Champ-RCU

My Feedback: (132)

Ok remember no cursing, degrading, insulting or embarassing. I purchased a DF airplane with an OS 91 VR-DF (Large head) you know Rear induction, rear exhaust. Revs to 22,000. The question is could this engine be used as a pattern prop type engine? Now that it's out remember the no cursing etc. Thanks for any imput.
Mark

NikolayTT

Pattern flying at high speed like the Jet-tubine engines yes, or if you wanted it slow, then you have to add a gear-box which
sometimes ago were made by 3rd party and still maybe avaiable on eBay in order to use the power of that engine, which
seems delivered only when the rpm are at the higher end. If you think that neither of those is what you wanted and the
engine is new, then you might consider to sale it and buy another which has the power at lower rpm. If the engine
has never been started its price is very nicely high on eBay, but if it has been run couple times then the price might
go even 50% down.

jaka

Hi!
No! It rews to much...and it does so using small 10-11" props.
It doesn't like larger props...So in a smaller ...faster airplane OK!
But not in a modern 2m pattern ship.

Flyer95

My Feedback: (1)

Pattern flying with that engine is not a problem. There are two things you must do to make it work good. You must lenghten the header with an extra piece of alu-pipe of the same dimension as the original header and retune the pipe so that it will stage on pipe at say 10-11k instead of 20k. and you gonna need to use a Perry VP-30 pump/regulator to feed the engine and place the tank on the CG. APC 12x12, 12x14, 13x12, 13x10....are all good. The engine weights more and will burn more fuel than a say OS91FX but should also be stronger, so not that bad. A hot plug like the OS-F plug is also great for instant throttle response on heavy prop loads, set the main needle slightly rich.

NikolayTT

In fact one can rotate lot BIGGER propeller by using the simplest
classic approach = GEAR BOX, have a look at the OS www-page:

http://www.osengines.com/history/ostimeline09.html

and that was the Approach in 1982. It is not heavier than the pipe and
gives lot easier Start of the engine + Smooth Idle and Stable RPM with
the Big propeller, and Efficiency is maybe better especially if the Reductor
is of good quality.

On eBay one can still find third party Reductors with gears or with belts
since that time, mid 80s.

In fact the Pipe as a mean to reduce the RPM is maybe a bit too "exotic"
solution, isn't it ?!

Flyer95

My Feedback: (1)

OS91VRDF is not much different than any other 91-sized twostroke engine other than higher exhaust timing and larger carburetor. It will waste more fuel than a sport engine but on a tuned pipe "as it should be" it is stronger than a sport engine on a tuned pipe. Even the much smaller OS77VRDF on a pipe is stronger than a say OS91FX on a tuned pipe. Backdraws for DF engines are the higher weight, higher fuel consumption and the need for a pump if the engine is loaded with a large prop.

NikolayTT

I would like to disagree quite a lot here. It is true that .91DF "understands" better the "Pipe-music" but you can make .91FX
understanding it too by changing its gas-diagra too. The high RPM engines are very(!) different than normal RPM ones, not
only by the light piston and sleeve but also by the angle of opening the exhaust, it is almost 180% for those fuel-hungry
"monsters" because those 4 bhp do come from the fuel "handling". There was a good statement on RCU that the 2S in in
fact a "fuel-pump" and those fuel pumps are very different inside althoug similar outside; weight is the smallest difference.

You can check for example what pipe .91FX and .91DF will need if you use the calculator on

http://www.mh-aerotools.de/airfoils/javapipe_en.htm

and then find in fact how different they are.

Anyway, the Pipe cannot (!) replace the Gear Box in efficiency handling;
I guess we can all agree on that.


Cheers,
Nick

dhal22

i'm trying to recall exactly what the setup was that my uncle (an aeronautical engineer and a lifelong rc enthusiast) designed. he scratch built a very thin, narrow profile, high wingload speed plane and shoehorned in this very same engine with a pipe exiting under the canopy area. a world class pattern pilot who was a member of his club flew this plane on the straight and level in the 180's. on a dive well over 200 mph was obtained if i remember correctly. the plane still exists, i should ask him to chime in on this thread and perhaps offer what prop size he used, rpms obtained, etc. maybe i can get him to photograph it and post a picture.

NikolayTT

Yes, please post that - it is interesting to see.

Meanwhile please refer in the PARALLEL discussion which was started earlier:
http://www.rcuniverse.com/forum/m_62..._1/key_/tm.htm

Also, here are some more photos of those DF engines in Airplanes(no one of them is mine):

jaka

Hi!
Nicolay! That OS reduction system you are referring to was only made for a specific 10cc engine. And all other belt reduction systems sold in the eighties and nineties is not going to work in the DF .91 as those hasn't got the reliablility and durance needed. None is sold today.
Of course the DF .91 could be used in a pattern plane if enough work is invested in it but why mess with it when there are more sutiable pattern engine awailable...!??

NikolayTT

Hi Jan,

There have been many exceptional things made continuosly with a lot of Gear boxes. Here is something hard
to believe but it is real, but it even works - have a look, for the guys who made this there will be no problem
making such reductor for .91DF if he had to. Of course I agree what you said and I have MVVS-26cc instead
of modifying for slow pattern flying .91DF; 91Df is for Speedy Aerobatics if one wants to do that indeed.

Champ-RCU

My Feedback: (132)

Thanks to everyone for their answers and suggestions. As far as prop reduction devices go - more weight and $$. As far as machining my own, well let's say ol Ben Franklin had more equipment than me. (That didn't come out right, should say more metal working equipment.) Speed planes (No Mark NOOO!) I guess that leaves sell engine or build a DF Jet. Winter is coming I'll have to give that some thought.

Again thanks for all the effort.

Mark

nsmjim

My Feedback: (52)

Help if possible. I'm looking to do just the opposite! I want to turn a lot of RPM's as I'm thinking of a ducted fan for my next delta wing. I already have a Jett sport .50 and another rear exhaust .50. I was trying to determine if I put a .91 ducted fan engine on a pipe what prop I'd need that would a) be able to pull away on a hand launch, and b) turn max RPMs without leaning out too much???
Any ideas out there?

Champ-RCU

My Feedback: (132)

nsmjim,
I know where you can get a nice OS 91VR-DF with large head and a Byron Fan andtuned pipe. See my ad in the 2 stroke For Sale area here at RCU.

MTY