IS THERE A SIMPLE WAY TO DETERMINE THE SIZE OF THE EXHAUST OUTLET (THRUST TUBE)??
TO MAKE A THRUST TUBE, WHAT WEIGHT OF CLOTH?? HOW MANY LAYERS?? WHAT'S THE BEST (EASIEST) WAY USING GLASS??
IS AN INLET DUCT REQUIRED 0N A SABRE?? OR KEEP EVERYTHING OUT OF THE WAY?
HOW CAN I BE SURE THAT THE INTAKE OPENING ON THE SABRE WILL BE BIG ENOUGH TO ALOW ENOUGH AIR IN WITHOUT CHEATER HOLES.
I KNOW EVERYBODY HAS AN OPINION, SO LETS HEAR IT.
THANKS GEORGE

As a general rule of thumb, the outlet should be about 75% of the fan area. Say you have a 5" fan, 5" x .75" = 3.75" for your outlet.

In most cases you will always benefit from ducting to the fan. Open it up as much as you can....the more air to your fan the beter!! Just roll 2oz cloth over some wax paper, and then brush or spread on the resin!!

Good Luck!!

Joey.

JOEY;
THANKS FOR THE TIPS.
AM I CORRECT TO ASSUME THAT A SMALLER EXIT WILL PRODUCE GREATER SPEED AND A LARGER WILL GIVE A BETTER STATIC THRUST?? IF IT IS TRUE, HOW SMALL CAN YOU GO BEFORE IT GETS "CHOKED OUT" ?? HOPE THIS MAKES SENSE.
THANKS
GEORGE

A 5" diameter fan has 19.62 square inches of area. 75% of this would be 14.73 square inches or a diameter of 4.33". It is a bit more math, you have to use pi and square roots.

Steven

The smallest single outlet for a 5" Dynamax fan that I have seen used is 3 3/8". The JMP Starfire used this size. I've built a couple of Regal Eagles with a single 3 1/2" outlet. That seemed to be a good compromise between speed and static thrust (for that particular plane used off of grass).

2 5/8" seems to be the magic number for a bifurcated pipe. I built one that was 2 5/16" for an F-4 one time and it had just enough power to get off the ground. But hard as I tried, I couldn't keep it off the ground very long.

PHANTOM2:

As you choke the outlet your compression losses are a lot greater than your thrust gains from higher air velocity.

And on the left side of your keyboard, about 1/2 way up, there is a key labeled "Caps Lock." Press it.

Bill.

Everything I've read about the math gives an outlet of about 90% of the fans swept area.
This figure from both the books:

"Ducted fans for model jets" by David James,
And

" R/C Ducted fans" by Frank Fanelli.

Great books...spend the money!

Hey Phantom,
Curious, what kinda plane are you building? Anyway, for a "simple" formula to determine outlet size, the fan diameter x .75 works like a charm almost every time.

regards,
Joey

joey;
first, sorry for the small print. i don't want bill to get upset lol.
i was'nt yelling.
The kit is a "D&B" models F-86 SABRE.
It was made in the early '70s. It shows to be a very scale outline.
But it gives you the option to make a scale exhaust outlet (size wise) or have it huge.
They (D&B) say to use a 40-60 DF with a "SCOZZI FAN" I think thats how you spell it.
They don't use an inlet tube. But i wanted to use one. They show a cheater hole through the nose gear door. The wing is one piece, which bolts to the fuse. No hatch on top, the engine is mounted inverted.
At this point, it will have leading edge slats, flaps, speed brakes, retracts. But skill and ambition will play a part in that decision. It will be built as the "H" version, 3 guns each side and dihedral at the horizontal.
It is roughly 1/7 scale.
64" span, 62" length
If anybody has any info or experience with this kit and or suggestions. let me know. I plan on starting to build it in a week or so, I'm finishing up my 35% CARDEN EXTRA 300s
Thanks for the replies.
GEORGE

George:

First, thank you.

The outlet area will probably be best somewhere between scale and "Huge." As has been almost said, the outlet free area should be at least as large as the fan's swept area. Any restrictiion causes a real drop in efficiency, and can cause overheating due to reduced air flow.

Down and dirty 100% accurate is the area of the fan circle, and subtract the area of the hub. 75% of the fan diameter is usually close enough.

On the front side, the more intake area, and the smoother the flow, the better. There is a practical limit, of course. A rule of thumb, again - inlet area twice the outlet area.

Enjoy the plane.

Bill.

Bill;
When you say "Inlet area twice the outlet area" do you mean that the inlet should be twice as big as the outlet?? or the other way around??
How do other scale Sabres with DF's deal with this??
I don't want it to look like a "Parkinson Jet".
TIA
George

George:

Let's use some real numbers, maybe it will come clear - not that tou couldn't get it, but that my explanations and pontifications were not as clear as they should have been.

We have a 5" Dynamax fan. The area of the fan circle is 19.63 sq". The hub is 1.5" diameter. area 7.07 sq". Now subtract the hub area from the total fan circle area, you get the "Fan swept area," 12.57 sq". Now working that backwards, we find an open circle 4" in diameter has the same area. So our outlet should be a 4" circle. Pretty close to our 75% at 3.75" diameter, and close enough.

The intake duct needs to have at least the same size as the swept area of the fan, and since pulling the air is not as effiecient as pushing it, combined with air drag in the duct, it works best if the intake area is 1.5 times the swept area. And here we have a pleasant surprise. If you can make the inlet duct a smooth tube the same size as the outside diameter of the fan, you are near as D****t. With the setup we've taken the numbers for, 150% of the swept area is 18.85 sq", and that's a diameter of 4.9". How's that for close to 5"?

