|Type:||Single Cylinder, Rotary Valve, Four Stroke Glow|
|Displacement:||0.91 ci. (15cc)|
|Total Weight:||26.96oz with muffler|
|Engine (Only) Weight:||25.2 oz.|
|Crankshaft Threads:||5/16 UNF|
|Prop Range:||13 x 6 through 14 x 8|
|RPM Range:||2,200 - 12,000|
|Fuel:||Glow fuel containing: 10% Nitro, 15% Oil (including between 3 and 6% Castor)|
|Muffler Type:||Spun Aluminum|
|Manufacturer:||RCV Engines Ltd|
|Available From:||RCV Engines Ltd|
RCV Engines .91CD is a completely different and amazing approach to RC engines. Based on their successful .58CD, it incorporates the same rotating sleeve design in a larger size engine. This makes for a very compact and powerful four stroke engine with virtually no maintenance.
RCV Engines Ltd (Rotating Cylinder Valve) has been producing four stroke engines in the United Kingdom since 1997. They are based on the rotating sleeve design that utilizes the rotating cylinder itself to control the intake and exhaust porting. This design eliminates the need for cam, lifters, pushrods, rocker arms, valves, valve springs, and spring keepers. Basically doing away with the entire valve train and its required maintenance.
RCV has five different engines in their lineup. The CD series with the .58CD and .91CD engine in their conventional design, and the SP series .60SP, .90SP and 1.20SP inline engines with geared output.
I had to do a little more research on how this fascinating design functioned. It is almost hard to believe this is a four stroke engine since it lacks the usual cam, push rods and other components I was used to seeing.
Pages could be written on the .91 CDs piston being machined from an aluminum casting, the crankshaft being machined from bar stock steel, close internal tolerances, etc. But when I'm looking for an engine, I'm always more interested in how an engine performs, it's durability and its looks rather than what it's made of. So I'll leave that to the engineers and the folks that just have to know.
|Crankshaft thread size:||5/16 UNF|
|Distance Between Mounting Holes Same Side:||1.69" (43mm)|
|Distance Between Mounting Holes Opposite Side:||2.16" (55mm)|
|Distance Carb Rear to Drive Hub:||4.71" (119.7mm)|
|Distance Rear Engine to Drive Hub:||3.76" (95.7mm)|
|Height Bottom of Crankcase to Top:||4.05" (103mm)|
|Engine Total Width Including Mounts:||2.55" (65mm)|
|Engine Width Between Bearers:||1.77Ē (45mm)|
|Engine||25.2 oz||26.5 oz|
|Muffler with header||1.76 oz||1.55 oz|
|Totals:||26.96 oz||28.05 oz|
Note from RCV: The RCV91CD has recently undergone some changes including a larger carb and intake manifold. This is the reason for the slight weight increase. Our sales literature hasn't caught up yet.
|Engine:||OS .91FS II||RCV 91CD||Saito FA.91|
|Weight less muffler:||22.6 oz (640 grams)||25.2 oz. (715.grams)||18.34 oz. (520 grams)|
|Height/Bottom of Mounting Tabs/Top of Cylinder Head:||3.97" (101mm)||3.20" (81.4mm)||3.81" (97mm)|
|Length from Backplate to Drive Hub:||3.60" (91.5mm)||3.76"(95.7mm)||3.66"(93mm)|
|Width Between Mounting Tabs:||2.36" (60mm)||2.56" (65mm)||2.36" (60mm)|
|Prop:||11X11-16X6||13 x 6 through 14 x 8||12x8 - 15x4|
|RPM:||2,000 - 12,000||2,200-12,000||2,000 - 11,000|
|Horse Power:||1.6 HP||1.5 HP||1.6 HP|
It was 78 degrees with 80% humidity when I arrived at my engine test area. I mounted the 91-CD to the test stand and installed an APC 13X6 prop for break-in. I filled the tank with 15% Byron fuel and spun the engine a couple times by hand to prime. I noticed it was very soft on compression. I hooked up the glow driver and within a couple revolutions of my trusty Sullivan starter it was purring away.
As per the manual I let the engine warm up a bit at idle then increased the throttle setting to about 1/4 and tuned the high-speed needle valve for slightly rich running around 6,000 rpm. I ran it this way for 15 minutes and shut it down to let it cool for 15 minutes. I repeated this four times and the compression was very good. It never requiring more than a slight bump of the starter to restart.
