I am going to use a rossi 80 or k/b 82 what is my inlet area and the exhaust area. I know that there is 5 1/4 in Di. I was thinking to use 5in inlet and 4 3/4 in outlet. does that sound right. please let me know the correct measurement.

Hi Angel,
I'm amazed that no one has responded to your question before now. There are some very knowlegable ducted fan people on this forum, and I don't consider myself one of them, but I'm confident that what I'm going to tell you is pretty close to right.

First, you need to think in terms of area rather than diameter. The fan has what is known as a "swept area". That is the area of the fan, minus the area of the cone in the center. On a Dynamax that area is approx. 16.25 sq. in. Most airframes that I've seen use an outlet diameter of 3.5" to 3.75". That equates to approx. 60 to 68% of the swept area.
If you were to have an outlet diameter of 4.75" you would have more outlet area than swept area, and you wouldn't get much speed out of the fan unit.

Some people may disagree with my figures, but I got them straight from a Dynamax fan using a vernier caliper to get the measurements, so unless I made some kind of mathmetical error, I believe they are accurate.
JA

The advice I'm about to give is more in regard to the fan and engine. You might consider getting a OS engine for that fan - more powerful and definitely more reliable. Also, the fan will give you better performance if you remove every other stator from the fan housing.

Adil

HI GUYS I thought more blades will give you more speed and thrust, for the reply the inlet 16.5 outlet about 3.5 to 3.75 if i was to plit them up in to two outlet ,would i keep the same area outlet as they are one . if so i am looking at 1.75 or 1.89 outlet each side. that pretty small.

I agree with Adil on the OS engine for the Dynamax although I have seen a couple of planes powered by the K&B .82 in the Dynamax that did well. Adil is saying to remove every other stator, not rotor blade. I have heard of this modification but I've never tried it myself. It does look like it would flow more air.
Are you scratch building an airframe or are you installing this in a kit?
I have made several bifurcated thrust tubes for Dynamax powered Miller F-4s and you want about 2 5/8" outlets. I built one that was only 2 1/4" outlets and the static thrust was reduced dramatically. On a single outlet tailpipe you can go a s small as 3 3/8". On my Regal Eagle bifurcated thrust tube, the 2 oulets are different sizes. I used an angled header to run the tuned pipe out the left outlet. I made that outlet larger to allow for the size of the tuned pipe. It works very well.

Hi Angel,
You're still thinking in terms of diameters. The 16.25 sq. in. is AREA, not diameter. the 3.5" -3.75" is diameter. They represent approx 9.62 - 11.00 square inches...
If you need two exhaust openings, they should equal the same AREA as the single openings.
two 2.5" diameter openings would give about the same AREA as one 3.5" diameter opening.
(2.5x2.5)x.7854x2=9.81 square inches.......3.5x3.5x.7854=9.62 square inches)

As far as the idea of removing every other stator vane, I have also heard of people doing this, and I can't see why it wouldn't help. A Ramtec fan has many fewer stator vanes than a Dynamax has.
JA

I am flying a JHH F-4 with the Dynamax + OS 91 with every 2nd stator removed. This combination has worked flawlessly for me and really pushes the F-4 around sky with authority. I have never run the Dynamax with all the stators so I really can't comment on the performanc differences, if any.

Regards,
Peter

You can't go wrong with the .91 O.S DF motor on a Dynamax fan. However you may want to do some testing on the ground if you plan to remove every other stator. A good bud of mine tried this with his YA A-4. Performance was YUCK![:'(]

Good luck!
Joey.

I Build a F-15 from scratch , i thing it has to much area inlet, and the outlet is 2.50in. on each side and i am use a mini pipe on a K/B 092082 wich i think eng. old but still has a very good compr,

How About the turbax-II from Jet Hanger bobbies can you compare the two?

I have flown Ducted Fans for many years. I used BVM, Dynamax, Turbax, and Ramtec. The Turbax is not close to the performance of the Dynamax or Ramtec. BVM is in a class of its own, both in price and performance.

Let me stick with your original question regarding the Dynamax. The K&B 82 and OS 91 operate the Dynamax unit just fine. There is no need to remove every other stator for operation. I have tried it both ways and didn't experience any significant performance change.

I'm not an aircraft designer, however, I measured the intake and exhause diameters of two of my old ducted fan jets in my shop. The intake areas average 22 square inches. The exhaust diameter is 3 5/8 inches. I think you will find this combination works great.

Hope this helps,
Greg

thank you very much guys , very helpfull. thanks.

eric dainty has practical test data showing improved thrust using 8 (i think) rotor blades.

no idea how many stator blades he used...im pretty sure they went unchanged.

his goal was static thrust though.

IMO you'd use the biggest engine possible with any fan design. i.e. always size the fan to the engine.

for a k &b 82 it'd yield around 3 hp wouldnt it?

since the dynamax gobbles up 4.8 hp at 23k or so and gives around 15lb at the effluex side of the fan. (this is actually true, wshen measured directly next to the cylinder head).

by the time it moves down a small thrust tube, it's more like 12-13lb.

now, 15lb at 23k means the thrust coefficient is found by:

Thrust = Thcoeff x (RPM/10000)^2

therefore 15 = tc x (2.3)^2
therefore tc = 2.84

assuming that 4.8 hp is absorbed at this rpm.

The power coefficient is

Power = Pcoeff x (RPM/10000)^3

4.8 = Pcoeff x (2.3)^3

Pcoeff = 0.3945

So, rpm for 3hp is 19,600.


the trick here is to ensure that your k&b gives 3 hp at 19,600 rpm.
thrust at 19600 is 11lb at efflux side.

and so on..

thanks for the math I keep this in mind for my next project.