But that would look terrible on a Sabrejet, wouldn't it? So make the nose intake 3" diameter, and leave the door off the nose gear, use that as your cheater hole. Total of the two holes should be at least our 18.85 sq".

A good test for duct sizes. Run the engine with fan in the open, check full throttle rpm. Then install the power unit and recheck the rpm. Higher, the intake is restricted. Lower, the exhaust is too small. This is not totally accurate, though, the inlet and exhaust could both be too small, and one would cancel the other.

All this is aimed at having zero restriction on both sides of the fan, and all I have given is empirical, with just a small grounding in real engineering with your calculators and computational machinery. But it works.

Have fun with it, DF is a different world.

Bill.

WOW!!
I have a headache now.
Strange as it seems, that made sense. I will see how large i can make the inlet without hacking the scale looks.
thanks
George

Take two aspirin...

Haw.

Bill.

Bill;
What about this, be patient.
if i had the cheater through the door, it would be hard to have a inlet duct-- correct??.
what if for take off , let's say. the door is connected to a "door" on the inlet tube. then when the wheel door closes, the "door" on the inlet tube closes also, keeping the inlet clean. then work off of "ram" pressure. while i will still enlarge the inlet itself, to a certain degree. whatcha think??
if you respond, put the dictionary down. i don't think i could go through that again.
thanks
George

George:

My almost unbelievable vocabulary is such that bibliographic reference is not required for the formation of an obfuscatory response.

On first thought your idea sounds like a real "Eureka" moment.

On second thought I remember reduced intake area equals reduced air flow equals reduced thrust and reduced engine cooling.

In other words, if you have enough thrust with the door open to take off, and 60% power, or however much you have with the door shut, is plenty for normal flight, go for it.

You might try it first with the nose gear door off, get everything trimmed, so forth, get used to the plane, and then try it with the door shut.

If it works, you're home free. But do listen for engine sag, or any other overheat signs. If there's a lack of power you wont have any trouble seeing that.

So overall, I think it's a great idea to play with.

Bill.

Not to throw a wrench into your calculations but, a hub with diameter of 1.5" has an area of pi(r*2)=1.77 sq" not pi(d*2)=7.07 sq" which makes an outlet diameter of 3.75" only 62% of the Swept Fan Area of the Dynamax fan. I've read that you shouldn't go below 75% SFA for the outlet area but grbaker said he has seen and used outlets smaller than 3.75" with the Dynamax. 100% SFA is plenty for intake area for a model flown off pavement. If you want to fly off grass, more intake area will help.

Craig:

Apologies, sir. I did indeed square the diameter instead of the radius.
Sorry.

Bill.

Craig & William:
Sorry to disturb you two.
Let me throw my "simpleton" observation in here.
A buddy of mine has a Y/A Starfire, the exhaust outlet is 3 3/8".
Back to me now. I plan on running an O.S 77 w/ Dynamax.
Would a bigger engine, for example, K&B 1.00 create more problems, such as, weight versus horsepower, and would any intake/exhaust issues arise. being a more powerful engine, would it require more air??
thanks George

I hope the OS.77 you have has an aftermarket connecting rod in it. The .77 had a problem with the stock rod. It won't live very long in a Dynamax. The K&B1.00 does not have that great of a reputation in the Dynamax either. It works well in a Byron or Ramtec where the RPMs are lower. The .77 will work well, if it has an upgraded rod in it. RPM was the manufacturer of the rod, but I don't think they're in business anymore. I would try to find a deal on a OS.91 if I could afford it. You won't regret it.

Make your inlet as smooth and large as possible. I Think on the Yellow Aircraft A-4 the nose wheel opening is used as a cheater inlet as William mentioned earlier. Some kit manufacturers used to recommend spring loaded "blow-in" doors on the cheater hole. Miller recommended 2 of them on his T-38 kit, just ahead of the Byron fan unit. I built it without any cheaters and decided if I determined I needed them, I could cut them out later. The plane got off the ground in about 150' without them. If you've got plenty of runway, try yours without a cheater. You can always go back and re-engineer.

One more word of advice: Seek advice from experienced ducted fan pilots. Sometimes it's hard to tell the difference between the person that has jet experience and the person that thinks he knows all about it.

PHANTOM2:

Ducting dimensions, both intake and exhaust, are governed by the fan's physical size, the power has no bearing. At least until your fan goes supersonic. With a 5" fan your limiting speed is right at 40,000 rpm, where the blade tips go over Mach. All sorts of strange things happen to the air flow when you get those shock waves bouncing around, up to and including total disintegration.

If you ever wondered what the intake doors were for on a supersonic plane, they limit the intake air flow to ensure the air velocity into the compressor is subsonic.

In other words, use you engine of choice, but keep the rpm below 40K with a 5" fan.

And just an additional note: your 5" fan, with a 12" pitch, has an air velocity well over 300 mph at 35K rpm. And a 10" pitch is right at 300 mph at the same rpm. I'm sure you can see that restriction of the flow, whether from being too small or a rough surface, will make for big losses.

Bill.

Gary;
Funny you mention the rod. on my second flight, with my y/a f-4, my K&B 1.00 decided to throw the rod. I had just taking off, about 20' up. needless to say, the engine siezed, spun my fan off, and down she went. not pretty.
I have experience with df's, but most of them were produced in "modern" facillities. a little more math might of been used.
maybe???

Bill;
I don't think i have to worry about my O.S 77 going over 40k.
as gary stated, the rod will fail before that.
I should start work on the sabre next week. my carden extra is just about done.

Thanks George