I set the high-speed needle a few clicks rich, then adjusted the low speed needle out about 1/4 turn rich and transition was excellent.
RCV states around one hour break-in time. The engine was getting stronger with each run and by the fourth run it was idling nicely with good, solid top end. It was then ready to mount and fly remembering to keep the engine running a bit rich for the first few flights.
The engine had a different sound than any four stroke engine I've run. It was very smooth with quick throttle response.
|PROP||WOT RPM||IDLE RPM|
|I'm sure RPM readings will increase as the engine continues to break-in.|
|Readings taken at app. 6 feet with an APC 13X8 prop installed.|
I decided to mount the engine in my trusty Four-Star 60. Since I had used this plane as a test bed for quite a few engines, I could compare their performance with the .91CD. Due to the slightly wider engine and longer mounting tabs of the .91CD, I swapped the engine mount for a 1.20 size adjustable mount. Then I moved the battery pack from behind the firewall to behind the servos to balance. I installed the 13.5x8 Bolley prop from the Saito .91 I had been flying with, and was ready to charge the batteries and head off to the field.
When I arrived at the field it was around 72 degrees and sunny with just a touch of wind. I unloaded and assembled the Four-Star and was ready to fuel and fly. After fueling, I spun the RCV with the starter a touch to prime the engine and hooked up the glow driver. A touch of the starter and it was sitting there purring. After a brief warm up, I ran the throttle up to wide open and set the high end three clicks rich from maximum rpm. Transition from idle to wide open was excellent. I let the engine sit and idle as I went through my radio check. I then snapped it wide open. There was no stumble or hesitation even after a prolonged idle. Great!
I taxied out to the middle of the runway and advanced the throttle. The Four-Starís tail came up almost immediately and was airborne in about 20 feet at a little over half throttle! So I just nailed it and went straight up to about 75 feet and turned into the pattern.
I flew a couple laps around the field at half throttle to trim the plane and suddenly she quit. I dead sticked it in and restarted the engine. Everything seemed fine as I ran it up, so I richened the mixture a bit more and it performed flawlessly the rest of the day. The engine seemed to be getting stronger with each flight. I'm sure it just needs a bit more break-in time before being peaked.
I flew the plane through every attitude I could think of that might cause an engine to complain: Inverted, loops, spins, fast and slow roll, snaps, power on dives, etc. It just kept purring along with no stumbling or sign it knew it wasn't sitting straight up.
Half to 3/4 throttle was all that was required for knife edge or any other maneuver I threw at it. Loops from level flight required no throttle change, just add elevator. Full throttle would pull the plane straight up till it was a speck. The torque was amazing.
I did my usual: Climb straight up till it was a speck, then idle all the way back down and snap the throttle wide open test. It passed with flying colors.
Fuel consumption on the .91 is excellent. After the second flight that day I realized I had yet to refill the 12 oz. tank! (Your mileage may vary.)
The torque was amazing and the engine did everything I asked of it. There's just nothing more you could expect from any engine.
Ok, I'm a total engine addict\nut. Engines to me are as much a part of the enjoyment of the hobby as building and flying. The RCV line of engines is amazing. Not only from a design viewpoint but also from the point of functionality. It was very easy to set up and adjust, performing flawlessly thru everything I threw at it.
Comparing it to the other normally aspirated .91 four strokes I've flown, the RCV .91s power is right up there: lots of torque and very smooth.
One of the remarks I had from a fellow at the field was that the engine sounded as if it had a bit "snappier response" than most four strokes.
The facts that the engine is pre-run and set up at the factory, comes with a two year warranty and lifetime repair guarantee, speaks volumes about the quality of the engine and the RCV company itself.
I had planned to put the original engine I had in the Four Star 60 back into it. But the RCV .91 has found a permanent home, at least till I find the perfect scale project for it. It's a keeper!
RCV is bringing out a 130CD early next year! (Feb 2007)
The Lifetime Repair Guarantee: All new RCV Engines purchased after 1st August 2005 are now covered by a lifetime repair guarantee which means that whatever happens to your engine, whilst it is still under your ownership, (even if you crash the engine in 10 years' time) you will never have to pay more than 50% of a new engine cost for repair or replacement. All you have to do to qualify for this offer is to send your completed warranty registration form back to RCV within 1 month of purchase from anauthorized RCV dealer!!Last edited by RichN; Oct 02, 2006 at 06:38 AM..
|Oct 03, 2006, 08:27 PM|
Dallas, Texas area
Joined Sep 2003
I have one OS 91 and several Magnum 91 four strokes they all turn a 14x6 APC @ 10k sustained, this engine turns a 14x6 APC @ 8950.
