Growing a Grumman - Part 13


Just to clarify - when one speaks of an 'experimental' aircraft, many people think of something built from plans, or even better, from a kit of parts. I should be so lucky as to actually have a proven set of plans, with all the kinks worked out, or a kit to work with!

No - with a true 'experimental', one where we really are experimenting with just about everything, and particularly of course firewall-forward, then the process we are suffering at present is the actual experimentation - thus we find that many things need a rethink/redo.

So - one step forward and two steps back! That's life I guess - but I have been busy with rectifications for the past few weeks, putting right my own blunder as a result of bad planning and lack of foresight!

Once the wings were attached to the aircraft and some fuel in the tanks, it was discovered that the prop-clearance was too little at about 6 inches. This appears to be a result of the CG having moved forward once the wings were in place, thereby putting more weight onto and compressing the nose leg.With only 6 inches prop-clearance the airplane will not pass muster with CAA and rightly so - it is bound to happen that I will ding that prop on any less-than-smooth runway, and certainly the blades will pick up stones etc on run up, damaging the tips.

After much pondering (there's lot's of that involved in doing this conversion!) it seemed the simple way to remedy this was to buy a slightly longer HTD belt for the PSRU. This would allow the assembly of the driven sprocket to which the prop is attached to move upwards by a few inches - et voila, problem solved.

Whoa!, not so fast there pardner! The next longer size in belts available (maybe available here in SA, maybe not!) would increase the centre distance between sprockets by more than 4 inches and that much extra height I do not want or need - darn it.No way could it be so easy - silly me! If I went to the longer belt therefore and wanted to add less than 4 ins to prop clearance, I would also need larger sprockets to take up belt length whilst maintaining drive ratio - snowball effect again!

Another solution proposed by an aviator friend was to make a new nose-gear fork with the necessary angles built in to get the clearance required - looking at this I did not fancy the idea, as there seem to be lots of possible pitfalls in this approach, not to mention suspected large costs!

Therefore I simply had to embark on a process of mounting the engine some 2 - 3 inches higher, which meant disconnecting the motor, lifting it off it's mount-points and welding in new and longer mounting brackets.

First though I had to buy an arc welder to get this job going and I had also to learn to use it - 'learn' describes an ongoing process by the way - one which is incomplete at this time, as can be seen from my welds made with this device! Remember, as mentioned earlier, this engine mount will not fly as it is a prototype in it's third of fourth iteration!

This 'simple' job took two days of solid and bloody hard sweat - cutting, grinding, shaping, bending and welding 5mm steel is hard and dirty work, let me tell you, when everything has to be done by hand!

But now at least I can now measure a good 9 inches clearance at the prop tips from the ground, which should help protect my prop and gives me peace of mind.

Of course, as they say about mods to airplanes, 'no change will go unpunished' - and my top cowling and nosebowl now need to be remade to suit

On the upside, the thrust line is now closer to the original, which cannot be a bad thing. In the looks department the airie's nose will look better, closer to the original.

Right now I have made cutouts to the top cowl so that I can install it, and the carbs and the distributor can poke through.I can either remount the carbs on longer inlet tubes to get them tucked away in a lower position where there is space behind the engine, instead of on top, or I can simply form a bigger 'speed-bulge', about 2 inches higher than before, on the top of the cowling.This would also have to be remade to cover the distributor which used to be fully under the standard cowling level but which is now also raised. The first 'longer inlets' option seems best, although again a lot of work - but then the risk of carb-ice is increased and we are adding complexity and weight - I always recall Colin Chapman's motto (he who started Lotus Cars and won many Grand Prix championships) - 'Simplificate and add lightness!"

After a little more 'ponder-time' I will have an answer to these questions as to what to do next!


Lots of pondering later, I see that a higher cowl bulge to accommodate the carbs is not the way to go - so I started to remount the carbs on extension tubes, repositioning them from the very top of the motor to the sides, sitting just above the rocker-boxes.

It should be possible to mount these carbs this way at a low enough level to provide clearance from the underside of the top cowl - but then the fuel may have to be sucked uphill, and SU's are not at all designed to act as updraft carbs.

I also will make a new nosebowl, as the previous one suited the height of the previous engine mounting and the extra length occasioned by the nosepiece housing of the PSRU - there is no way it can be easily modified for the new height of the drive assembly. Luckily I still have a mould for the original Grumman AA1 nosebowl.


