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Technical Tips
This page is dedicated to reproducing those useful little bits of
information which crop up all over the place, but which you can never find
when you need them! Most of this information comes from the excellent BMAA
eGroup. I have reproduced these without first seeking permission as the eGroup
is public domain. Should anyone prefer their words are not used, please let me
know and I'll remove them immediately. Clearly, I make no copyright claim to
any of these entries!
Also, it is important to note that neither the authors of the bits of
advice, nor the author of this web-site may be held responsible for any events
which occur as a result of any of this advice being followed. Go sue someone
else!
Cold Seizures explained
How to change the monopole on an XL
Can you make money flying a Microlight?
The 'Drag Curve' explained
What is the '90-day rule'
Diagnosing electric start problems
Cold Seizures explained
The text below is reproduced from a posting to the BMAA eGroup, where a
member helpfully shared this information - which comes from a Rotax expert -
with the group.
First.. the term "cold seizure" is a bit of a misnomer..
All seizures are caused by heat/friction. A cold seizure is where
the piston expands faster than the bore it is travelling in and contacts the
sides of that bore. These are also known as four corner seizures.
The worst case scenario is that the engine can just lose power and stop.
In the case of a mild (mini) seizure the engine may just lose power for a
second or two, but will respond to throttle inputs and will recover when the
throttle is advanced. This may happen a few times before a major seizure
occurs.
Cold seizures "usually" occur after a full throttle run when the
engine is powered back to a cruise throttle setting. If the engine
has experienced some previous mini seizures, the stoppage can occur
anytime in flight as there is already some aluminum (off the piston) attached
to the cylinder wall and galling (unwanted removal of aluminum from the piston
to the cylinder wall) will be occurring at a variable rate.
In a cold seizure scenario the engine may just sputter and lose rpm for a
second or two, or it may bring the engine to a complete stop. Once the
engine has cooled down a bit it will appear to re-start and run
properly. Don't let this fool you. More than one person has tried to fly
his plane out of a field where they had to land because the engine quit, only
to have
the engine fail again in short order.. unfortunately, the second failure
usually happens when the pilot has fewer options for a safe off field landing.
Don't let "gethomeitis" bite you. Find out why the engine
stopped before you carry on.. After all, the "self fixing engine"
has not yet been invented.
The cause of the cold seizure can be variable. One obvious one, would be
a lack of warm up prior to going to full throttle. Also, long extended
descents at low power settings followed by a high power run (go around) can
also contribute/cause the problem. A major cold seizure (complete
stoppage of the engine) can occur as a result of multiple mini seizures
finally causing a big time stoppage of the engine.
An easy check for a four corner seizure (cold seizure) is to remove the
exhaust "Y" pipe (manifold) and take a peek at the sides of the
pistons. If the engine has experienced a seizure, the pistons will tell you
the story. On a cold seizure, there will be vertical scuffing towards to
outside edges of the piston as viewed through the exhaust port. There
will be two corresponding vertical scuffs on the intake side, but these cannot
be viewed without removing the cylinders.. If the engine has experienced a
cold seizure these marks will be visible through the exhaust ports.
If you have any thoughts that your engine might have experienced a mini
through a full seizure, its far cheaper to fix the engine problem than fix
both an engine and airframe problem because the engine failed again. The
"through the exhaust port" test is simple and definitive!
How to change the monopole on an XL
Bob Hood posted this information to the eGroup, after finding out
himself the hard
way:
I had to change the monopole on my XL recently. There are no shortcuts. The
job took about an hour, and after removing the propellor to give better access
to the engine and tank involved the following;
-
Drop the fuel tank by removing the strap and
petrol pipe, then pulling backwards. If you have the metal tank then note
that the support lugs at the top/front end of the tank fit over spacers on
the lower of two bolts that go through the monopole. DO NOT LOSE THE
SPACERS!!
-
Disconnect the engine electrics from the wires
that go to the kill switch and any ancillary gauges etc.
