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  1. #1

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    OS 91 VRDF experts wanted

    Gentlemen,

    Need some advice on my OS 91 VRDF. Here the facts

    Fuel 16 % syntetic oel, no nitro !
    Tuned pipe, length 43cm
    Prop 11x8 Zinger shorten to 10
    RPM 16.600
    Transition from idel to max RPM is excellent

    I have run the engine a few times on the stand, most likely 30 min in total.

    What worries me is

    a) The quite low RPM..was expecting to be closer to 18.000 or 19.000
    b) the fact that at the high speed needel is only half turn open (never had to close a needle that much to peak an engine)
    c) the enormous pressure that the pipe is producing, I was scared my tank will explode


    Could it be that the pipe is to small ? that this will cause the huge pressure and this also explains the strange needel setup ?

    Please share your thoughts with me.

    rgds
    Gordon


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  2. #2

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    RE: OS 91 VRDF experts wanted

    no nitro, less head shim

  3. #3

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    RE: OS 91 VRDF experts wanted

    keep in mind this engine needs atleast 25% nitro to perform as it should, rpms 18 to 25k of course thats a ducted fan
    smooth pass

  4. #4

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    RE: OS 91 VRDF experts wanted

    so which one of you two is the expert lol one says no nitro and the other says way more.....?

    I dont think that is a DF pipe. It might be to small. Yes that RPM is way down. I have seen 55's turn that rpm
    AMA # 126183
    Fly light, fly fast and fly low.

  5. #5

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    RE: OS 91 VRDF experts wanted

    I ay more nitro in my experience with DF engines, and yes it might not be the right pipe either, straight through nitro pipes is usually whats used
    smooth pass

  6. #6
    combatpigg's Avatar
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    RE: OS 91 VRDF experts wanted

    I've never held a DF engine in my hand, especially not .90 sized so I've got no specific knowledge about this engine.
    What I do know is if the exhaust timing is set for high rpm on a tuned pipe, the exhaust timing will not allow the engine to develope as much torque as a regular sport engine of the same size.
    A 11x8 prop might not be feasible, try a 10x8 to see if it makes a big difference with how the engine hits the pipe. You are trying to match the load to the engine to get an explosive jump in rpm.
    I would find out what the optimum high rpm range was for this engine as a ducted fan model and then try to match that rpm with a carbon fiber speed prop.
    To really be prepared for a tuning session with an unknown engine, you need to have the ability to experiment with various compression settings, various nitro percentages, various props that meet some sort of target speed you are trying to acheive and of course the ability to adjust the length of your exhaust system [if the exhaust system isn't already "pre tuned" for a specific application].
    If I'm playing with an unknown engine, I start with the lowest compression, lowest nitro and least amount of load...then gradually work up from there.
    WHO GUNNA FEED MAW KEEEIDS..???

  7. #7

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    RE: OS 91 VRDF experts wanted

    NOTES FROM OUR TECH DEPARTMENT
    The O.S. Ducted Fan Engines are designed to develop high power at high RPM.
    These are used in Jet (ducted fan) Models where the engine drives a turbine-
    like impeller (compressor) that draws a large quantity of air through a shroud
    to create thrust. These engines are NOT covered under the O.S. 2-year warranty.
    Because this is a high-performance engine, reliability is sacrificed.
    The Ducted Fan does NOT come with this engine. One must purchase the fan to fit
    BOTH the engine size and the specific plane (these fan units are typically
    manufactured by the same people who make the plane kit - a company like
    Jet Hangar Hobbies (562) 467-0260 )

    FEATURES: Rear Exhaust .
    Schnuerle Porting
    2-piece Crankcase
    2-Crankshaft Ball Bearings
    Rear-Mounted Carburetor
    Drum Rotor Induction
    ABN piston/cylinder (aluminum piston with a nickle plated brass
    cylinder
    INCLUDES: One Assembled .91 VR-DF Engine w/small head
    One Set of Instructions and Poster
    One Exhaust Adaptor w/screws and O-ring (OSMG2709)
    Bellcrank for use with the remote needle assembly (OSMG7258-comes
    with the engine!)