Have spent the last few weeks making a new nosebowl as per the original Grumman design - and having fitted that to the aircraft I found that the top cowling would no longer fit properly, since it is slightly longer front-to-back as it had a downward slope before! So had to make a new one of those as well! Anyhow, now we have a cowling exactly as Grumman intended - problem is that it reaches about 10 inches short of the spinner! So that is the next task, to make it extend to the back of the spinner.

The new nosebowl has no holes for incoming cooling air as I have not yet cut those - and I am thinking I will try it without those holes since my radiator is under the belly- Maybe we will need a small hole just to let some sweep air into the cowling for general cooling draught - again some experimentation will be needed.

Meanwhile I stripped the PSRU for examination as it has run 12 hours or more - lo and behold I discovered that both bearings in the nosepiece housing were rough and noisy - they had failed. Puzzling to say the least, since these are both large size deep-groove bearings which should last for thousands of hours.

I took both to a bearing specialist who took one look at them and noticed the manufacturer's name engraved on them - "BTC, What is this rubbish?" he says,"Better Than Chinese?"!

I think we have some gash bearings here. At the time of purchase these were all that was readily available - I should have held out for a decent brand I reckon, so NOW we will have to find a good brand like Timken, FAG, SFK etc. Then try again.

Re. the carburettors - after yet more 'pondering' I have been moving these around the engine compartment trying to find the right position - not too close to heat sources like exhaust manifolds and aligned facing each other so as to keep a simple linkage. Also have to consider carb-heat to these carbs. I found the place which will achieve this, but will need some fancy curvy pipes. They will be placed right behind the rocker boxes of the V6 and slightly lower. In this position they do not want to stick out of the cowling anywhere at all! Without pipe-bending equipment I will have to fabricate a mockup of the whole catastrophe, possibly using PVC pipe (easier to bend with a heat gun and sand). With the mockup in hand my tame aluminium welding specialist can copy these exactly in ally tubing.

That's the plan then - now to see if it will work!


Several months down the line we have finally finished the engine raising project by completing the new cowlings necessitated by that change.

Re. the cowling I reckon the real answer would be to get hold of cowls from a Grumman Tiger - these will have the additional length that I need and are aerodynamically good, being designed by Roy LoPresti - however with only about 4 or 5 of those Tigers in SA chances do not seem good!

I found an auto-exhaust shop which does mandrel bends in steel pipe - so that is what we are going to use - no chance of cracking through vibration, and all around much simpler to make up! This place can also make up profile-cut mounting adaptor plate and carb flanges.

So the ally tubing I previously had in mind will not be seen on my engine - weight wise there will be little difference anyway, since there is no thin-wall ally tubing to be had in SA!

Of course having ordered the adaptor-plates and carb-flanges last week and sent drawings, there was some kind of problem with the fabricator who promptly lost the drawings but failed to inform me of this fact! Only when I called to arrange to collect the parts, did I discover that work had not even started! Hopefully we may see these parts soon - who knows?

Had to hacksaw the carb-plinth off the top of the manifold of course - managed this time to do this without the hospital bills - nice one! Then I took the manifold for refacing, since it had previously been welded and needed a clean up, so that I can get a seal between the steel adaptor, attached with studs and the aluminium surface of the manifold.

Managed to get decent quality bearings for the PSRU - the brand is RHP from Britain - boy, these are expensive too at R380 for the big bearing! On the other hand the "better than Chinese" rubbish was also very costly even at half that price, given it is now only good for scrap metal!


More pics! Click the image for larger (1024x768) view - I have provided the large size images as I know you guys want to see DETAILS!

>>> Next - Part 14


Nosebowl as per the original - a bit too short!
Nosebowl as per the original - a bit too short!
Nosebowl as per the original - a bit too short!
With prop and spinner installed - now to start filing the gap!
With prop and spinner installed - now to start filing the gap!
With prop and spinner installed - now to start filing the gap!
Urethane Foam slabs hot-glued into place
Urethane Foam slabs hot-glued into place.
Urethane Foam slabs hot-glued into place.
After more slabs in place and carved/sanded to shape The carburettor visible is temprarily placed to check for clearance height
Getting there!
One last bit missing.
Gap is filled!
Hole cut for PSRU nosepiece to stick through.
Prop and spinner remounted - gap between spinner and cowling will need to increase i think
Amazing what a spot of paint can do for looks!
In all it's glory!
Still some lumps and bumps to get rid of - let's worry about the cosmetics later!
With the basic shape we can later look at smoothing a bit more.