Fora 15D Pioneer Engines

[flyingkiw1]

Andrew and I have been working on getting a Fora 15D Pioneer up to snuff for Classic FAI and come up against a few engine design problems. The engine itself appears to be quite well made but there are a few hidden snags in the engine (between Andrew and I we have 4 of them, 2 from the UK and two directly from the FORA factory.

1. Contrary to popular belief, and urban legion, both sources have exactly the same engine timing problems. When questioned on email recently directly about the problem with intake crankshaft timing being out of skew the Fora guys claimed they knew nothing about it and nothing, absolutely has changed on the recent engines, so they say.

2. The compression setting bottoms out so one cannot get a good setting in the engine unless one changes the fuel mix or does surgery on the head. Looking at photo's on the net of other peoples engine after running I can see that the comp screw is screwed almost completely in in most photo's so the problem seems universal. The head space in the whole system is too large at TDC so the only way to solve the problem is a combination of shimming down and a light skim on the head itself ie metal removal.

3. The con rod are really really easy to bend, in fact as we found out they can bend when one is running the engine in under very light running-in conditions. Some users have broken rods, some bent them, but I suspect there is a universal problem here.

4. One of my engines had the plastic venturi blocked with plastic swarf from new, easy to fix, but a little annoying.

Can you guys give me feed back on any problems you have had with this engine as I want to get back to FORA about it. It is basically a good little engine but if we can get some of these annoying basic design faults fixed by FORA then we will have a 1/2 decent engine.

Feedback... lots of feedback please. Email me on smithlw at optusnet dot com dot au if you do not want to go into print here.

Many thanks in advance.

[locktite401]

Greggles and myself have been playing with a Fora Junior/Pioneer for KMFC Diesel Goodyear. It was our first attempt at a DGY model and engine and a bit disappointing.
One practice day we ran out of compression altogether, so removed the (0.35mm?) head shim. It helped but not much. Airspeed was something like 29-29.5 sec/10 laps.

Rather discouraged we moved on to PAW 15TBR/GTS2 and R250 powered models.

Fora fuel was standard 20% Castrol M based OT brew.

After Maris's ACLN article I changed the timing, on the bench it achieved +17000 rpm, however it wouldn't fly with the over 5mm venturi hole. So I tried an existing R250 venturi assembly bored to 4.5mm, and on advice dropped the oil to first 15% Benol, then 13% (Nelson mix). IPN was about 12.5 ml/ litre, kero was Supercheap brand.

In the air everything seemed better, starts improved, airspeed was right down to the "speed limit" of 27/10. The Compression problem disappeared, in fact I forgot about it. It was a happy little engine. No sign of any distress.

However I was a bit worried about the crank strength, and people pointing out that it was in fact a low powered sports engine!.

fora001.jpg (77.59 KiB) Viewed 866 times

It hasn't been run since the last trial and I've just pulled it apart.

Yes the comp adjustment is right at the limit, however the contra-piston is dead level with the head surface.

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fora_head002.jpg (28.58 KiB) Viewed 866 times

CP does not intrude into combustion chamber

fora_head003.jpg (17.07 KiB) Viewed 866 times

[locktite401]

The Fora rod didn't particularly worry me because it's just as spindly as that of the Norvel range of glow engines. It must be an old European Eastern Bloc thing!

The Norvel 15 glow is similar in many ways to the Fora Junior. Coated Aluminum integrated finned cylinder, and the rods are both "spindly". The Norvel 15's are real screamers. Over 20000 rpm on an APC 7x4 on 30% nitro. The rods don't seem to fail despite their thickness.

Both Norvel and Fora Junior rods are 4.5 mm crank pin end and 4 mm the other. They even "eyeball" to about the same length.

I've been experimenting with a Norvel 15 BB conversion to diesel using a modified Parra 15D head button. It works fine but the rod is a bit of a worry. In fact I've bent one in an stupid accident that was all my own fault.

Norvel 15 BB diesel conversion.

norvel_diesel001.jpg (72.59 KiB) Viewed 864 times

Norvel diesel with bent rod.

norvelwith bent rod.jpg (39.67 KiB) Viewed 864 times

Now possibly some good news. An Enya 15 SS glow conrod available for less than $10 from Enya Direct is an almost straight replacement for a Norvel 15 rod. The rod needs a bit of thinning with a small file across the flats, so it will fit into the piston. Otherwise it's a perfect fit.

Chances are that it will also fit the Fora. The holes are the same size and the length looks right. Even if it's not quite the right length, it may still be usable.

Measurements to follow.

Left, Norvel piston with Enya rod. Centre bent stock Norvel rod, right Fora piston and rod. Big end 4.5 mm, little end 4 mm.

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[flyingkiw1]

Many thanks locktite401, that is all really good information, especially the confirmation that the Enya 15 SS rod might fit, I do have one in my box of spares so I will give it a careful measure up. The other info is great too, our contra-piston was in exactly the same position, full down and a very very sad very under-compressed misfire. Andrew's engine rod is now shorter by 0.2mm and strangely bent with a bit of a twist on, and rubbing on the backplate too.

We will fly the repaired skimmed head engine tomorrow in the Classic FAI so we will be able to see, and report how it goes.

Many thanks again

-Lance

[doc]

I bought a couple of Foras about 18 months ago, at the time when the Brits were reporting that they were equal to the best Oliver Tigers and were to be the only engines allowed in their Classic 15 teamracing. I wanted to try them in my first Classic FAI racer, so you might like to add my observations to the ones you already have.