-
Undo the six engine mounting bolts (2 at the
top and 2 on each side at the bottom of the engine)
-
Undo the jubilee clip and remove the carb
-
Put the engine, complete with exhaust onto a
chair out of the way (NOTE: it is heavy, so if you have a friend handy,
then get him to help you)
-
Make sure there is nothing still attached to
the engine mountings (like fuel tank breather tube etc) then remove the
upper and lower bolts that go through the monopole and support both the
engine mountings, and the plate with the fuel switch on it.
-
Find the bolt on the monopole pivot bracket
that has the safety strap attached to it, remove the nut and pull off the
strap.
-
Undo the single pivot bolt at the base of the
monopole and remove the pole complete with engine mountings.
-
You may be lucky and have mountings that slide
easily off the pole. I didn't, and spent about 10 minutes with a hammer
and a block of wood "persuading" the engine mount to leave the
monopole. Some WD40 helped with this, but not much.
-
Grease the inside of the engine mounting and
the outside of the monopole, then slide (tap?) the mounting onto the new
pole until the bolt holes line up. This sounds easy but wasn't. It proved
a bit fiddly getting the holes to line up, but I can't remember why.
-
Line up base of monopole with bolt holes in
pivot bracket, then insert bolt and add nut. Don't do this up too tightly
or you may not be able to pivot the pole easily. Just do up the bolt until
there is no sideways slack between the pivot and the pole.
-
Now assemble the rest of the items you took
off in reverse order until it is all in place. Then go round and check
that no washers have been left off. DON'T FORGET THE SPACERS for the fuel
tank lugs if you have a metal tank.
Like I said, it took me about an hour, working on my own, with no special
tools, just a couple of 13mm ring spanners, a couple of 10mm and 11mm
spanners, a flat head screwdriver, a cross head screwdriver, a hammer and a
block of wood. Oh and some WD40 and grease.
Best of luck!
Bob Hood.
Can you make money flying a Microlight?
The message below was posted by Chris Finnigan of the BMAA to the eGroup, and
addresses the question of can you charge for aerial work.:
It is the purpose of the flight that helps to define whether it is aerial
work or not. If you take off for a planned photographic sortie
with the guy in the back handling the camera to take shots that he intends to
sell I believe that is aerial work.
If you take off for a private flight with a friend in the back bringing his
camera along to record the experience and that friend contributes to the
direct cost of the flight that is not aerial work. If at some stage in
the future he is offered money for one of the photographs he has taken that is
probably OK too, but if he advertises his services as an aerial photographer
selling photographs he has shot from your aircraft, you may be in breach of
the law.
To give you a bit more detail an extract from one of my advice letters
reads as follows:
PUBLIC TRANSPORT, AERIAL WORK AND PASSENGER CARRYING BY MICROLIGHT PILOTS
A microlight pilot who does not hold an instructor rating cannot undertake
either public transport flying or aerial work but may conduct flying tests
provided a strict set of criteria are observed.
Microlight Instructors cannot undertake public transport flying but may
undertake aerial work in respect of flight instruction only.
Holders of private pilot's licences are not allowed to receive remuneration
for their flying activities.
Aerial work means any purpose (other than public transport) for which an
aircraft is flown if valuable consideration is given or promised in respect of
the flight or purpose of the flight. Valuable consideration is any
payment in cash or kind that is of a more than nominal nature. The
definition of a nominal nature is not clear but for a payment in kind could
probably interpreted along the lines of:
 | A small gift not exceeding, say, £20 in value. |
| Being brought lunch or a drink or two. |
| Having a 20 litre jerry can of fuel brought for you, but not being given
cash to buy it yourself. |
In summary, as a PPL holder you cannot receive remuneration for your
services as a pilot and you cannot undertake aerial work for which valuable
consideration is given.
The article of the Air Navigation Order that covers the subject of public
transport and aerial work is Article 130, a complex four page plus set of
clauses and sub-clauses written in legal terminology. It does take
a bit of careful reading to extract the parts relevant to microlights but
essentially if public transport flying and aerial work are both ruled out then
only private flying remains.
