I need help please. I have a BVM 91 with a Viojett fan on the way to me. I would like to use it in a YA F18 single but am having problems with a pipe setup that would work directing the exhaust out one of the nozzles.I have been told to use an O.S. 91 and get an angled header but there has to be a way to use the BVM. Or am I wrong? I've been flying for a long time but this is my first ducted fan and I could use some advice. Please don't tell me to get a turbine ( I wish ), that just isn't a financial option.

Thanks much,
Dan

JMP SELLS A PIPE SPECIFICALLY FOR THAT MODEL AND IT SHOULD WORK WITH THE BVM .91

VISIT WWW.JETMODELPRODUCTS.COM

DAVID

They are the people who told me to use an O.S 91 since the header on the BVM is part of the cylinder casting.

Dan

The pipe should fit on the header fine, whether it'll tune properly is another question since the BVM wants to spin faster than a Dynamx setup.

Hi Chris, I sold you a set of Agressor retracts a while ago, hope they served you well. Saw a picture of your E-Viper and man it kicks *****.
Im curious to know your E-Viper setup compares to the new Viofan setup cost-wise.

Thanks,

David

Last time I checked, the Cook pipes were made to fit over Cook's headers. The BVM does not have a seperate header, the "header" is cast with the cylinder in one piece.

Also, the Cook pipes are made to fit over a special silicone sealing coupler that goes onto the special bolt-on Cook header.

Has anyone actually run a Cook pipe on a BVM 91??

A little less dollars, little less power, sourced from multiple places and assembly required

Working on a Starfire now - custom ducts, ready for a test flight in the next week or so...

Hi,

You can use an O.S. .91 in a Violett fan, but doing it the other way around is a hassle, I hear. If the BVM setup is the only one you have, then you'll have to make it work, but it's not worth the trouble if you have a Dynamax setup in that particular plane. The mounting and the inlet are gonna be bears. Good luck!

You're right, I'm confused.

I've used the BVM pipe on the OS but of course when I fitted the JMP pipe the header needed to be changed to the JMP part. It MIGHT work if you used the JMP rubber piece over the BVM stub header since the pipe ID's are the same but it might not without the JMP ledge on the end of the header which protects the end of the seal somewhat from hot gasses...

The BVM setup is the only one I have. I don't as yet have the Hornet though - I'm very close to the "buy now" button. Shaun, what exactly do you mean by the inlets and mounting being a bear? Is it possible to run the BVM pipe in the Hornet being straight back?

Dan

The two fans are different diameters, the Dynamax is bigger.

Dan,
What these guys aren't making clear to you is that the Yellow F-18 ductwork is sized for a Dynamax fan. The BVM fan is a smaller diameter and will not fit without major modification.
A Dynamax is about 5 1/4" diameter, and a BVM fan is about 4 7/8". I don't recommend trying to cobble up the ductwork to make the fan fit. If not done exactly right you won't get adequate thrust.

So if you want the F-18 you should get a Dynamax and OS91 for it. If you want to use the BVM set-up then you should buy a BVM plane.

Joe

Deltron,

What JoeFlyer is saying is correct--mostly. The diameters are the issue. The modification to make the inlet fit is not as major as it may seem, but still not necessarily worth the trouble. The amount of thrust you'll get is going to me more than enough to fly the bird, even if not done exactly right, as evidenced by an inlet mod I saw done by a fellow I sold a used Viofan system to. He totally boogered it up (his words), but the performance was still identical to what I'm used to seeing with this bird.

The inlet and the engine mounts were designed with the Dynamax in mind (like Joeflyer said), and the BVM fan--in addition to being a significantly smaller diameter--has mounting tabs which are positioned higher up on the shroud (and angled). All of this would require not only modifying the inlet, but also the mounting system. When you're done with that, the engine sits up taller than the O.S. installation, so there's that issue to deal with. None of these things are unsurmountable, but the question remains--is it worth the trouble?

So far, it seems to me that the installation of BVM fans into Yellow Aircraft DF airframes does, in fact, yeild you a slight performance enhancement. Again, it doesn't necessarily seem worth it to do the work to make it fit in all cases. The Starfire, for example, with a BVM fan, seems to be noticeably faster. So does the A-4. After having done the mods, I didn't feel it was worth the difference in either case.

Feel free to call if you have questions about that bird. 619-944-7574.

