RCV 120sp investigation

[Jon H]

RCV engines with the reduction drive have always interested me and while i have always been keen to grab one and have a play, i never did. Why didnt i just buy one? At the time i had a chance to other things seemed to have greater priority and since then..well they have gone. 2nd hand engines always seemed to end up at a much higher price than i thought was justified for an unknown ebay job, so my itch went unscratched. 

 

A little while ago i made a passing comment on a thread about my desire to play with an RCV 120sp as it was an engine i had not had any real experience with and i was curious to try out a whole bunch of stuff with it. Following that passing comment i received a very generous offer from Mike here on the forum and now, i have an RCV 120 sat on the bench. 

 

The full back story from Mike is that the engine was a replacement for one that gave trouble. This one has only half hour running on it but after repeated noise test failures it went in the drawer and i think has sat there since about 2005 if the paperwork is anything to go by. Currently the engine is locked solid with castor so i suspect it will need new bearings all around and general service before i can run it, but that should not be a big issue.  

 

My main curiosity is props. Looking at the RCV blurb they recommend a prop RPM of between 5000 and 5600 at full power on props of 12 and 14 inches pitch. Thats over 11k on the crank, and to my mind much too fast for an engine of this size. I would normally run a 120 at 8500 ish on a prop with 8 inch pitch so 12 and 14 (equivalent to 6 and 7 inch) are bit thin on pitch in my view and i am keen to test out props with 16 inches pitch and prop/crank RPM of around 4250/8500. 

 

I am just interested in how the thing behaves and if the issues many had with noise/heat were related to this high recommended RPM. Its possible i have the wrong end of the stick, but im always keen to test things and see what the data tells me. If anyone else has experience with these engines by all means comment, i am curious to find out how it was received by others and what their experiences were. 

 

Before all that, i need to flush out some castor! 

 

 

[Steve_Wright]

Always wanted to put one of these in my Wot 4 XL, I like intereting enginering solutions, I have fallen into the dark side of electrickery now so doubt I will ever run one.

[J D 8]

Hi Jon, I have run an SP 60 in my Seagull SNJ [ navy AT6 ] for a good few years.

I had one of the early one's and when it gave up [ broken gear teeth ] I bought the later version with helical cut teeth. 

  The prop I use is a Master 15 x 10, on which it has been happy. Could go more diameter but ground clearance is only an inch with tail up so care has to be taken on take off. 

   I just like it because it is different, the starting from behind with a helli like hex driver has people looking at the strip [ starter has to run in reverse for engine to turn the right way ] starting can be the normal way if you want.

  The sound as it goes by does remind me of those large American radials.?

  Cooling has to be right with baffle sending the air across the engine through the fins.   Cheers, John.

[Denis Watkins]

At my place of flying, we often have a group project to split the cost.

Maybe of no help to you, but will comment, although the motor was sold on some years ago.

We picked one of these up second hand from a show, intending to fly it in an 8 foot span, lightly built Maule.

We just never got into the air.

The motor started and ran, but was so loud, and sounded like the proverbial " bag of spanners "

The mechanical sympathy built into us lads, just prevented us from running the motor, fearing a blast of some sort at some stage.

I have heard opposite comments of these motors running smoothly.

[J D 8]

 Found the instructions for my RCV SP60, It is generic for 60, 90 and 120. Covers the usual running instructions and extras like the cooling baffle, heavy duty mounting method, dismantling and re assembly, timing the engine and recommended prop size's.

        Prop                          Idle                                  Max RPM 

60  16 X 12               1200-1500                           5000-5300

90  18 X 12               1200-1500                           5100-5500

120 18 X 12               1200-1500                          5300-5600

 

Given the last set of figures covers an engine twice the size of my SP60 I find them rather odd. Bigger engines running at higher RPM's on same or similar size props seems wrong.     

I think it was a shame that RCV brought out the unusual innovative  SP series first, and had the more conventional CD models been marketed first they would have had more success. 

 

P.S. sort you a copy if you want.  John.

Edited May 24, 2021 by J D 8
more info.

[Jon H]

Thanks for the comments. They seem to echo what i have heard over the years in that some have an awful experience, and others a good one. Can the engines vary that much? 

 

JD8, i agree that something looks odd about the props and rpm's. Take the idle for example, 1500rpm is something i can achieve with a conventional engine and yet in this case it means the engine is idling at 3000rpm. Seems fast to me and i would not expect more than 1000rpm at the prop on a geared engine like this. 

 

Your 60 is also interesting as you are only running 5 inches of pitch in normal terms. Have you tried 12 inch pitch over the years? Using a laser 62 as a representative 60fs, i saw 10000rpm on 12x6 when i tested one a few weeks back. In RCV terms that is a 12 inch pitch at 5000rpm on the prop. Using a thrust calculator i get a hp match using 17x12 at 4950rpm. No idea if the power will scale like that, but its what i am using to guide my investigation. 

