Hi, guys I have some question about the exit hole in a canister's tunnel.

I'm building the H9 Edge 540 with the ZDZ 80, have a KS 89 canister muffler to be placed inside the fuse.

I have made a tunnel for it with a diameter of 85 mm, the canister has a diameter of 65 mm, look at the picture!

My problem is that I have also to make a curve for the air to exit on the bottom of the fuselage. No problem about the construction, but I wish to have some advice about the area and shape.

Do you think is better to gain higher pressure or higher speed just before the exit out of the fuselage ??

Because I have to shape the duct with an exit area bigger or smaller to obtain that.

I have seen many picture of german plane with a tunnel and exit curve like the one below, I understand that this is made by a german company, but I whant to do it myself because I think is not so difficult and I already have the duct now!

What can you tell me about that ?

Also a constant diameter till the exit can be considered, and is actually what I'm planning to do!

Moreover I don't like to live the hole at the exit as it is, but instead I wish to close the hole and make 10 opening divided in 2 row at 45 deg, just like the second picture but with a different number and dimension of the aperture.

Now the number and area of the 10 opening equalize the area of the duct (so no change in speed or pressure at the very and of the exit), but to obtain a sufficent area for cutting this opening, I have had to enlarge the duct at the end (gradually till the duct encounter the fuselage) so In this portion I have a pressure increase, but the air can't go anywere because the bottom of the fuselage let only escape the air allowed by the aperture size (that as said before is equal to the area of the duct).

Does it make any sense ?

I hope to have been clear enough, sorry for my english !!




Best Regards

Gianluca

The exit hole size and shape is best if it is made to create low pressure . louvers would work better than the flush cutouts you show.
The bottom of the plane is NOT a low pressure area so the exit holes should have some sort of angled edge/plate at front of the hole - to create the low pressure.
Air is not being blown in from the front!!!
It flows only when a strong low pressure area is at end of tunnel.
The air speed thru the tunnel will be constant -if the effective space remains constant.

Turbo ex outlet on the belly of the P47 Thunderbolt is a good example.

You definately do not want the exit hole area to increase, as that will lower the fluid velocity & raise the pressure. There will be some fluidic expansion due to heat rejection from the muffler, so either a constant area, or slightly convergent outlet will be OK.

Don't do the exit louvers -- this will act as a diffuse matrix & create a significant exit obstruction.

Thank you guys for your inputs.

The way I see this thing is as follow, thaking into account the Venturi effects and Bernulli rules :

Imagine the airplane flying, level flight at a constant speed:

Air enter the cowl from the hole on its front and most of the air follow its way to the exit hole of the cowl.

Some air enter the tunnel. (only at the initial stage of the process this is sure, because when to regime the dynamics should be different and it depends on how much air exit the tunnel at the end because as an extreme no air can enter the tunnel if no air is leaving it at the end, right!

Inside the tunnel the canister muffler exchange heat with the air passing (I don't know how much) and the air become hot, consequently it expands, but the space inside the tunnel doesn't allow this (well this is allowed only at the start of the process, but not when to regime), so the pressure and speed of the air increase!

At the end of the tunnel occupied by the canister muffler, the fluid see an increasing section because although the diameter of the tunnel is still constant, all the area occupied by the canister is now free for the air.

Consequently the air pressure increase more but the speed decrease!

At the end of the tunnel, if the tunnel section is still constant, what the air will do depends to the relation from this pressure and speed and the outside pressure and speed.

If the inside pressure is higher of the outside, it is just like if we have a depressure outside the exit hole (no need to create some sort of angled plate)

If the speed of the air inside is higher than outside, the air should naturally flow out due too the energy of the inside air, even if the outside air try to limit this flow because is at 90° of it.

The angled plates in my opinion are not ment to create a low pressure, but to guide the flow outside and limit the reaction of the external flow to this exit.

I don't know if this correspond to the real world, but is what I think about it.

Somebody know why there is a rule about the relation between the in and out area, I ear somthing like 1 to 3, why ??? Like if the game is to obtain a divergent tunnel !!

I don't know

Stop with the theory!
The idea of the large outlet is to make the job simple
More air can flow out of a larger hole
unless air flow is happening there - (you already know this )--no air flows in at front.
You DO want low pressure at outlet -
Make a lip in front of outlet - disturb the air at the lip and low pressure happens just above -in the hole
Cowl flaps on full scale , function in exactly this manner .

Dick,

regarding the support for the canister, I'm thinking to use only one support position at almost the center of the canister.

Where the cylinder is more thick, as you can see in the picture.

It seems to be the balancing point of the canister.

I will use 4 point to support the canister in that position, just like another guy have done.

What to you think ??

Do I'll need 2 supporting position, initial and end of the canister, or one is ok ??


Regards

Gianluca