Carrying a passenger on a private flight in a microlight is permissible if
the only valuable consideration given or promised in respect of the flight or
the purpose of the flight is a contribution to the direct costs of the flight
that would otherwise be paid by the pilot in command. The
proportion that such a contribution should bear to the total direct costs of
the flight shall not exceed the proportion that the number of persons carried
on the flight (excluding the pilot) bears to the number of persons carried on
the flight (including the pilot). In other words the direct costs
of the flight can only be split equally between the pilot and the passenger.
The rules also state that, "no information concerning the flight
shall have been published or advertised prior to the commencement of the
flight".
In summary, the BMAA advice about carrying passengers is as follows:
| If any microlight pilot is receiving more than 50% of the direct costs
of a private flight from a person carried on that flight then the law is
being broken. |
| If the carriage of persons on such flights is being advertised, or
bookings for this carriage are being taken, then again the law is being
broken. |
| If any microlight pilot takes a passenger on a private flight, (whether
valuable consideration in terms of 50% of the direct costs is received or
not), without having passenger liability insurance, then he or she is
being irresponsible in the extreme and could face punitive claims for
damages in the event of an accident. |
I hope this clarifies the situation rather than confusing you further!
Regards,
Chris
The 'Drag Curve' explained
Posted to the eGroup by Jeremy Harris:
If you remember back to the "Aeroplanes Technical" bit you will
recall that there are two principal forms of drag, induced drag and profile or
form drag. Induced drag is essentially due to the production of lift and
so is only really dependent on the amount of lift produced and hence the angle
of attack of the wing. Profile drag is a direct result of the pressure
exerted on the aircraft by the moving airflow and is proportional to the
square of the airspeed.
If you plot the variation of induced drag with speed, with drag on the Y
axis and speed on the X axis, you get a curve that slopes downward to the
right lower corner. If you then plot the profile drag on the same graph
you get a square law curve that slopes upward to the right upper corner.
The total drag is effectively the sum of these two curves and is therefore U
shaped. If you approach the minimum drag point from the left hand side
(i.e. climb to cruise from a slower airspeed) you may need more power to get
there than if you approach from the right hand side of the curve (descend, at
a slightly faster airspeed). This is where the term "flying on the
wrong side of the drag curve" comes from.
When you look at this situation closely you can see that this is due to the
change in angle of attack. When climbing the A of A is high, so the
induced drag is high. Levelling off from this doesn't take you through
the dip in the curve. Descending results in a lower A of A, so reducing
induced drag and getting you straight on to the right side of the dip.
Hope this makes sense!
Jeremy
What is the '90-day rule'
Posted to the eGroup by Chris Finnigan:
The 90 day rule is contained in the Section on Licences at Schedule 8 of
the Air Navigation Order and, referring to the holder of a Private Pilot's
Licence Aeroplanes, states that "He shall not fly as pilot in command of
such an aeroplane carrying passengers unless within the preceding 90 days he
has made three take offs and three landings as sole manipulator of the
controls of an aeroplane of the same type or class....."
It does apply to microlights (I have checked) and came into force when the
CAA last amended the ANO in August 2000 to take account of the JAR changes
required. While we were unhappy about the way it was sneaked in to
Air Law without us being consulted, and I told the CAA so, we do actually
think it is a sensible rule and so we haven't opposed it.
If you haven't done three take offs and three landings (always try to
maintain the ratio as 1:1!) in the last 90 days as pilot in command then you
should not take off with a passenger until you have done three take offs and
landings which can be achieved solo in about 15 minutes at the average
microlight strip. Having a quick fly around on your own is often a
good way of shaking off the rust if you haven't flown for a while and if
something is going to go bang on your aeroplane it is always easier, in my
view, to cope with if you are on your own and haven't got someone screaming at
you from the other seat.
The principle of the 90 day rules is to ensure that pilots have both
currency and recency when carrying passengers and aren't trying to cope with
their own rustiness or lack of practice while at the same time talking to a
passenger. I had to ask a passenger I was flying recently who was
clearly enjoying it a lot if she would mind being quiet for a few seconds
while I concentrated on landing at a busy GA airfield and I am very current
and recent!
