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Semi unducted fan.

Posted: Thu Feb 11, 2021 9:16 am
by Taxi645
Semi-unducted fan.

Ok, my knowledge of turbojets is very limited, but I’m still curious so went along with it anyway. From the open rotor wiki page (as a true wiki ranger), we see a number of ways to reduce the noise of an open rotor design:

General methods for reducing noise include:

1. lowering the blade tip speeds
2. decreasing the blade loading, or the amount of thrust per unit of blade surface area.
For contra-rotating propfans, which can be louder than turboprops or single-rotating propfans, noise can also be lowered by:
3. increasing the gap between the front propeller and the back propeller;
4. making sure the blade lengths of the back propeller are shorter than those of the front propeller, so that the back propeller blades avoid cutting through the blade top vortices of the front propeller (blade-vortex interaction);
5. using a different number of blades on the front propeller compared to the back propeller, to avoid acoustic reinforcement; and
6. turning the front propeller and back propeller at different rotational speeds, also to prevent acoustic reinforcement.


I was thinking wouldn’t a semi-unducted fan fix all these issues (at the expense of ultimate efficiency off course)? This would be a turbofan, unducted fan hybrid with a very low (old school) inter-nacelle bypass ratio to keep nacelle size small and reduce noise from the fast spinning ungeared fan inside the nacelle. The unducted fan is connected through a direction and revolution changing gearbox. The inner-nacelle part of the unducted fan is doing extra work by converting rotational energy from the ducted fan into forward thrust.

Don’t be too harsh on the exact proportions in the image. It was meant as an indication how it would look like.

Image

I will address the above 6 points one by one:
1. lowering the blade tip speeds
Work from the unducted fan can be offloaded to the ducted fan, so the rotational speed of the unducted fan can be reduced because it doesn’t have to provide all the power.

2. decreasing the blade loading, or the amount of thrust per unit of blade surface area.
Work from the unducted fan can be offloaded to the ducted fan, so blade loading can be reduced.

3. increasing the gap between the front propeller and the back propeller;
The blade tip vortices of the unducted fan don’t interact with the rest of the engine because it’s at the rear of the engine and has a far larger diameter anyway. The gap between the ducted fan and the inner-nacelle part of the unducted fan is very large because both are ate the other end of the engine (also they are inside the nacelle aiding noise reduction).

4. making sure the blade lengths of the back propeller are shorter than those of the front propeller, so that the back propeller blades avoid cutting through the blade top vortices of the front propeller (blade-vortex interaction);
The blade tip vortices of the unducted fan don’t interact with the rest of the engine because it’s at the rear of the engine and has a far larger diameter anyway. (The ducted fan and inner-nacelle part of the unducted fan do interact but over a larger distance and inside the nacelle aiding noise reduction).

5. using a different number of blades on the front propeller compared to the back propeller, to avoid acoustic reinforcement; and
The unducted fan has no contra rotating counter part so that’s no issue. Inside the nacelle this is easily achievable as well.

6. turning the front propeller and back propeller at different rotational speeds, also to prevent acoustic reinforcement.
This is accomplished in this design because the inner-nacelle art of the unducted fan is connected to the gearbox like the unducted part of the fan.


Basically what your doing is avoiding all the noise issues of two contra-rotating unducted fans by instead offloading work to an small bypass ratio fan inside the nacelle. What do the engine experts reckon? Why would this design not work? Not enough gain compared to a GTF? Impossible to have a proper bearing system? Gearbox too hot? Other reasons?

Re: Semi unducted fan.

Posted: Fri Feb 12, 2021 10:19 am
by Armadillo1
still dont understand concept.
compared to undectef fan GE36/578-DX you want to accelerate inner flow instead of using it to power fan?

Re: Semi unducted fan.

Posted: Fri Feb 12, 2021 12:16 pm
by Taxi645
Armadillo1 wrote:
still dont understand concept.
compared to undectef fan GE36/578-DX you want to accelerate inner flow instead of using it to power fan?


