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GROWING A GRUMMAN - Home-Part 1 - Part 2 - Part 3 - Part 4 - Part 5 - Part 6 - Part 7 - Part 8 - Part 9 - Part 10- Part 11- Part 12 - Part13 - Part14

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!)

  See those cowling bulges to clear carbs and header tank? - they will have to go and they can go now that the rad has been relocated and the header tank lowered on the firewall. Carbs will eventually be repositioned to sit behind the motor where the rad used to be and the top bulge will be gone. I call it my Prop-powered dragster - we have seen those jet powered dragsters so why not indeed?
 
What's that foam in the cowl inlet? Glad you asked - I tried taxiing around with the cowl inlets blocked off just to see if it made any diff to water temps - it did not. However my engineer mate advised that I should allow for some "sweep air" to pass over the motor as all auto engine manufacturers specs require this - so I took the plugs out for the high-speed runs. The round hole below and to the left of the spinner is to bring cooling air onto the PSRU drive belt  
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GROWING A GRUMMAN - Home-Part 1 - Part 2 - Part 3 - Part 4 - Part 5 - Part 6 - Part 7 - Part 8 - Part 9 - Part 10- Part 11- Part 12- Part 13
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