Hi Folks
I am new to DF and when I went looking for my first Df engine to power my aeroplane I of course went for the OS91 VRDF. I had great difficulty getting one but i finally did. Now i believe the 91 has all but been dropped by OS. You certainly cant get one through the Uk importers and I believe the US importers have also dropped it.
So my question is what now for DF now that our favourite engine is all but gone.
The obvious answer is BVM but to match this engine up to anything other that the Viojet fan it was designed for would be a waste of suberb piece of engineering. However the Viojet is 4.9 inches and other than BVM all the DF kits are designed around the 5.25 ( ie Yellow).
So what now or should I just resign myself to going the way of the dinosaurs before I have even begun.

Mike

Mike

You could go down the 45 route. There is no sign of the OS 46 VXDF going out of production, after all it's just a boat engine with an aircooled head on it. Thorjets do a fan and a few plans for various jets, trouble is you have to like Balsa bashing and you have to build light, no more than 8lbs if you fly of grass you can go a bit heavier if you fly off Tarmac.

I am well into the build of my second 45 sized D/F (Mirage F-1C) and my F-20 is still going strong.

Stuart

you can use the BVM fan in most 5" applications with just a little fabricating of the inlets, My idea is to take a piece of hard foam "beer cooler thickness" cut it out in a circle to match the 5" inlet, then cut the center out to match bobs fan. Then "ramp" the difference by sanding, so the air wont be distrubed any more than is absolutley needed and epoxy it into your inlet. Just a thought, probably someone has a better way. russ

Mike - Be of good cheer. A couple of months ago, there was a string on RCU about the O. S. .91 becoming extinct. User feedback was so strenuous that the engine was placed back into O. S. factory production when the order levels call for a run. My feeling is that the engine will be around for quite some time. There will always be a group of modelers who want to fly jets but for one reason or another are not willing to put the money into buying a turbine. Dynamax, Ramtec, and Turbax ducted fan setups offer a less expensive (than turbines) way to get the thrill of flying a "jet".

As always, though, this is just my opinion based on recent trends. Your mileage may vary.

DFF

I just happen to hear today from my LHS owner that his "rep" confirmed a shipment of DF's coming to Tower as early as this month possibly.
I take him as very reliable, but that information is only as good as the one who told him. I can't speak to the reliability past him, therefore it's the "grapevine" and we all know how that can go. Hopefully for DF side, it's true.

George
SimJet

Well I just bought a new in box OS 91 VR off this board from a member for a good price I thought. And I am building my first "Jet" now its a Jet Hangar FJ-3 Fury or the Navy version of the F86 Sabre. I bought a new OS 46VX and Turbax for it and just ordered two Violett remote Needle valves for both engines. I have been flying for 16 years but this will be my first ducted fan and I am installing scale landing gear struts that work and I also will have flaps and speed brakes in my Fury. Now that we can get smaller and higher torque micro servos these functions dont weigh as much. I too was worried about the OS 91 VR not being available but I was able to get my first one last week in only hours of me putting a wanted add on the jet board here. When I see a second one for sale I will most likely buy it as well. I am about 1/3 of the way done with my Fury and then I am buying a JHH Phantom with the scale landing gear struts. Since I live fairly close to Jet Hangar I have known Larry for about 12 years so I have him to help me if I get stuck(like building the speed brakes) I also started a turbine fund this month. Every month I am putting away some money for my first turbine. So by the time I have both my ducted fans flying for about two years then I should be ready to take my test for a waiver and buy my new Turbine and scale aircraft for it. I dont think OS will stop making the 91 there are still plenty of people that cant afford a turbine right away or dont feel they should fly one as their first jet. So I dont think you have to much to worry about, infact Larry is working on a 46 size F100 that I will buy when its ready so I will get another 46VX as well. I cant wait to get my Fury in the air,
Viper

Hi Folks
My interest in DF is simply that although I could afford to go Turbine I feel I need a few DF under my belt and that frankly this is a hobby, and as such I don't particularly want to have several thousand pounds invested in one model which I could so easily lose in one go.

Yes I like the idea of Turbine I also like driving nice cars and many of the other pleasures of life.
I also enjoy flying Heli and building planes in general.( namely anything with wings fixed or rotating of any dimension as long as it will fit in my car)

I think what I am trying to say is that there will always be many modellers out there who for one reason or another do not want to go turbine and the engine and DF Kit manufacturers should not ignore us.

