Solved Question: The Physics of Thrust Tubes (Solved?)

[Air-headed Aviator]

Hello Forums 🖐️ I was wondering if any of you have a resource of combination of resources that can help me understand the physics of thrust tubes on EDFs.

I of course understand that their use along with a properly designed inlet improves/increases the thrust you receive out of a fan. I understand the general sizing guidelines, and have even adjusted those guidelines based of real world turbofan aircraft. I know that the thrust tube helps increase the effective pressure of a fan, and reduces the swirl from the blades, but that's nearly where my understanding ends. While I have guidelines for sizing, I don't know what forces encourage those guidelines. I don't know what physics can help specify how long or how tapered you would want to make a thrust tube on a Ducted Fan, and my current research provides little clarity.
Do any of y'all have more insight? Resources on how to calculate the pressure build up of a fan? Information on what qualities of a fan determine the length of the thrust tube? Equations that describe how much performance is lost or gained from their inclusion? For my preference I prefer substantiated info over anecdotes. Anecdotes are plenty useful and effective, but not clear on their origin and how they're based on reality. Thanks to anyone who can assist.

 

[Merv]

That sounds like a question for @Mid7night .

 

[quorneng]

Air-headed Aviator
I can't help with any clever mathematics but what you are asking involves a huge number of variables.
What I can say from observation & experience is that a bare EDF (case but no thrust tube) and with a bell mouth generates the maximum static thrust but it will have the lowest efflux velocity. This suggests any additional duct length generates a velocity loss and a thrust reduction.
How fast the plane needs to fly determines the required efflux velocity.
A thrust tube with a "nozzle" adds to the efflux velocity but decreases static thrust although some thrust may be regained from the dynamic pressure resulting from the planes forward motion through the air. How much pressure recovery will depend on the inlet geometry as well as the length and configuration of the inlet duct.
You can see that the "best" EDF installation depends on many features some of which might well be limited by the physical configuration of the plane, its ducting and wing loading.
Finally remember that a duct configuration that works well for fast full size jets may not be relevant at the speed RC EDFs fly.

 

[SlingShot]

I know that @Mid7night wrestled with this problem and you should let him tell you his conclusions. My limited experience revealed that the thrust tube reduction should be very little if not zero. The more air available to breathe, the better.

 

[SlingShot]

Finally remember that a duct configuration that works well for fast full size jets may not be relevant at the speed RC EDFs fly.

I believe this to be the critical declaration. The fancy calcs don't really apply to our speeds.

 

[Piotrsko]

Calcs actually apply. Issue is the data is so tiny that it is close to not measurable so you cant ascertain if you measured it or not. From my experiences, you invariably choose a wrong data point outside the target data

 

[quorneng]

I have experimented quite a bit following the principle that air viscosity is very significant at model sizes as the slow moving boundary layer is the same regardless of duct diameter. It follows that the smaller the duct diameter then proportionally greater is the effect of the boundary layer.
As the exhaust duct diameter is set by the EDF diameter to minimise boundary layer effect the exhaust duct should be short when compared to the inlet duct where the diameter is not restricted.
Taking this to the extreme the EDF should be placed close to the exhaust nozzle and the inlet made as large a diameter as possible. An inlet duct with an area 1.2 x the FSA is practical.
In a scale plane true scale inlets can be rather small meaning either over size inlets and duct or be limited to a suitable smaller EDF. The common practise to get over this is to incorporate cheat holes in the fuselage to allow a bigger EDF to breathe more freely however such an arrangement does not fully restore the airflow from an adequate duct.
My scale DH "Swiss" Venom follows this theory using a small for its size 50mm EDF such that scale wing root inlets give 1.2 x the FSA. In addition the EDF is set well back in the fuselage with a short 25mm long thrust tube reducing to just the FSA. The long and complex inlet duct is very carefully 3D printed to give a constant 1.2x FSA and to be as 'free flowing' as possible smoothly changing from twin "triangular" wing root inlets combining to match the EDF casing.

It has a span of 1016mm.
It is worth noting that with the limited thrust of turbo jets in the 1950s duct length losses were an issue so the layout of the Venom actually favours an "efficient" EDF installation.
With a light foam airframe it flies very well requiring only a modest 1400mAh 4s for 4+ minute flights!

 

[Piotrsko]

Hmmm. fsa of 1.2 hints that the airspeed decreases and static pressure increases. Or not... this is model aerodynamics

 

[SlingShot]

I have experimented quite a bit following the principle that air viscosity is very significant at model sizes as the slow moving boundary layer is the same regardless of duct diameter. It follows that the smaller the duct diameter then proportionally greater is the effect of the boundary layer.
As the exhaust duct diameter is set by the EDF diameter to minimise boundary layer effect the exhaust duct should be short when compared to the inlet duct where the diameter is not restricted.
Taking this to the extreme the EDF should be placed close to the exhaust nozzle and the inlet made as large a diameter as possible. An inlet duct with an area 1.2 x the FSA is practical.
In a scale plane true scale inlets can be rather small meaning either over size inlets and duct or be limited to a suitable smaller EDF. The common practise to get over this is to incorporate cheat holes in the fuselage to allow a bigger EDF to breathe more freely however such an arrangement does not fully restore the airflow from an adequate duct.
My scale DH "Swiss" Venom follows this theory using a small for its size 50mm EDF such that scale wing root inlets give 1.2 x the FSA. In addition the EDF is set well back in the fuselage with a short 25mm long thrust tube reducing to just the FSA. The long and complex inlet duct is very carefully 3D printed to give a constant 1.2x FSA and to be as 'free flowing' as possible smoothly changing from twin "triangular" wing root inlets combining to match the EDF casing.
View attachment 256854

View attachment 256855
It has a span of 1016mm.
It is worth noting that with the limited thrust of turbo jets in the 1950s duct length losses were an issue so the layout of the Venom actually favours an "efficient" EDF installation.
With a light foam airframe it flies very well requiring only a modest 1400mAh 4s for 4+ minute flights!

I like the look of that thing. Fly good?

 

[Air-headed Aviator]

Physics Based Theory on EDF Thrust Tubes and Inlets

Forward: For a week I've been attempting to research whether there were physics based rules one could use to size the inlet and exhaust of EDF power systems. I've searched through a variety of theorems, principles and concepts and after many brain straining nights I think I've finally crafted a...

forum.flitetest.com

 

[quorneng]

SlingShot
Indeed it does! And rather unusual for an EDF it does not even need full power for a hand launch despite only using a 50mm EDF.
A low wing loading and modest speed does allow manoeuvring in quite a small field with plenty of tall trees!

The Swiss aerobatic team colour scheme stands out well and it glides pretty well too.

 

[Piotrsko]

It's model sized. Most nicely derived calculations are incorrect mostly because that data you've collected is not accurate for using in full scale formulations.