Dave03B

I have recently purchased a NIB OS91vrdf and BVM Viojett fan unit. I am planning to use this powerplant in my PJP 1/8th BAe Hawk however It has been suggested to me that the OS91 cannot handle the revs that the the bvm fan will bring about. Also that the fan blades may stall?? and cause damage to the engine. And finally that the ducting may not be sufficiently designed to work with the viojett.......

I do not want to damage my engine so I'm open to suggestions, and will consider anything, just would like to hear a few opinions before i sell the two viojetts that i have, to buy a Ramtec or Dynamax.

Thanks for your help
Dave

EDIT: LET'S VOTE.
For a model with small inlets which combo will perform better and get my hawk off a grass field?

Scott Douglas

According to the BV website, the Viojett accepts the OS91 ok. The only thing is the Viojett is a sligtly smaller diameter than the Dynamax or Ramtec so you may have to modify your ducting slightly.

The OS 91 Rotor Spool adaptor is Part #6015. I'm not sure if the standard OS exhaust header would fit the BVM pipe, maybe someone else can help there.

Cheers
Scott

PS, that F16's almost ready for the maiden now... I'll have video on the website when it flies

Dave03B

Hi Scott,
Thanks for your reply. I saw that on the BVM site too, which is why i'm slightly perplexed. I have the rotor adaptor and the the 91 came with an unused pipe and header which all goes hand in hand.
Like you I only saw the ducting differences as the main problem. The ducting currently in the Hawk was for the thorjet 45 fan, which is considerably larger in diameter to the Viofan. A Ramtec or Dynamax would simply slot in very snugly to the current inlet duct. However I think that the OS91/Viofan combo will produce more thrust than the Ramtec or Dynamax. (perhaps someone will chime in to confirm/argue this?)

Hope to see that F16 video soon. Best of Luck, hope all goes well
Cheers

joeflyer

My Feedback: (48)

It's been done, but the OS91/Viojett is not well matched. It's like putting a too small propeller on a prop plane. It revs too high for the engine and doesn't put out optimum power. It might fly OK but the OS91 will not be very durable at the higher rpm's. Don't expect the engine to last more than 25-30 flights. The only way to get them to live longer is to run rich enough to keep the revs down. If you do that, then you won't be getting the kind of power you'd like. The BVM engines are designed to be durable at 26-28K, whereas the OS91 won't last too long if you run it over 24K.

The ducting is your other problem. The smaller BVM fan needs nice smooth ductwork to work efficiently. If you go cobbling up the existing ductwork it may not work too well.

I'm sure that there are some that could chime in and say that the combination worked out for them, but that would be rare. My advice is to go with what works. After putting in a lot of time, work, and money you want a plane that you will enjoy. Sell the Viojett and get a Ramtec or Dynamax for that plane. It will perform better.

Joe

CobraJet

My Feedback: (4)

Dave,
That NIB OS 91 will last a long time in a Ramtec fan which keeps the OS in it's RPM range. Sell the Viojett fans to fund a new Ramtec and an inflight mixture control or sell the OS and get the BVM .96 motor. I think you'll be happy either way.

Dave03B

Guys, thanks for your help. I will part with the Viojett and seek the more suitable Ramtec or Dynamax.
May I now ask which fan unit you would suggest that I use instead.... Ramtec or Dynamax?

Thank you once again
Dave

Miniflyer

In combination with OS91 i would go with the ramtec. It has an higeher efficiency than the dynamax, providing a slightly higher static and loosing a lot less thrust as speed increases.

So much for the theory in numbers.....in practice you probably won't notice a big difference unless you have two identical planes, one ramtec and one dynamax, in the air at the same time. Personal preference remains ramtec, though....

joeflyer

My Feedback: (48)

Dave,
I'm also partial to Ramtec. It's a later design and was specifically designed to match the OS91. It will produce the same thrust as a Dynamax at 1000 or so less rpm, so your engine will last longer. The only drawback to a Ramtec is that they don't work as well in a plane that has small inlet ducts.

I noticed this in my Yellow F-16A, which has a small inlet. When my Dynamax threw some fan blades I replaced it with a Ramtec and could notice the performance drop. On the other hand my TGA F-15 (large inlets) flies great with a Ramtec.

If this plane has decent sized inlets I'd recommend a Ramtec. You should do a RCU search on Ramtec and Dynamax fan for more information.

Joe

Dave03B

My Hawk has scale inlets. Which may cause a problem. I'm totally against adding cheater holes to the model as it will kill the look. I've been reading the book 'ducted fans for model jets' and it seems that cheater inlets whilst increasing static, can kill dynamic thrust. It was concluded that, if unavoidable, blow in door would be best.