[J D 8]

 I purchased the SNJ from a fellow club member who found it a handful, tuned down the control [ elevator ] movement and fly it for what it is, a warbird trainer [ not for yank, bank or slow low ] With its original SP 60 it had a 3 blade prop but I remember no details.  The engine gave up when the 4 bolts holding the main body to the lower crankcase came loose in the air resulting in chewed gears and then broken prop on forced landing.

   I had a recommended 16 x 12 on the newer engine and but ground clearance was a problem. The 15 x 10 went on because that is what I had and is happy with it.  I have not measured the RPM at either end but recon I have quite a low tick over as it is needed to lower residual thrust for landing.

  Recon there must be some power loss due to the gear system [ as with any gear system ] but offset by larger slower prop gains. Have seen your post on gear system development like the Webra's had.

  Interested to see how you get on with the 120

[Jon H]

After quite a fight the engine is apart. Its an interesting engine to dismantle and some of the design choices left me scratching my head, and yet some were very well thought out. It also differs from my exploded parts view in a few areas. 

 

 

As you can see here, the piston ring was pretty well welded into its slot by castor. My solution to this is always the same. Soak in fuel for a little while, and then try and get a feeler gauge into the ring gap. Often this will give enough leverage to move a small section of the ring. Once moving, keep gently wiggling it while frequently dunking it in fresh fuel. This will slowly (very slowly) soften the castor and after about 20 minutes of wiggling and dunking the ring was free. I then removed it, cleaned the groove and gave the ring a rub on a surface plate to clean its faces. Refitted with some oil, and that was that. 

 

 

The rest of the bits got a clean up as well and i now just need to order the bearings

 

[J D 8]

 Prototypes.

[J D 8]

       Not great pics but these are the prototype RCV engines developed by Keith Lawes.

        Top is number 4 the engine this was built to a better standard by John Oliver and Ray Ivey.

         Second is 1st prototype made by Keith  as were no's 2 and 3.

         Third is the 2nd prototype and first to have prop on the cylinder.

          Forth is 3rd prototype testing a set up more like the production engine.

[J D 8]

  Correction, this is the second prototype.

[Jon H]

Those are cool. What era are they? early 90's? 

[J D 8]

   They were running in 96.

[Jon H]

Bit of a delay getting this on the move as i have had some other issues to deal with. 

 

But, new bearings are on the way and in the process of ordering them i was met by a few strange design choices. Of the 4 bearings in the engine 3 are metric but one is imperial. I dont quite know why they have done this as there are plenty of metric bearings that would fit this location. 

 

The other strange choice is that the front bearing for the crank (the one with the blue oil seal) is tiny. As it is this bearing that takes all the propeller load i do not understand why they have used a bearing that is so small. The black sealed bearing is for the crankshaft and is more in line with what i would expect to see on the front bearing on an engine this size. 

 

Anyway, it is what it is and new ones are on the way ? 

[J D 8]

Yes there are some odd design choices, I have said before that the SP series could have done with more development time and CD should have come on the market first. 

Only bearing I have replaced on my well run SP60 is the large cylinder bearing and only because it felt a little bit rough.

[PeterF]

You said that the engine was gummed up with castor, i ran a pair of SP60 engines for a while and i recall that castor was not recommended, my memory is no longer perfect so you should check for yourself. I ran this pair of engines in a Brian Taylor 81" Mosquito , they were noisy and rattly but really hauled it around on 16x12 props at about 5,500rpm and gave good flight times. I had to add cooling scoops to take air into the inlet opening on the cowl and divert it through 90deg over the fins and then out of slots where the exhaust stubs were. Never had any overheating problems. I also had the TwinSync system with glow driver and this kept the engines in sync and sounded good. However, the problem was the vibration and i had an aileron servo malfunction that i put down to this, i was struggling to get the plane back with random servo inputs on the ailerons and landed short in the rough. I have since rebuilt the wing and gone electric.

[PeterF]

Here is my Mosquito, it weighed around 17 to 18lb so was hauled along with authority by a couple of 4 stroke 60s. This shows how different they sound.

 

 

[Jon H]

I think when RCV were making the engine they initially recommended castor. I have the 2004 dated instructions here and the section on fuels is shown below. Once RCV were passed to weston uk weston naturally recommended their own fuel for the job and said no castor. Its a bit of a contradiction. 

 

In fairness, i wouldnt like to use castor in any engine either, and the recommended duraglo fuel is just awful. Also we used to recommend castor for Laser engines but time moves on. 

 

i will be using the laser 5 fuel i use in everything else once i get the engine back together. 