Don't overestimate my own understanding of turbojets. The idea is to use a small old school low bypass inner nacelle fan to take load off the unducted fan (say 35-40%) and to avoid noise interference between two contra rotating unducted fans. The schematic is just a very rough indication.

A rough calculation seems to indicate that theoretical fuel savings compared to a GTF would only be about 5%, so probably not worth the extra noise. Compared to two contrarotating unducted fans the consumption would be around 12% higher although you would save the weight, resistance and cost of a gearbox for one of the two contra rotating unducted fans.

Re: Semi unducted fan.

Posted: Sat Feb 13, 2021 8:22 pm
by 77west
Would having the gearbox right in the hottest area of the core not be problematic?

Re: Semi unducted fan.

Posted: Sun Feb 14, 2021 12:00 am
by Aircellist
Somehow your drawing reminds me of the engines on the Convair 990 Coronado… Except for the open part of the fan.

Re: Semi unducted fan.

Posted: Sun Feb 14, 2021 9:01 am
by Taxi645
77west wrote:
Would having the gearbox right in the hottest area of the core not be problematic?


Yes, as pointed out in the opening post. I don't know how well they would be able to isolate it from the exhaust heat without sacrificing efficiency or weight too much.

Alternatively a design with the unducted fan at the front could also be possible. That said, I don't know if the idea has merit in the first place, hence my question in the first post..


Aircellist wrote:
Somehow your drawing reminds me of the engines on the Convair 990 Coronado… Except for the open part of the fan.


it is not really my drawing, it was a quick photoshop job on the different turbine engines on the unducted fan wiki page: https://en.wikipedia.org/wiki/Propfan

Image

Re: Semi unducted fan.

Posted: Mon Feb 15, 2021 2:42 am
by MohawkWeekend
And the GE700's from the Falcon 20. Your AFT fan is bigger though and outside the cowl. Neat idea.

Re: Semi unducted fan.

Posted: Wed Feb 17, 2021 11:06 am
by seahawk
So kind like a new General Electric CJ805-23

Re: Semi unducted fan.

Posted: Sun Feb 21, 2021 10:32 pm
by CowAnon
Basically what your doing is avoiding all the noise issues of two contra-rotating unducted fans by instead offloading work to an small bypass ratio fan inside the nacelle. What do the engine experts reckon? Why would this design not work? Not enough gain compared to a GTF? Impossible to have a proper bearing system? Gearbox too hot? Other reasons?

Not an engine expert, but anything with a small bypass ratio makes me skeptical. Propulsion through the expansion of hot air is less efficient than through a fan/propeller pushing air backward.

I think the noise and vibration issues from open rotor engines are overblown. The General Electric GE36 from the late 1980s was designed to met and exceed the noise regulations at the time, and it still would've nominally met the tighter regs that went into effect in 2006. The complaints today are more about noise quality than noise quantity, in my understanding.

Re: Semi unducted fan.

Posted: Mon Feb 22, 2021 5:20 pm
by Taxi645
CowAnon wrote:
Basically what your doing is avoiding all the noise issues of two contra-rotating unducted fans by instead offloading work to an small bypass ratio fan inside the nacelle. What do the engine experts reckon? Why would this design not work? Not enough gain compared to a GTF? Impossible to have a proper bearing system? Gearbox too hot? Other reasons?

Not an engine expert, but anything with a small bypass ratio makes me skeptical. Propulsion through the expansion of hot air is less efficient than through a fan/propeller pushing air backward.


Yeah, that's why you would keep the work of the low bypass fan inside the nacelle to the minimum needed to take just enough load of the propeller.

CowAnon wrote:
I think the noise and vibration issues from open rotor engines are overblown. The General Electric GE36 from the late 1980s was designed to met and exceed the noise regulations at the time, and it still would've nominally met the tighter regs that went into effect in 2006. The complaints today are more about noise quality than noise quantity, in my understanding.


I wouldn't be able to say. Like you I don't know enough about jet engines to make useful estimates.