[&o]

Dave03B

My total inlet area comes to 54cm2. And the Fan swept area of the Ramtec fan is approx 104.9cm2.
So my hawk's intake area is 51% of the total fan swept area......from what I've read, this is not a good thing. I fear that all 91 fans will encounter the same problem....What is the desirable % for intake area vs FSA?

sonic bang

What i've come to know from this forum, inlet duct area should be 125% of the effective fan unit area for it to work properly. I'll reconfirm this by reading the literature that came with my dynamax fan unit. Dynamax should give you more static thrust and less dynamic thrust as compared to Ramtec fan unit.

Irfan.

Dave03B

Has anyone successfully flown a model with small cheek inlets from grass using an OS91 and Ramtec fan unit? Am I doomed to fly this model with a 91 DF because of the inlet area? I'm running out of ideas and really want to get this flying.

Thanks all,
Dave

Miniflyer

What i would do is install the sucker and run him up SLOWLY, checking the RPM every few throttle clicks to ensure that the fan won't over-revv due to lack of air in the intake ducts. If you are ok in the RPM spectrum (ramtec should stay at around 22k or below with os, anything over 24k i would change the ducting) then go for it.
You won't reach such high static thrust levels as other os/ramtec models, but due to the enormous intake velocitys you'll really get shining in the upper speed envelope (where the fan suddenly gets optimum air through the intakes).

I've had a ramtec sucking through roughly 70cm², and it was doing just fine...my guess is: so will your system. You'll definetly be getting better thrust than with the "optimum fan" for your inlets.
I have also seen a pair of Jim Fox 1/8 Hawks with .91s on ramtec, and they performed awesome for d/f models.....no problem in the grass and out of sight verticals. I believe PJP has the fox moulds, so you should be in bussiness.

Dave03B

Miniflyer, thanks for that info and advice. If you've seen other 1/8th Hawks go on the 91/ramtec combo with scale inlets, all should be ok for me [8D]


Still welcome any further advice others may have, keep it coming.

Thanks.

Dave03B

btw, miniflyer are we talking about the same hawk? I know PJP has the fox 1/6th Hawk moulds, but I did not know that the 1/8th where from fox desent.........? My 1/8th is originally designed for .45 power...

Miniflyer

I'm not sure, but from what i hear the PJP stuff all came from jim fox. And the size of both jets is also identical....

Dave03B

There are two sizes of Hawk sold by PJPjets.

1/8th Hawk (which I have) designed for 45DF or MW44 and this model is 48" span 52" long
1/6th Hawk designed for 91DF or i believe 22lb and up thrust turbines. span: 60" length: 78"

I'm looking to put a 91/fan into my 1/8th Hawk. Hope thats cleared things up

However, upon looking at photos of the 1/6th Hawk, the inlets look like they'd be almost identical in inlet area of the 1/8th.

Miniflyer

check your mailbox
I was also talking about the 1/8 version....

Dave03B

cheers

Dave03B

I'll try the ramtec in this model then. Will let you know how it goes. Any estimates on how much 5% fuel i'll need onboard for approx 10mins flight time?

duthie

dave,
this item on ebay may be of interest: 5946474624

duthie.

Miniflyer

expect to be getting around 1 minute of flight time with every 100ccm you carry along. Will fill a normal flight envelope and leaves a bit extra for a safety go-around or two....

Marty Snell

My Feedback: (2)

Dave,
The Dynamax is the most forgiving fan for less than perfect ducting. You can use less than 75% swept area for the inlets if they are fairly clean. Use 2 of the 11 oz. df fuel tanks from sullivan this should get you about ten minutes. At full throttle the os91 uses about 3 oz/minute.
Always tune your engine to see a slight vapor trail in flight. Running this way should give you 50 runs before replacing the rod and front bearing.
In STRAIT AND LEVEL flight all three fans are within 15mph of each other only in steep dives is there much difference and that is more of an airframe thing.
I hope this helps.

joeflyer

My Feedback: (48)

Dave,
With inlets that small you would be better off with a Dynamax. I just measured the inlet on my Yellow F-16A. It's 75 cm2. This plane is considered to have a small inlet for a 91 DF. Origonally it flew fine with a Dynamax fan, but based on feedback from a few others I added two cheater slots about 9 cm2 each, for a total of 93 cm2, and noticed a minor improvement in performance. Later after putting a Ramtec in the plane performance was significantly worse, and I ended up buying a new Dynamax for it.

Summary: Dynamax performed good with 75 cm2, Ramtec was marginal with 93 cm2 inlet area.

The recommended inlet is 125% of FSA, so you are pushing it putting a 91 in that plane. Your best bet would be a Dynamax, and you still might need a cheater hole or blow in doors.

By the way for 10 minute flights you should carry about 32 oz. of fuel. The extra weight won't help with those small inlets though.

Good luck,
Joe

Dave03B

With so many different opinions of which combo to use (all opinions I value) it gets slightly confusing. (so please vote)
I may have to cut a cheater inlet infront of the fan, through the wing no matter which fan I use. But please vote on which fan would be more suited to the small inlets.