That is a huge difference in power. I think it is very misleading to state
"Comparing it to the other normally aspirated .91 four strokes I've flown, the RCV .91s power is right up there: ..."
That is just my opinion.
|Oct 03, 2006, 09:23 PM|
USA, FL, Eustis
Joined Jun 2004
Thanks for the great post and look into the compact RCV engines. I have been keeping my eye on these engines for quite some time now and will probably end up purchasing one. The engine appeared to run flawlessly on the test stand and I have no doubt that RPMs will continue to increase as the engine breaks in. Although the rotating sleeve design may produce more parasitic internal drag than some engines, this engine will have less airborne drag due to its more compact profile. I like it!
|Oct 03, 2006, 10:11 PM|
Joined Mar 2004
I had flown the same plane with the OS 91 and noticed no difference in performance between it and the RCV. I'm sure with a bit more time on the RCV, its top RPM will increase. I've found peak RPM is not always the true indicator of an engines power and performance.
|Oct 07, 2006, 09:00 PM|
Joined Oct 2006
"I've found peak RPM is not always the true indicator of an engines power and performance."
What do you find is an indicator of engine power and performance then?
...Just trying to learn...
|Oct 08, 2006, 01:36 AM|
All of this conjecture may very well be explained by differing performance in the air. Don't forget before we start slamming people that engines behave differently in the air than they do on the ground. Peak RPM on the ground does not totally indicate what an egine will do when it "unloads" in the air. As different as this engine is it may "unload" more in the air than other brands which would explain the author seeing very little difference. If you need anymore proof ask yourself why we set our engines rich on the ground.
BTW nice review I have often wondered about these engines thanks for going through the trouble to write up your experiences.
|Oct 08, 2006, 04:31 AM|
the engine should be kept on 11000-11500 RPM s max on ground to obtain best result due to port timing,so propeller should be chosen accordingly.The torque is the first factor here because of the port timing issue.If you overrevv then overheat problems occur.
If you want more info pls. take a look at the threead:
Especially Post #:49,106,113
|Oct 08, 2006, 07:04 AM|
Joined Oct 2006
I wasn't "slamming" anyone so please don't turn the blow torch my way.
My question is if you are seeing 1,000+ rpm difference on the ground, all things being equal such as fuel, prop, weather, etc., isn't that a big difference?
As far as unloading in the air, unless somehow mechanically limited such as valve float, etc. wouldn't the two engines still show a significant difference in flight performance?
As far as "torque" and "horsepower", when grounding running the engine with flying prop isn't that a "loaded" condition and the engine characteristics in flight, except for extreme orientations, such prop hanging, etc. be pretty much the same?
I read the linked thread. Interesting about the tappered bore not being able to tolerate too rich running and needing heat to expand. I wonder if the engine would be safe to run in cold climates like here in the northeast in winter?
Like I said...
Just trying to learn.
|Oct 08, 2006, 10:24 AM|
Joined Aug 2004
I just had to try one of these designs so picked up the .58 RCV about a year ago. At first I was dissappointed with the high idle speed necessary to keep it running. However, now, after a gallon of fuel, it is outstanding in performance, low idle, low fuel consumption, lots of power, instantainious throttle respons, great fuel draw etc. It is now my favorite 4 stroke and will run well over 12 minutes on an 8 oz. tank at full throttle. I actually have it mounted with the throttle assembly at least an inch above the top of the fuel tank with no fuel draw problems and no problems with inverted flight with the throttle assembly now an inch below the tank, no change in RPM or power nor idle--fantastic.
|Oct 08, 2006, 10:35 AM|
Joined Mar 2004
I see you just joined, so first of all let me welcome you to RCGroups! There are a lot of great people here that are always more than happy to help anyway they can.