 

 

[Andy Stephenson]

When Neil was making Laser engines in the 80s, to start with, he recommended only castor fuel, no nitro and 2 stroke plugs, how things have changed.

[Jon H]

Yep, time and tides 

 

To my joy a little packet of bearings arrived in the post so I set about getting the engine back together. I was going to document it, but I ended up needing to pay attention! I also found something interesting with the rod as the video will show. 

 

I was able to reassemble the engine and I did put it on the bench briefly. Again I was going to video it but I had a short window of time and I suspected it was going to a faff. I was partly right, but I got it going and it's not that bad. The ring is finished though and compression is very poor. I'm Sure it's the ring as there was a high pressure jet of red hot oil and smoke coming from the breather and that suggests excessive ring blow by. 

 

I will give it another run to see if it beds in, if not it's new ring time. Either way, I will get a video of the action this time 

Edited June 11, 2021 by Jon - Laser Engines

[Frank Day]

I have a 90sp and a 120sp, the prop sizes on the chart are odd. Use regular diameter items with double the pitch.  Though some use larger diameter and reduce the pitch. 120 break in prop is 20in. Try different ones until you get what you need from it. 

Never put castor oil in them it is truly the devil's work. Use prosynth from Westons. Starter- use ball end hex starter as it buys you more knuckle room behind the prop. Duct air across the engine unless you have a wide open cowl. Faired in P51/Spitfire good ducting is a must. Westons have a diagram. 

[Jon H]

Yea it was the pitch thing that started me on this adventure. They state clearly in the blurb that you need double the pitch, and yet only recommend 12'' which is not double the 8 inch pitch i would normally expect on a 120. 

 

I have been faffing about in the background but so far no video as its mostly me fiddling and grumbling. The original ring seems past it so i knocked up a replacement. Its not as perfect as i would have liked to be honest but it was quicker than ordering one. I ran the engine again and its better, but inspection of my knocked up ring (modified ring from a dead laser 150) showed that my original candidate ring was in the scrap box for a reason! So, i will do a proper one this time and try again. 

 

That said, i was testing it on a 21x10 and it ran well at 4400 but that is about .4hp down on what i would expect from a 120 at 8800 crank speed. 

 

I will order up some more props once i am happy it is working the way it should. 

[Jon H]

well it been a while. I didnt realise my being too cheap to buy a propeller lasted so long. 

 

Anyway i did finally order the 20x16 apc prop that i thought might be the holy grail ans it has been very interesting

 

To recap, a normal 120 runs around 8500rpm on a 15x8 and this engine with a 2:1 reduction should run about 4250 on the prop to give the same crank speed and 16 inches of pitch as we have halved rpm. This is contrary to the RCV instructions which call for equivalent pitch's of 6 or 7 inches and up to 11000 rpm on the crank, which i deem too fast. 

 

The result? 4200 steady with peaks of 4300 and once i saw 4400. With my less than perfect test bench setup i think it will hold 4300 with a better installation. This is exactly in line with my predictions and above half throttle the engine ran quite well. It was fairly smooth and sounded very content. Below half throttle is another story as the carb seems unsuited to the engine with  rich band mid throttle and a very lean idle. This could be due to my sub optimal test setup as the tank is a smidge high due to my bench currently being set up for lasers. I would change it but the engine shook so badly its completely destroyed my already tired test bench. I elected to shut it down before it decided to exit stage left at 90mph. 

 

So, i am confident that this prop is a winner and my limited testing so far backs up my thinking that the recommended props are too small. I also have some ideas for improving the idle performance beyond just carb settings. All i need to do now is find an airframe to fit it to for some flight testing. I am curious to see how it performs with a more optimum setup, installation, and cooling. The only problem is finding a model as i do not think i have anything suitable on hand and there is no room at the inn for anything new...he says again having added 3 models to his fleet after saying that last time. 

 

Is buying models an addiction? if so i think i need one of those support groups to help me. 

[Engine Doctor]

At the introduction of the SP range at the Sandown Syposium, I think it was the 60 fitted on a Wot4 with no cowl fitted. It overheated and stopped  after a couple of minutes . . The ducting to get air to  flow through the fins is essential. The early ones also sounded like a bag of spanners with a whine! 

Not a bad idea but like all the alternative inc Webra,HB, Falcon etc four stroke engines they just were not as good as the poppet valve design.

There was talk of RCV making a version for small motorcycles , probably  killed off by emission as although technically a fourstroke it still required a two stroke fuel mix.

[Jon H]

the design has its inherent weaknesses for sure and the biggest one is the need for exceptionally accurate machining and i am not sure the mass produced ones ever achieved the required level of accuracy. Still, i am just happy to play with it and see if any improvements can be made