Hi Guys,
I have acquired an old (new condition) OS 77VR. I believe it was a very early Ducted fan engine, but does not have the oversize head. It looks like an OS 61VR, but just a bit bigger. I was curious if there was any experience running this engine with a prop. The carb intake is quite large, but the OS 7D carb is also a perfect fit. The engine does not have any instructions which might clarify propeller use. About 30% more displacement than a .60; so it should really haul something like an EU1-A nicely.
Thanks,
Will

The only specs I have is from a OS 1992 catalogue for the 65VR-DF and 91VR-DF.

65VR-DF was supposedly good for 2.5k-25k with 3hp @22k
91VR-DF " " 2.5k-25k with 4.8hp@22k

Your probably stuck with 9" or 10" diameter props unless you want other club members peeling off the EU1-A stickers and replacing them with XF-84H :-)

I think the engine may be as much as 10 years older than that; so early '80s. I know they (purpose built ducted fan engines) were designed to run at very high rpm, but this is clearly a very early effort by OS. I suspect that using the 7D carb will 'tame' the beast somewhat, but still allow it to produce lots of power on more reasonable prop sizes like a 12x10.

The O.S. Max .77 VR-DF engine was intended to run in the high 'teens' with a tuned pipe and 30%+ nitro. Oil content needs to be in excess of 22% oil or you'll likely destroy the connecting rod. Castor oil is the preferred oil for best lubrication...no synthetic at all is strongly suggested. If the engine's not run at a high-enough RPM, the engine won't develop its designed power. Don't expect much in the way of throttle response if you're not using a ducted fan. The carb's not really intended to go very low, so idle with a prop will be problematic.

Overall, while a fine engine for its purpose, it's not a good choice for Pattern.

Thanks, Bill.
Just looking at the exhaust timing, that's kinda what I thought!

What's wrong flying a EU1-A at 22,000 rpm? Start your roll at one far end of the field and finish it at the other end, all at 22,000 rpms. Sounds like fun to me.

If thatcase is the the same size as a 61 FSR just swap the 77vr's case piston and head for the 61's.The only troucble I can think of with this swap is the 77 Conrod's diameter for the crankpin. So you would have to use the 61FSR conrod.

The biggest difference on the Ducted Fan version's is the crankshaft rotor timing which is significantly advanced as compared to a non ducted fan.

Here's a chart I put together on various engine timings on the 61FSR, 61RF, and the 61RF Hanno. They are not real accurate but you could at least use them as a reference to comparethe 77 sleeve portings to see how much difference.

pic of my degree wheel setup</p>

actually using the smaller carb with a macs quiet pipe set long would actually work well, the quiet pipe has enough back pressure to use the longer exhaust duration on the vr77... a 11x10 or 11x11 or a 12x9 would give some serious performance. no its not the normal pattern setup, but the smaller carb will throttle well with the above props as long as you use a pipe with more back pressure.

a examination of the helicopter engines used today will show that they generate huge torque with 10-11 MM.5 carbs and have good throttle response.

I'm missing something. A ducted fan engine doesn't idle below 3k rpm which means you would have a really fast taxi run.

Or, one could put a break on it...

David

Of course we are kidding but you could do a takeoff like a pylon racer as well.

I think the .77 was an adapted or shared marine engine. It was a rear disc rotor induction unlike the the .65VRDF and .91VRDFs which were drum rotor. I know you could use a .77Marine case with the .77DF internals as the only difference was the bosses around the exhaust port were drilled and tapped for water cooling. They did have weak rods. An RPM rod was the rod to have when the .77s were popular. The VRDF series would idle much lower when setup properly. They also didnt need 30% nitro to make the power which further leads me to believe the .,77s started life as marine engines. Those boys LOVE their nitro.

Hey Will,

Check out this postatextreme speed prop planes thread http://www.rcuniverse.com/forum/fb.asp?m=832046

r-c guy posted "I used to get 25000-26000 in the air with 20% fuel with an 11-8 narrow" with an OS77 VRDF. That would be a scary beast.

I wonder if the 11-8 narrow he's refering to is the Top flight wood?

Bryan</p>

Hey Roary,
I agree that stock DF engines don't idle very slowly (and don't need to) which is why I planned to use a smaller (7D) carb than the stock carb which has a huge throat diameter and probably can't draw fuel at less than 3000rpm. As for the weak conrod - I don't plan to run the engine at 18,000 rpm either [X(]. When I get around to experimenting with it I will post results (good or bad) here.
Interesting to hear other views on the subject.
-Will

the rod will be fine, boats are brutal on rods as the boat often lets the engine free run then buries the prop in the water many times per run, and yes you wont be running it anywhere near the rpm range of a ducted fan, some guys use a throttle curve to make the carb feel more linear

I flywilly
I will very much like to see how your experiment will end, because I have a rossi 105 DF with any use for almost 13 years
and I'm curious to see if instead of "silent stone" in the workshop it will be something screaming.

Best regards
Avi

So running a smaller venturi carb on a DF will allow it to come closer to a taxiable idle? I didn't know this. Would you also need to run a certain pitch/diameter prop to accomodate the flywheel action in order to maintain a slower idle as well? I'm asking since I have a few DF engines and might try some of these tricks, will stay tuned.

yes the diameter makes a difference in flywheel action.

Hi Roary,
Prop mass has a lot to do with flywheel effect. Typically, more diameter means more mass, however, increasing pitch for a given diameter will also increase the mass of the prop although more at the center than the tips. I would love to see the performance curves (torque especially) for this engine. Does anybody remember seeing it reviewed in any of the magazines?
-Will

Anything to report on the OS 77 DF?

Jim

Also have a os 77 sm head duct fan, was for dynamax fan setup, Yellow f-16, Bought it in 95, setting in a box, ran it on bench with intake and ex ducts, what a screaming little motor.