The RCV I tested is a ringed engine. And a little different sort of ringed engine to boot. At the time of the RPM readings it was approximately half way through the recommended break-in. I'm sure RPM will increase as it continues to break-in. As I flew the plane the engine did seem to be getting stronger with each flight. Iíve had some engines take close to over two gallons of fuel before they were performing at their peak. Iím sure it will attain the mystery RPM difference as it finishes breaking in..
All engines are designed to have a torque curve. This can be altered by port\valve timing, carburetion, exhaust back pressure, air intake, etc. Actually manufacturers could probably provide much more rpm than the engine produces, but at a sacrifice to the ďusableĒ power band of the engine. (i.e. low end verses midrange, verses top end, etc.)
There have been many threads and entire books written on engine performance. And all of them by people a lot sharper than me on the physics and statistics.
Unless you have an onboard data recorder, it would be impossible to determine what is actually happening RPM wise in flight compared to on the ground. Iíve had a good strong wind come up and change an engine a few hundred RPM on the test stand. It would be interesting to see what actually does happen to a glow engine comparing flying upwind to downwind.
ō What do you find is an indicator of engine power and performance then?
I had flown this plane with around five or six different engines, both four stroke and two stroke. So had a good indicator of how the plane performed with each engine. IĎve had engines that turned a third more rpm than the other on the same prop, that didnít fly the plane as well as the slower engine. All related to the torque curve and type of flying being done.
Putting the physics issues aside, and maybe Iím old fashioned. But to me how the engine actually flys the airframe I install it in, is the biggest indicator. Not very scientific, but it has always worked for me.
Once again welcome to RCGroups!
|Oct 08, 2006, 06:33 PM|
I looks like a similar design to the HP VT.21 and VT.49 four strokes. The Hp had just the combustion chamber that rotated, the glow plug and exhaust are on opposite sides of the engine, and also gains better performance after about 2 hrs of breakin time. The only problem The HP four strokes had was their power to weight ratio. They were heavy engines. I hope the RCV's overcame that problem.
|Oct 10, 2006, 03:46 PM|
Dallas, Texas area
Joined Sep 2003
Not meant as a flame
It was not intended to be.
The review of the engine is excellent, well written, and with some real RPM numbers included on several props, a lot of work went into it and this is very much appreciated.
The RPM numbers I posted for the OS and Magnum 91 four strokes are not peak numbers, the engines fly at these RPMs.
Since the fuel I use and the fuel used in the review is both 15% nitro and the prop is APC 14x6 the setups are roughly the same, I am near Dallas, TX so altitude is not helping me.
I think the RCV is a really neat engine and if it fits in a cowl where you would have to cut a hole to make a different 91 fit and the RCV has enough oomph to get the job done then it could be a very good choice.
After the engine gets a couple of gallons of fuel through it I would like to know if the high end improves.
Until then I still think 8950 compared to 10k+ is a lot of difference.
|Oct 10, 2006, 05:39 PM|
One little sort of "warning" for anyone buying an RCV .91CD...
Be sure and remove the little vinyl cover over the crankcase vent before you run it!
I could NOT get my new .91CD to run properly on the break-in stand. After a lot of frustration, buyer's regret, etc., I noticed I had not removed the vent cover (and I think it IS mentioned in the manual).
Once I removed it, everything was fine.
I haven't mounted the engine in a model yet, but I'm hoping to find a home for it over the winter and put it to use next season.
|Oct 10, 2006, 10:12 PM|
USA, FL, Eustis
Joined Jun 2004
In the RCV engine thread over in RCU the representatives from RCV state to expect 9,000-9,200 (minimum) with the 14x6 on it. The engines are run, tached out, and are shipped if the engine meets the minimum RPM of 9,000. Unlike at least one reader in here, I understand the value of torque and how well torque will fly an airplane and how that airplane will respond to an engine with good torque. The engine appears to have instantaneous throttle response and comes up extremely quick to max RPM. This feature will really make a plane respond quickly to the pilot's input and should put a pretty good smile on his or her face!
|Nov 16, 2006, 06:22 AM|
Joined Feb 2006
I thought I would put my twopence worth in here, even though late.
The RCV design means that power keeps increasing with use unlike conventional engines.
The other thing that is highlighted, which is correct is that RCV produce very torquey engines, this is just as important as rpm in how a plane performs.
We get loads of feedback on our engines from existing customers, and truthfully, I have never heard anyone disappointed with the power in comparison to conventional 4-strokes in the same size, even when the engine is hardly run.
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