So Mr OS please please dont stop giving us excellent DF engines
and Mr DF Kit manufacturer how about making your kits Viojet friendly.( it only after all entitles a small tooling change)

Mike

mikehannah , I agree totally!
To put it frankly, I cannot afford a turbine but I have had DF for quite awhile and quite happy with them. Just wish DF engines were as reliable as a turbine!
Hopefullt all these OS .91 rumors are true!
Myles

BVM use to sell a fiberglass adaptor that would attach to Dynamax or Ramtec sized inlet ducting and neck down to accomodate a Viojett fan. I'm not sure if it's still available.

Also I noticed recently that BVM now offers a spool adaptor to adapt a BVM 91 or 96 to a Rantec or Dynamax fan. I assume that this is in response to the short supply of OS 91's. That combination would work but I doubt that the BVM engines are a good match for a Dynamax or Ramtec fan.

Joe

Joe why would the BVM engines not be a good match for the Ramtec or Dynamax fans? I am just curious since I don't know much about those engines?
Thanks Joe

i would think the BVM engines would work fine in dynamax or ramtec because the viojett fan turns MORE rpm than they do.

Who makes the BVM Engines? Are they made by Nelson? Or someone else? Just curious
Viper

BVM engines are made by nelson and fitted by BVM. 99% sure of this.

Wow the spool adapter for the BVM/Dynamax is new!!! That is awesome! This opens up new reliable power plants for various older planes! Hmmmmmm Should have never sold that........darn

Thanks BV!
Dustin

Hi Folk
Well I am glad to see that I am not the only one who feels this way. I would like to think that the manufacturers, both engines and kit wise read these forums and take not of the views of the people. Because it is fairly clear to me that there is a strong group of people who wish to stay DF.
The idea of putting a BVM into a Ramtec I would do if I was stuck but for the results this animal is renowned for it really needs to be driving the viojet fan which has been clearly optimized for the BVM.
But I will do anything including reworking ducts if it keeps me flying and enjoying my hobby .
Modelers are an inventive crowd at the best of times


Mike

I'll try to explain the comments in my prior post a little better. For optimum performance a fan needs to be matched to an engine much in the same manner that we pick the right diameter and pitch for a propellor plane. If the propellor is too big you load the engine too much, the rpm is low and you loose performance. If the propellor is too small the engine will over rev, use extra fuel, loose performance, run hot, and put a lot of extra wear and tear on the engine.

If you've ever seen an engine performance curve you know that as rpm increases hp increases until you reach the hp peak, after that power drops off as rpm increases. For best thrust you would typically end up with a larger diameter/lower pitch propellor that probably runs at less than hp peak rpm. For best speed (air velocity) you would typically pick a smaller diameter/larger pitch propeller that runs very close to the hp peak rpm.

In the case of a ducted fan there other factors associated with air flow and ducting that enter in, but everything else being equal you basically still want to run the engine close to it's horsepower peak for best velocity.

The information that comes with the OS91 engine says that the engine puts out 4.8 hp @ 22K rpm. Now 22K is pretty close to what it will run with a Ramtec fan. This fits with the story that the Ramtec was designed to match an OS91. The Dynamax was designed back when 65-77 sized engines were available, so it doesn't load an OS91 as much as a Ramtec. Consequently a Dynamax will let the engine rev up to about 24K rpm, producing about the same amount of thrust as the Ramtec does at 22K. Some that have run the Viojett/OS91 combination tell me that the OS91 over revs (maybe 26K) and the engines don't last very long. The only way around that is to run overly rich to slow the engine down, thus loosing performance. That would suggest that a Viojett/OS91 is not well matched.

BVM does not advertise their engine perfomance numbers, but I would guess that the BVM 91 is about 5.5-6.0 hp at 28K, and pretty well matched to the Viojett. These engines will hold up well running at much higher rpm that an OS91.

Based on all my years of engine design and development (automotive), I know that for a given displacement the kind of things you do to boost high rpm hp typically degrades output at lower rpms. There is no free lunch, aside from efficiency improvements you basically tilt the hp curve trading off low rpm output for high rpm improvement.

This is speculation on my part since I have not seen the hp curves for each engine nor have I tested them, but I would expect a BVM 91 to have less output at 15K and 20K that a OS91.

If that is true then a BVM91 would not perform any better, and probably worse, than an OS91 in a Ramtec fan. A Dynamax fan would be a better match for a BVM 91 since it would allow the engine to rev a little higher, but I doubt you'd see any noticeable improvement over an OS91. Neither combination would be up to par with a BVM91/Viojett.

Again this is speculation on my part. The rpm numbers quoted above are estimates, your actual results may vary. It would be interesting to hear from someone who has tried a BVM91 in either a Dynamax or Ramtec fan.

Joe