Thanks to everyone for the advice. My answer to "is it worth the trouble" is yes. I have what I have, finances ARE a reality I have to deal with, so getting another engine and fan is something I'd rather not do. I went with the BVM setup because just about everything I read strongly suggested the BVM was the better powerplant with the fan an added bonus. I really didn't realize though how much trouble it would be. The mounting aside, I'm still up against the pipe. Will a pipe that does not angle towards one side or another work?

Thanks again,
Dan

Dan,

I don't know what you're going to end up doing about that problem. There are actually a couple of problems there. The first one is that the distance between the header and the place where the thrust tube splits is too short for your BVM pipe. You'll end up with the pipe sticking through one of the two holes. This may give you a certain amount of differential in how much thrust is coming out of one side of the outlet or the other. This may or may not be troublesome in flight, but it's moot until you figure out how to get the right down-angle. Maybe someone could machine an adapter for you?

Shaun,
I agree with everything you said. It definitely can be done. As a matter of fact I had a BVM powered Yellow F-4 that performed quite well.

I was intentionally being negative since this would be Deltron's first ducted fan and this is a complication that might prove difficult for him without much benefit.

Joe

Joe,

Good point. Thanks!

If I did this right, attached is a pic of a scratch built 1/29 scale Shuttle Endeavour. When complete it's going on permanent display at the Reading Public Museum here where I live. I've built stuff like this all my life so I'm ready to do some mods on the Hornet to get the BVM in there. The pipe still gets me though. If a JMP bifurcated pipe is tuned for the O.S. would it work on the BVM - could you possibly tune a pre-tuned pipe?

Dan

Dan,
If you are determined to go with the BVM powerplant there are two ways to address your pipe concern. You could heat up and bend the BVM pipe. That might take a few iterations to get just right.

I had the same concern with asymmetric thrust that Shaun mentioned on my F-4. The pipe blocks most of the area of one of the exhaust cones, so most of the thrust comes out the other side. You can offset this with a bit of rudder trim. My solution was to use a Weston Mini-pipe.

The Weston pipe is about 9 inches long so you do get exhaust residue in the rear ductwork, which is not a big deal. The performance was very good with a BVM 81 engine. I have no experience with a BVM 91 and Weston mini-pipe, but it should work fine. If you want to try it I still have the Weston pipe and can give you a good deal on it.

Joe

Here's the link to Weston's website if you're interested.
http://www.westonuk.co.uk/index_197.htm

Joe

I've never had a rc engine that required a pipe, so I'm a bit ignorant on the properties. I thought the length of the pipe determined the "tune"?

Dan

Dan,
You might try doing a web search to learn more about tuned pipes and how they work. You are basically correct in that you vary length to optimize a particular pipe to a specific application, but other factors also influence it. Some pipes work better than others on specific applications because rpm, engine displacement, engine port timing, pipe volume, internal geometry, fuel, weather, etc. all affect tuning.

The idea is to set up a pressure wave inside the pipe so that it scavenges the exhaust giving a supercharged effect. Most DF pipes on the market are pre-tuned to work well on a OS91/Dynamax. They may not be optimized but usually work well on similar sized engines operating at high rpm.

Most tuned pipes you see are full wave pipes (one scavenging wave per exhaust event). Mini-pipes are half wave pipes (two scavenging waves per exhaust event - so one of these waves is essentially wasted since the exhaust port is closed when it hits it). If everything else is equal a full wave pipe will perform slightly better than a half wave pipe). Weston seems to have done an exceptional job of mastering the science of half wave pipes and in many cases they perform as well or better than full wave pipes.

If you do a search in the Jets Forum you will find many who are amazed and very pleased with the performance of the Weston DF Mini-pipe.

Joe

One question, how would you seal the area between the shroud and the inlet? RTV?

BVM has just come out with a Viofan adapter for 5" ducts (why o why now?). It is design to be used with the new E-V-Fan. That may take care of the inlet, however you would still need to seal the area between the shroud and the Exhaust Duct.

You could offer to trade your BVM fan unit and engine for a dynamax fan and os engine. Someone like me might take you up on it.

You're the third person to offer me that. I have an arrangement in place to do just that, and if for any reason they would fall apart I'll certainly be getting back to you. Thanks to all for their help and generosity. There are some really cool people on the forum.

Dan