Firstly, there are evidently some important variations between one Fora and another, which makes them an unsuitable choice for a racing class in which no modifications or corrections are allowed (as the Brits specified for their Classic 15, and as some people would like for Australian Classic FAI). Of my two engines, one leaked excessive amounts of fuel out through the front bearing when running, whilst the second engine seems OK in this respect. The leaky engine has now been shifted to a Rookie Trainer where it has performed admirably, with no breakages despite numerous crashes. Both my engines had insufficient compression adjustment to run without misfiring on a 7X6 prop and fuel containing 15% oil, 20% ether, 2% DII, so I skimmed a little off the head to allow it to sit a little further into the cylinder. Problem solved.

Fora mounted in fibreglass pan with moulded in aluminium engine mounts.

If the Fora's skinny conrod is going to bend, I reckon the time it will do so is when the engine is accidentally flooded whilst starting. With a Fora mounted inverted, flooding is quite likely to occur I fear, because of the shape moulded into the exhaust port, angled to guide any excess exhaust prime into the cylinder rather than away from it. Has anybody come to grips with this problem yet?

[flyingkiw1]

Thanks David for that information. We have found like you that there is quire serious variation between engine regardless of were they originated, either the UK or directly from Fora. One of Andrews engine has a really poor fit with the crankshaft and the other one really good. My Fora's have small variation in the head clearance, one from the UK and one from Fora directly, whilst Andrews has one with huge clearance and we will have to skim the liner now, having done the cylinder head, just to get down to 5-6 thou clearance. One engine has bent a rod during a very very gentle run in session. No seizing or flooding but the rod is bent.

Our testing today confirmed that we have to skim the cylinder, the performance has improved but we are still not there yet, and the engine will not go on song as the contra-pistion is still full down. Increasing the IPN and the Kero content helped with the lack of compression but did not get anywhere near solving the problem. So more surgery.

A possible replacement is the Nelson 15D rod, but it will need the big end reamed out and a replacement bronze bearing inserted to bring it up to 4.5mm. That's a very robust rod and not many of them break or even bend in use. A CS Oliver rod seems to fit also and it is larger in the body diameter than the Fora original, but it is also a plain bearing with no bronze big end insert. The CS rod does not have as good a history as the Nelson rod.

Keep the information rolling in.

[locktite401]

Apparently an OS 15 RX rod will fit the Norvel 15. Do you have a distance between centres for the Fora?

Part number O.S. 21805000; Cost 2 for $39.60, 4 for $58.88 (?)

From Tower hobbies:

This is the replacement connecting rod for the O.S. .15RX buggy engine.


FEATURES: Made of aluminum
Has brass bushings installed


INCLUDES: One connecting rod


SPECS: overall length - 35mm
length between centers of holes - 27mm



Also, if a CS Tiger rod is close then so should a genuine rectangular c/s R250 rod. Steve will sell them to Victorians.

[flyingkiw1]

Yes, I measured up the centre to centre measurement for the Fora Con rod holes and they are 27mm, over-all length 34mm.

The CS C/C was the same but the big end is actually 5mm not 4.5, my mistake, still OK for a bronze bush though.

Still, I'll take a look at the OS15 RX rod. Could be the bees knees.

Many thanks

Lance

[doc]

Yes the comp adjustment is right at the limit, however the contra-piston is dead level with the head surface...

CP does not intrude into combustion chamber

Your photos seem to show that the outer edge of CP are level with the outer edge of the head... but because of the conical shape of the squish band the inner edge of the head is not level with the outer edge of CP, so the outer edge of the CP actually is protruding into the combustion space.

Enlargement of locktite401's photo attempting to show the step between CP and head

foracp.jpg (46 KiB) Viewed 814 times

Is this important? According to theory, gases will be forced to flow up over the step to join the remaining gases in the concave region of the CP. My initial thoughts were that it would be best to have no step (easily achieved by skimming more off the head so the squish band sits closer to the piston, which will result in the CP sitting further inside the head at running settings). Does anyone know of any research done into the best shape for this kind of design for 2-piece head/contra piston? With his knowledge of head shapes for larger glowplug engines, Lance may have some suggestions.

[locktite401]

Yes the comp adjustment is right at the limit, however the contra-piston is dead level with the head surface...

CP does not intrude into combustion chamber

Your photos seem to show that the outer edge of CP are level with the outer edge of the head... but because of the conical shape of the squish band the inner edge of the head is not level with the outer edge of CP, so the outer edge of the CP actually is protruding into the combustion space.

foracp.jpg

Is this important? According to theory, gases will be forced to flow up over the step to join the remaining gases in the concave region of the CP. My initial thoughts were that it would be best to have no step (easily achieved by skimming more off the head so the squish band sits closer to the piston, which will result in the CP sitting further inside the head at running settings). Does anyone know of any research done into the best shape for this kind of design for 2-piece head/contra piston? With his knowledge of head shapes for larger glowplug engines, Lance may have some suggestions.

Yes, it is protruding, but only about half the diameter of a mosquito's piss!

While it looks quite dramatic in the blow up, in fact you only just notice it when you run a finger nail over it.


Ideal CP position on a push/pull head (Overheard around the outside table at StIves)

My understanding is that you establish your best setting in the air with the push/pull headed engine. Take the head off and if the CP is clear of the head (like this example is) you shim it back, and keep retrying and shimming until it runs best with the CP flush, otherwise if it's up in the head you remove shims until you get the optimum run with the CP flush.

So the "target" combustion chamber shape is smooth.

My Fora isn't too bad as regards compression settings. It was right on song at 27/10 in a full DGY at that setting, about as good as an R250 (with standard venturi) on the same APC 7x6. This one was from the first batch. It does leak fuel out the front bearing however and may not last too much longer at that speed.