    SPECS: Construction: Aluminum and Aluminum alloys
    Displacement: 0.91 cu in
    Bore: 27.7mm (1.09")
    Stroke: 24.5mm (.97")
    Max Output: 4.8 PS, 4.73 HP at 22,000 RPM
    Practical RPM Range: 2,500 - 25,000 RPM
    Weight: 662g (23.4oz)
    Length: 133.5mm (5.26") from carb intake to front of drive washer
    Width: 43.5mm (1.71") neglecting the engine mounting flanges
    52mm (2.05") distance between mounting hole centers
    Height: 103.3mm (4.07")
    Crankshaft: 5/16-24

    Needle Settings: High Speed: 2 turns out from fully closed
    Low Speed: 2mm gap between the needle and spray bar
    (looking down neck of carb) at full
    _ throttle.

    REQUIRES: Glow Plug (O.S. #8 or #9 (OSMG2691 is recommended)
    Tuned pipe (we don't carry, contact the manufacturer of the kit).
    Fuel with 20% MINIMUM lubricant content and 20%-50% Nitromethane
    Wildcat Ducted Fan fuel exceeds these requirements

    COMMENTS: This engine is the "small head" version of the .91 VR-DF engines.
    This engine is designed to operate where space is at a minimum.
    This engine works w/ Dynamax or Turbax fan units.
    JBN 9.23.93
    ir/jl
    ADDITIONAL COMMENTS: It is suggested that the cutomer uses a Ramtec fan unit
    as opposed to the Dynamax unit. The Ramtec has a much
    higher pitch which "loads" the engine better.
    updt lmm 4/14/09


    – If you need more information, use
    smooth pass

  8. #8
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    RE: OS 91 VRDF experts wanted

    I see a few things that need a change. First, I would say 20% nitro is minimum. On a glow engine there are 3 components to ignition " Timing " Compression, load and nitro. Running too little nitro will " Retard " the timing. Bumping up the compression would help with the lower nitro as previously stated but adding nitro will have more benifit. While we are on the subject of timing, the pipe is too small ( Looks like a .60 size pipe ) and too short. So you now have an engine that is firing too late and the pipe pulse is hitting too early. This will really knock the RPMs down. Zinger props are not all that great either. I would bump up the nitro first. Some YS 20/20 would work well, run an OS 8 or Enya 3 plug, an APC 10X10, please get rid of that spinner before you kill yourself, it was never intended to be spun that fast. You need a .90 size pipe and start with a 3" header. This should get you into the 21-22K range. To get max power everything has to work together right now you have a few components working in opposite directions.
    Of course it's true, I read it on the Internet.

  9. #9
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    RE: OS 91 VRDF experts wanted

    An APC 10-10 isn't meant to spin that fast either - 19k tops according to APC safety warnings (190,000 divided by diameter). Cutting down a wooden 11" prop is a good idea for bench running so you have lots of hub strength. Not ideal for a flight prop but it will get you in the air. Also note that for setting up the pipe is it normal practice to use your intended flight prop cut to 85% diameter to allow the engine to achieve flight rpm on the bench and thus allow you to set up the pipe to optimum length on the bench.

    It is standard practice in prop speed applications for 15cc engines to run CF props only of no more than 9" diameter, typically in the 9-10 range, or smaller/higher pitch.

    Sources:

    RocketRob (CF 9-10 based on trimmed/reshaped APC 10-10 I believe)

    These two have props designed for AMA D C/L speed which are pretty much what you want:
    Steve Wilk / Eliminator props
    Mike Hazel / ZZ Prop

    and there are European sources at EU $18+ per prop and shipping [:-]

    Sorry I'm late dear, I had to help my uncle Jack off his horse.

    Revver Bro #231

  10. #10

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    RE: OS 91 VRDF experts wanted

    Hardly anyone ran 20% nitro in OS91DFs back in the day. I ran 5% in mine during the summer and 10% in the winter with about 22% oil. Your pipe looks a bit small, but that is just a guess.

  11. #11
    iron eagel's Avatar
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    RE: OS 91 VRDF experts wanted

    This is a OS .91VRDF pipe. It is 18. 5 inches (45.72 cm) long and the major diameter is 2 inches (5.08 cm) with a ruler for scale. This pipe is pretty big compared to the one you have shown so I would go along with the assessment that your is for a .60 size engine which would explain the high pressure you mentioned. This pipe is for use with a os.91vrdf with a dynamax 5.25 df unit and should spin a 10" prop to at least 22k rpm. And I would also add that I don't think a wood prop is a good idea at that high of a rpm as mentioned before.
    You should not run that engine with lower than 22% oil content!
    As far as nitro I know they run good on 10-15 %, as far as what type of tricks can be done to the engine I'm not sure, but it is my understanding there are a lot of mods that they can handle.

