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Growing a Grumman
- Part 9
2009-09-06
Having sorted
out the engine cooling three problems remained before we could begin
taxi-trials - a damaged wheel, very noisy wheel bearings and poor
brakes.
After carefully inspecting
the actual wheel bearings and finding no apparent damage to the taper-rollers
or cage, I purchased a set of four new bearing tracks (races) - as these
had suffered pitting and corrosion and some surface distortion caused
by the aircraft standing on it's wheels immobile for lengthy periods.
In addition damage to one wheel was found - where the snap ring would
fit into it's groove, holding the felt grease seal and washers in place,
the groove was worn away - luckily I managed to obtain new inner and outer
wheel halves (used - serviceable) and today assembled the wheel and replaced
these on the aircraft, along with the brake calipers. Now I know the brake
calipers weep fluid from the bleed nipples, but the rate of loss is low.
Having fitted the
cowls as you can see by the pics, we were ready to try some taxi-trials
and if all went well, then some high-speed runs along the 1.1 Km runway.
Having taxied around
for a while and finding the brakes adequate and everyyhing else good to
go, we taxied to the runway threshold - only to, by habit and training
halt at the holding point, ready for run-ups! What on earth for?, I asked
myself, there is nothing to check that I have not already been checking
on the longish taxi out to the threshold. Let's go - and we lined up on
Rwy 31.
Holding the brakes
as hard as possible and slowly advancing the vernier throttle demonstrated
what I already knew - that the brakes hold only up to about 3500 rpm at
which point brute force overcomes them and the wheels turn, so we let
them - by this time as we began to accelerate I glanced at the rev counter
to see 4000 rpm showing - and we were moving very smartly down the runway
- in less time that it takes to tell the ASI was indicating 80 mph and
it felt like it! We had now covered perhaps just over one-third of the
runway length, and with brakes that are not fully confidence-inspiring
I decided to pul back on throttle and begin slowing.
So far so good. At
the opposite end of the field I simply swung around on to Rwy 13 since
there was no wind nor traffic today - and again held the brakes against
power - but this time I simply slapped power to full with the unlock button
on the vernier throttle - geez! The motor instantly went with a howl and
a roar to just over 4100 rpm and we charged off like a dragster going
for the quarter-mile record! This time the ASI reached 90 before half
the runway was used.
Later I repeated this
and again reached 90mph indicated in a very short time, each time - I
think we have sufficient power! And with wings fitted we would certainly
have been flying today.
However the left brake
again faded away -damn, so back to the hangar and pull off both brakes
- these darn things are gonna get fixed once and for all this week!
But a good day overall
- power in abundance - no radiator overheating problems, wheel bearing
rumble no longer competing with engine noise. Some progress and once brakes
are working properly I will need to slightly repitch the ground adjustable
Warp Drive prop so that I can get the revs I need - 4400 rpm on engine
= 2750 at the prop. Maybe we can crack that quarter-mile record yet!
2009-09-25
After bleeding the
brakes once again and having run the aircraft up and down the runway at
high speed many times now, a problem reared it's head - the prop - tip
clearance seemed lower than before! On investigation it was indeed lower,
as the forward engine mount brackets had actually drooped a little - probably
as a result of the forces generated whilst impacting the bumps in the
runway at great speed! Instead of 8 inches of tip clearance from the groun,
we had only about 5 inches - no good!
These brackets are
made of mild steel, welded-fabrication, and clearly need to be beefed
up, so that is the next task. Ultimately these mounts will be made from
solid aluminium block, once the final engine mount has been made up in
4130 steel tubing.
However the repitching
of the prop I did manage to get done - and found that rpm's at max throttle
increased to 4600 rpms - okay for now and once the additional weight and
drag of wings is added, perhaps that will become the targeted 4400 rpms.
For now she'll do!
As for the brakes,
these seem to be working okay - but the remaining problem is that the
mismatch between piston sizes i.r.o master cylinders and brake calipers
means that I have excess pedal travel - so I still need to find the correct
calipers for the Grumman. BTW did I mention that I took a wheel and axle
to a motorcycle shop to see if any bike calipers would fit? They will
not as even the smallest calipers available, which would not in any case
have the necessary braking power, are still too bulky to fit inside the
wheel rim as the Clevelands do.Pity about that!
The aircraft has now
run for over 4 hours in taxi tests - and of course we have discovered
some problems but that is why I wanted to run it about a lot! Better to
find those problems now than on the first flight! Quite soon I hope to
complete the taxi testing, at which point we can move on to:
Build and install
4130 tubing engine mount (copy of prototype now installed)
Build one flap/aileron set to replace the damaged one
Install wings
Fair in the underbelly scoop
Remake all water piping under the cowl - presently steel, needs to be
aluminium piping.
Replace brake calipers with correct part
Sigh...so much to
do!
2009-10-11
Got the engine mounts
beefed up by adding gussets - seem okay after 4 runs along our bumpy runway!
No sagging of the motor anymore.
Decided to run a static
max power test today - and I took a stout towrope to the airfield for
just this purpose. This time I hooked the tail tie-down ring (my engineer
friend has calculated the strength of this fitting at 7 tons!) with the
tow-rope to my wife's car and then gave the Chev full throttle. Went to
4800 rpm instead of the previous 4600 rpm.Standing hard on the brakes
held the aircraft as I slowly advanced the throttle, until I was seeing
3600 rpm engine speed - after that, as the revs increase, the rope and
the car has to do the job of holding the aircraft as the wheels on the
aircraft start turning - or simply start skidding across the ground!
The difference in
rpm from the previous high-power test runs could be as a result of cooler
weather - or simply that I have never been able to let the motor really
get into it's stride when charging along the runway, being worried about
running off the end of the strip! Of course 4800 rpm is probably a bit
too much according to power and torque curves for this engine - on the
other hand the motor will clearly turn this prop at just about any revs
I want - and the prop has no limiting speed according to the manufacturer
other than that the tips should not go supersonic!
A friend who was observing
this performance started waving madly at me in the cockpit and after throttling
down and shutting off the engine I asked what was wrong - he told me that
the aircraft was pulling the car along on the grass - with the wheels
locked. It had hauled it about 1 metre before I shut down - thought I
felt a slight lurch forward!
Hmmm! The car, a Honda
Ballade (Civic to non-SA folks) weighs apparently 2280 lbs empty - one
ton. I think we are generating some substantial thrust, since there is
no way any 150 or 160 hp Grumman will pull a car with the car handbrake
firmly on and in 1st gear!
More pics - taken
before the high speed runs (BTW aircraft looked exactly the same afterwards
in case you were wondering!)
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