There have been a spate of GA fatal accidents this year and sadly two
involving BMAA members too. A common trend throughout these
accidents is lack of recency where some pilots who have not flown for months
have suddenly taken to the air and found their performance as pilots wanting.
My article in the last Flight Safety Bulletin (the one with Colin Bodill in
his Blade on the cover) said something about this too.
I hope I have cleared this up for you. If you have any further
queries please e-mail me again or give me a ring at the office.
Regards,
Chris Finnigan
Diagnosing electric start problems
This was posted to the egroup by Ian Holt:
I run an Avid Speedwing 582 which has had periods of troublesome starting.
There are a pile of obvious factors that will prevent a start and a few
obscure ones. None of these may apply in others situations but here is what
I have learned the hard way.
1. Fuel always helpful for starting.
If your carbs have done more than 150 h look at the float chamber valves,
and the float heights. Both need to be spot on for consistent starts. I had 6
months of lousy starting that went away when I set the float height correctly
one carb was way too high in fuel level. Is your prop balance / track a
bit out ? Vibration will stuff your float valves in no time ( in one cases I
have seen the main jet worn to an elliptical cross section !)
Primer pump ( if you have one ) - check to see that fuel squirts into both
carbs the pipe layout on my machine resulted in 100% of the priming fuel
in one carb, zippo to the other. ( one pipe routed over the other ).
Avgas kiss of death for 582s in Avids at least. After six hours on
Avgas nothing will start my machine other that one new plug swop any
single plug and it fires first time with lousy mag drops until the combustion
has cleared the other plugs then it is fine. I have done a full forensic
on the affected plugs but found nothing to explain this no high voltage
leakage, no whiskers or shorts.
2. Electrics Ducati CDI ignition
mills need to crank at around 600 800 rpm to have any chance of
getting underway. To achieve this :
The battery must have a low internal resistance at high currents cheapo
m/cycle batteries can be poor in this respect. Buy a good one ! Those
chaps finding an easy start after adding a jump lead connection to an external
battery probably have an example of higher than desirable internal resistance.
Get a better ( not necessarily the same as bigger ) battery. Two batteries in
parallel means half the effective internal resistance and voltage loss.
The starter solenoid and cabling must be of low resistance. I had a year of
desperate starting until I happened to test crank the mill by wedging a huge
tommy bar across the solenoid starter motor terminals ( dont try this at
home folks!) . Result was a super brisk crank and immediate firing up and
an immediate pace backwards on my part. Remedy I fitted a Lucas truck
solenoid the size of a can of beans. Result good starting. I opened up the
old unit and found badly burned contacts, doubtless caused by five years of
prolonged cranking to get the mill fired up.
Alas none if this will be of any use unless your trigger gaps are correctly
set up at the workshop manual minimum ( or as close to the minimum as flywheel
eccentricity permits ! ).
Plug caps my machine has cheapo motorcycle 5k resistive caps they
are great, with good radio noise performance, for about 50 hours, when I
bin them.
3. And now for the weird one the gearbox ( the type B box
).
When I first acquired the machine it started cold, first time,
but would not start warm, or even on the same day. Every possible variable was
explored mainly by very expensive replacement to no avail. I happened to
speak to an American Skidoo repairer who told me he had often seen this
problem with 582 engined units with backlash in the drive train. He
suggested I look at the gearbox. I did the Bellville washers that apply
engagement pressure on the PTO gear dogs were totally shot the box was a
few hours from having the prop set off alone up the runway. I had the box
reconditioned starting problems vanished. I can only surmise that
after an overnight stand the cold gear oil settled in amongst the belville
washers and damped the backlash after one start the oil was flung around
and the full backlash was evident. My informant told me that the first
starting kick from the engine is reflected back to the crank by the
transmission slop and arrests the piston stroke , ergo no start. Well,
sounds crazy but it worked for me.
Hope this helps ! .
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