    I edited to correct the measurements...
    It is my understanding that these engines used with a remote needle valve setup (in flight variable) can spin up even higher with a prop and a tuned pipe.
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  12. #12

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    RE: OS 91 VRDF experts wanted

    The pipe length looks about right, but is it the right volume? Loose the deflector, at least for now. And loose the spinner, you can let the engine chuck it at a later date, just kidding, I don't know how well it's made or balanced it is but try it at another time, any vibration can suck up a lot of power. If the spinner's back plate is skeletonized, forget about it. 30% nitro would be better but not mandatory, I have ran them on FAI fuel with surprising results, but you need to take out all or some shims. You need to make sure the piston has some head clearance! 22% oil is a good idea, at least through the set up and break in time. I would add some castor to what you have to get it there. This link will help calculate mix: http://www.nitrorc.com/default2.asp?...orc.com/fuelws

    The 11X8 Zinger cut down is safer than a cut down 10X10 APC! At 10X8, that prop is still a lot of load, much more than a 9x10 APC carbon clone. For one thing, that wood prop is a heck of a lot thicker and at those speeds the disk area is more important than pitch when it comes to static engine loading. Think thin prop and small disk area. Thin it and start making the prop smaller in 1/8" increments. For what you have done, your engine is doing very well.
    Oooh eee oooh ah ah(the crowd) ting tang walla walla bing bang!(the plane crash)

  13. #13
    combatpigg's Avatar
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    RE: OS 91 VRDF experts wanted

    Hey FF..didn't know that you've wrestled with one of these engine before. I've seen them often for sale with no time on them. Compared to running .50 sized [and under] engines and that size of plane do you see any advantages with running this engine..?
    At 21-22,000 rpm is this engine built to take it over the long haul compared to let's say a .45 that is run at 24-25,000...?
    I've always liked the idea of trying one of these engines, but figured that the grass is always greener on the other side of the fence and have reached a point where if I want to add a new toy around here, I must get rid of 2 old toys first......
    WHO GUNNA FEED MAW KEEEIDS..???

  14. #14

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    RE: OS 91 VRDF experts wanted

    watch the rear main bearing like a hawk, as its the same bearing dimensionally as the VR 61 and up, you can still find the nelson piston and liner sets, which nearly everyone went to after a few flights, there is a better rod for it as well..the rear main lasts maybe 10-15 runs when run over 21k

  15. #15
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    RE: OS 91 VRDF experts wanted

    lfinney....not good news, but thanks for the info.
    It sounds like the rear bearing just doesn't have the size needed to put up with the forces applied. The best remedy besides running the highest quality bearing would be to remove as much dead weight from the piston as possible and to use the highest oil content the engine is happy to run..
    I would consider this engine a great candidate for one of my full complement bearings.
    WHO GUNNA FEED MAW KEEEIDS..???

  16. #16

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    RE: OS 91 VRDF experts wanted

    Thanks for all your answers and ideas...

    Unfortunatly is Nitro Methan the last thing that you want to be seen with with at my country, so my challenge will be to maximise the RPM without Nitro using a 10x10 APC. If I can get 18.000 on the ground I will be happy. I have a chance to borrow a pipe made for an 15ccm OPS , will try this next and keep you posted.

    @ MJD, you mentioned some source in Europe for carbon props for 18 USD...anything more specific ? Still have family back in germany..so that wouldn't be a problem



    cheers

  17. #17
    combatpigg's Avatar
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    RE: OS 91 VRDF experts wanted

    I've got 2 identical engines except 1 engine is designed to run on 0% nitro and the other engine can run as much nitro as you want. Each engine has it's own specially designed pipe.
    The 0% nitro pipe is much fatter in diameter. Both pipes are roughly the same length.
    The 0% nitro engine has a very small combustion chamber compared with the engine that burns nitro. Machining your own head is pretty basic work if you find that the stock head seems to let the engine run undercompressed. Undercompressed, it just wont handle much load or hit resonance on the pipe. There is a sweet spot that requires patience to find, unless the engine manufacturer already has taken the time to find it for you and has already got the right parts to make your choice of fuel and propellor work together.
    There is a German website that is dedicated to RC Speed Planes. At that site you will find the most knowledgable speed freaks who can tell you where to find propellors for your engine / plane combination.
    WHO GUNNA FEED MAW KEEEIDS..???

