im used
to better performance and I just need to adjust to this new plane. I think
thats alot of it.
The Twinstar is perfect for the reasons I bought it. Portability and stits
covering (I dont like dacron sails). I wish the performance was better but
Ill get used to it. It has a new dual carb 503 with a 2 blade 64" Powerfin
prop. If I install a tuned pipe, I can get 65HP out of the 503. I have a
tuned pipe on my Phantom. It gives you a very noticable difference at all
levels of performance.
My Twinstar does have the gapseals doped in on the ailerons. Im thinking
about increasing aileron travel a little. I had an incident where a rotor
rolled me inverted and there was not enough aileron to bring it back. The
plane kept rolling right with the stick on the stops to the left.
Ross
sandyh
jim
TWINSTAR. ONE IS TITLED " TWINSTAR". IT SHOWS DNE AS 75MPH. THE OTHER IS
TITLED " TWINSTAR, MARKII. IT SHOWS DNE AS 80 MPH.
"sandyh" <san...@dccnet.com> wrote in message
news:u0r03e4...@corp.supernews.com...
mojave...@gmail.com
Where dose one acquire a Tuned exhaust for a Rotax 503? It has always seemed to me that the 503 could benefit from dual expansion chamber exhaust. In the early 2 stroke dirt bikes the first modification was expansion chambers which provided significant improvement. It also seems like fuel injection, ceramic pistons and silicon cylinder bores similar to Hirth would further improve the Rotax 503.