iron eagel

Can anyone tell me what the diameter of a Dynamax fan unit for a 91 size engine is.

KC36330

been a while sense i had one but from memory i do believe 5 1/4"

kc

iron eagel

Thanks for the info...
I was told that they produce around 15 lbs of thrust, is that true?
Seems like a lot of force for such a small size, not that I am complaining. I was expecting them to be around 10 or 12 inches in diameter.

KC36330

they are very strong, but with the OS 91 VR I'd say closer to 13-14 with good ducting.

kc

FalconWings

My Feedback: (57)

I'm more inclined to say perhaps 10~11.5 lbs on a good day, less on a hot summer Texas day. Don't ask me how I know.

KC36330

in my old Maverick it got 12+ during the summer here in south Alabama, just as hot and humid as Texas, winter I'd gain a pound of thrust with the much denser air. i used 15% nitro, 20% oil in an OS 91 VR in that setup. with a BVM 96 and the fiberglass shrouds with clean ducting i'd say 15 is doable.

kc

Shaun Evans

Hi,

Depending on how you measure it, you're going to be doing well to get 11 pounds of static thrust with a Dynamax and O.S. 91. If your setup is in a model with a poor ducting design or small inlets, you can expect even less.

FalconWings

My Feedback: (57)

I never used more than 5% nitro on ym Dyanamax/OS .91's. I asked around to the local ex-DF addicts and it turns out costly to get just a little bit more of performacne out of an OS .91. There were guys out there that would sell aftermarket parts for the OS to "upgrade it" and we had a local guy (Mickey Edmunds) who could take a Dynamax and re-pitch the blades to get optimum performance.

A BVM .91/.96 would proably fall on the higher 11.5~12 lbs. For the Viofan is not so much about thrust but about holding 23K RPM for 10 minutes without a single hick-up.
You could take off full power and remain full-power all thoughtout your 4 minute flight.

iron eagel

Thanks to all who have taken the time to reply!

I have no knowledge at all of the function or setup of ducted fans and you replies have been very informative.
One thing I have garnered is that having the setup of the intake and ducting is an important consideration.

Are there any rules of thumb as far as the setup of ducted fans that you can give me.

KC36330

keep the intake clean and smooth to air flow and of sufficient size to not starve the fan, on the exhaust side the same goes for keeping it as clean to airflow as you can, it needs to be of sufficient size to not restrict the airflow excessively. tweaking the exhaust exit in relation to the fan size tweaks thrust to an extent.

kc

Dennis Crooks

Back in the day, we removed half of the stator vanes in the dynamax. This has a significant effect on thrust with a minimum amount of exhaust 'swirl'.

Although I never measured it, 13# of thrust might be a little optimistic. Although static thrust can be altered with exhaust diameter, 3 3/4 " seemed to be a real good compromise between static thrust and exhaust velocity.

DC

donniercjet

My Feedback: (10)

when making your ducting it will depend on your flying field, concrete or grass as to the diameter of exaust outlet. if you are on strictly concrete you can make the outlet a little smaller than if you were on grass.

Shaun Evans

Mr. Crooks,

What effect did removing those vanes have, exactly? Thanks in advance for your reply.

KC36330

i never tried it on a nitro fan, but on a brushless 6 blade EDF I've taken 3 blades off and it dramatically increases speed.

kc

iron eagel

Regarding the exhaust exit, if I am looking at a DF unit as being roughly on the order of 5 1/4 inches what size do I want to have the exit at?
For my application I know I may lose a lot of you jet guys here: max speed is not the issue (at least not with the design I am working on) thrust is the primary consideration.
Now the duct might be on the order of 5 feet long now I know this is going to cause a drop in velocity? Would it be better to have a longer intake of a large diameter tapering down to the DF unit? And a short duct to the exhaust, I really have no clue here, that much should be obvious to you guys. I am trying to learn on the fly, as I design my first airplane that uses ducted fan for power.

KC36330

max thrust on a ducted fan is with a bell mouth inlet and an unrestricted exhaust.

kc

iron eagel

Thanks KC

What I gather is: compress the air going into the compressor, and let it blow...

Your help has been invaluable!

KC36330

I'd prefer the term Funnel not compress.

kc

iron eagel

Point taken.

How much is it safe to funnel into the compressor?

Say the wide portion of the funnel is 3-5X the DF unit intake?

Again thanks for all the info...

If you are curious as to what all the questions are about check out "oddball design 101:name thruster", in the scratch build / design forum...

KC36330

a ducted fan is not a compressor, you only need 'enough' air to be feed to the fan so that you don't starve it of incoming air, if you try to ram air the intake you'll have all sorts of efficiency and mixture problems from the constant loading and unloading of the fan as air is forces into the duct, this is why ducted fans loose efficiency the faster they go, if they had a compressor stage that wouldn't be an issue.

kc

iron eagel

Thanks, that explains a lot.