  18. #18
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    RE: OS 91 VRDF experts wanted


    ORIGINAL: Headhunter70

    Thanks for all your answers and ideas...

    Unfortunatly is Nitro Methan the last thing that you want to be seen with with at my country, so my challenge will be to maximise the RPM without Nitro using a 10x10 APC. If I can get 18.000 on the ground I will be happy. I have a chance to borrow a pipe made for an 15ccm OPS , will try this next and keep you posted.

    @ MJD, you mentioned some source in Europe for carbon props for 18 USD...anything more specific ? Still have family back in germany..so that wouldn't be a problem

    cheers
    Sure, look up Erdt on RC-Network.de. But be advised these are for all out speed, on very clean airframes - the sizes are in the order of 8x11, 8x12, 8x13 etc. Not sure if he has his own version of the popular sport speed prop they use over there, a CF 9x10 with the pattern made from a trimmed and reshaped 10x10 APC. I wonder how fast that engine can really can swing a full 10-10, that's a big shaft load. The only benchmark I have is from the Jett site, a guy reporting that his piped Jett FIRE .90L is swinging a 10x10 16-16.3k on the bench. I wonder if 18k is a bit optimistic considering the timing of the engine, but don't let my thoughts discourage you as I don't have one myself and I don't know how far those numbers are off the powerband of that engine. With the right pipe length..?
    Sorry I'm late dear, I had to help my uncle Jack off his horse.

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  19. #19

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    RE: OS 91 VRDF experts wanted


    ORIGINAL: combatpigg

    Hey FF..didn't know that you've wrestled with one of these engine before. I've seen them often for sale with no time on them. Compared to running .50 sized [and under] engines and that size of plane do you see any advantages with running this engine..?
    At 21-22,000 rpm is this engine built to take it over the long haul compared to let's say a .45 that is run at 24-25,000...?
    I've always liked the idea of trying one of these engines, but figured that the grass is always greener on the other side of the fence and have reached a point where if I want to add a new toy around here, I must get rid of 2 old toys first......
    I first discovered how well they ran is when I'd run the engine through at the end of the day with FAI fuel. I would do this for DF and Marine engines. If you don't do this, the bearings wont last. The engine was down on power a tad and you really had to lean it. I used a clip on rolling IV clamp so I didn't have to mess with my flight needle setting. One day I was out of 30% and too lazy and didn't want to spend the gas money to get some more so I pulled all the gaskets and hoped the piston wouldn't hit the head....what the heck, roll the dice. Well it ran fine, but it was visibly down on power and the engine easier to fall off the needle setting. Perhaps It would have ran better with the pipe re-tuned for the low nitro, but it doesn't matter any more as the plane flew away from home some weeks later.

    As you know, you need a clean plane (dart) and flight time is limited by tank size, however before you even consider this, make sure you have a nice selection/candidates of CF props actually in your paws, not on order, not shown in stock and not promised, just in your hands and good luck with that. Cut down and thinned wood props work, but they are very fragile and time consuming.


    Oooh eee oooh ah ah(the crowd) ting tang walla walla bing bang!(the plane crash)

  20. #20
    combatpigg's Avatar
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    RE: OS 91 VRDF experts wanted

    ORIGINAL: freakingfast

    I first discovered how well they ran is when I'd run the engine through at the end of the day with FAI fuel. I would do this for DF and Marine engines. If you don't do this, the bearings wont last. The engine was down on power a tad and you really had to lean it. I used a clip on rolling IV clamp so I didn't have to mess with my flight needle setting. One day I was out of 30% and too lazy and didn't want to spend the gas money to get some more so I pulled all the gaskets and hoped the piston wouldn't hit the head....what the heck, roll the dice. Well it ran fine, but it was visibly down on power and the engine easier to fall off the needle setting. Perhaps It would have ran better with the pipe re-tuned for the low nitro, but it doesn't matter any more as the plane flew away from home some weeks later.

    As you know, you need a clean plane (dart) and flight time is limited by tank size, however before you even consider this, make sure you have a nice selection/candidates of CF props actually in your paws, not on order, not shown in stock and not promised, just in your hands and good luck with that. Cut down and thinned wood props work, but they are very fragile and time consuming.

    Thanks for the "low down" on this engine FF.
    Having an assortment of props and a way to quickly adjust compression is really a must.
    It sounds like a 9x9 [or similar load] would be a good "default" size to start out with on a 36 inch span delta.
    I'd look at something like this as like a Big Block Vega bracket racer that the driver "short shifts" to easy 10 second runs.
    WHO GUNNA FEED MAW KEEEIDS..???

  21. #21
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    RE: OS 91 VRDF experts wanted


    ORIGINAL: freakingfast

    I first discovered how well they ran is when I'd run the engine through at the end of the day with FAI fuel. I would do this for DF and Marine engines. If you don't do this, the bearings wont last. The engine was down on power a tad and you really had to lean it. I used a clip on rolling IV clamp so I didn't have to mess with my flight needle setting. One day I was out of 30% and too lazy and didn't want to spend the gas money to get some more so I pulled all the gaskets and hoped the piston wouldn't hit the head....what the heck, roll the dice. Well it ran fine, but it was visibly down on power and the engine easier to fall off the needle setting. Perhaps It would have ran better with the pipe re-tuned for the low nitro, but it doesn't matter any more as the plane flew away from home some weeks later.

    As you know, you need a clean plane (dart) and flight time is limited by tank size, however before you even consider this, make sure you have a nice selection/candidates of CF props actually in your paws, not on order, not shown in stock and not promised, just in your hands and good luck with that. Cut down and thinned wood props work, but they are very fragile and time consuming.
    Good practice, apparently this is SOP for the Euro speed cup fliers - run some FAI through the engine at the end of the day and pull the line at full tilt.

    The Tettra 4060 and 4061 bubble free tanks are a reasonable 58x54mm in cross section and hold enough fuel for 2.5-3.25 minutes of full bore speed flying, more if you throttle back now and again. I use the 4060 on an OPS Speed .65 (~4 oz//minute) and so far the runs are long enough for the attention span of my remaining brain cells.

    To the best of my knowledge the CF 9x10 APC rework/knockoff is a popular choice on 15cc DF engines in sport speed models overseas. By sport speed I mean the 190-210 mph stuff (Speed Cobra, other older speed designs, composite ME-163's etc) as opposed to the state of the art speed airframes that can make use of less diameter and more pitch. 20k on a 10" pitch prop has serious speed potential, and 9" diameter has the disc area to drag a little more airframe behind it.

    Sorry I'm late dear, I had to help my uncle Jack off his horse.

    Revver Bro #231

  22. #22
    iron eagel's Avatar
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    RE: OS 91 VRDF experts wanted

    Mike Connor and I had discussed the possibility of doing up a modified SD with one of these engines, the one that goes with the pipe pictured as a matter of fact. I had thought about going the DF route but a propped version makes a lot more sense because the fan isn't all that efficient. I was thinking that a SD airframe scaled up to a 36-38 inch span should be about right proportionally.

  23. #23

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    RE: OS 91 VRDF experts wanted

    Hi

    I have taken this engine in a Stiletto Mustang to 189+mph with 15% fuel....... gps/pitot/radar timed.....
    setup=15% marine fuel with castor; 10-10 or 11-8 cf props; TT ultimate spinner; Macs marine pipe; K&B 1L plug; Fully cowled engine and 1/2 pipe.
    I also used lots of after run oil after every session.....

    randy

  24. #24
    combatpigg's Avatar
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    RE: OS 91 VRDF experts wanted

    Sounds like a great bang for your buck deal if you don't go crazy on prop load, stick to light props. A 36-38 inch Screamin' Demon ought to be perfect for cheap thrills.
    I built a 36 inch "Diamond Dust" several years ago that a stock Webra .50 pulled around in the mid 150s with a 9x8 prop. This plane sold last year to a guy with a bunch of large hydroplane engines. Never did hear back from him how things went.......
    WHO GUNNA FEED MAW KEEEIDS..???

  25. #25
    iron eagel's Avatar
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    RE: OS 91 VRDF experts wanted

    The .91 VRDF I have was a deal too good to pass up several years ago. It's no Nelson, Jett, or OPS but still a fairly high performance engine and should work well propped properly.
    What is going to be the interesting part of the project is to fit an airframe to this engine. I'd like to keep the wing loading in the 12 oz/ft range, that's probably going to be the main design criteria.


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