Ducted Fan Efficiency discussions

[jsport]

This is LM Fury 1500, unveiled back in 2011 by - then - AME Unmanned Air Systems, flying in her previous iteration since 2008.

yeah a BWB designed for jet speed but a prop. oh and a thrust losing open prop at that . requires a major infrastructure launch and recover.

 

[AeroFranz]

I fail to see how the open propeller would then be universally used in applications ranging from zero to 400kts.

I can vouch for the cumbersome nature of launch and recovery. I heard people in the services calling the large and unwieldy mattress and net used for capture the 'circus tent'.

 

[jsport]

I fail to see how the open propeller would then be universally used in applications ranging from zero to 400kts.

I can vouch for the cumbersome nature of launch and recovery. I heard people in the services calling the large and unwieldy mattress and net used for capture the 'circus tent'.

Hoping your eluding to open propellers having low efficiency for both forward flight and VTOL.

 

[AeroFranz]

While it is true that you can design a ducted fan to produce more static thrust for the same amount of power of an open propeller of same diameter, typically the shape of the duct that you end up with does not perform well at higher speeds.
The duct itself represents a weight penalty as well as a source of additional friction drag.
You can widen the speed range over which the duct works efficiently by providing variable pitch for the fan, and/or controlling the exhaust nozzle area, but that of course adds complexity and weight.
Do you have technical references that support your theory?

 

[jsport]

While it is true that you can design a ducted fan to produce more static thrust for the same amount of power of an open propeller of same diameter, typically the shape of the duct that you end up with does not perform well at higher speeds.
The duct itself represents a weight penalty as well as a source of additional friction drag.
You can widen the speed range over which the duct works efficiently by providing variable pitch for the fan, and/or controlling the exhaust nozzle area, but that of course adds complexity and weight.
Do you have technical references that support your theory?

You appear to want to sideline facts as "my theory".
Fact is, as you stated, Duct fans produce more static thrust of the same diameter. Noone put speed in this equation. If you want speed use a jet. Yes, friction drag is an issue and doomed the Piaseki Uh-60. This drag and weight issue could be remedied by material science if industry were motivated to do such. Likewise complexity and weight issues hinder faster variable pitch duct fan or even fan/jet could also be overcome if industry were to be sufficiently motivated to accomplish such, given the current state of material science.

 

[_Del_]

This drag and weight issue could be remedied by material science if industry were motivated to do such.

You don't think there is sufficient motivation in the industry to find lighter, stronger, cheaper, more durable materials?

 

[jsport]

This drag and weight issue could be remedied by material science if industry were motivated to do such.

You don't think there is sufficient motivation in the industry to find lighter, stronger, cheaper, more durable materials?

w/ the exception of AVX Aircraft company helicopters have seen little in the way UAVs.

 

[AeroFranz]

You appear to want to sideline facts as "my theory".

TBH i was struggling to find a better word. How can i put this...
If these were "facts", then they would be documented in engineering textbooks, technical reports, and would find their way into hardware. Ducted fans have existed for at least a century and their principles are reasonably simple and well understood, yet only few specialized applications have emerged.

 

[sferrin]

You appear to want to sideline facts as "my theory".

TBH i was struggling to find a better word. How can i put this...
If these were "facts", then they would be documented in engineering textbooks, technical reports, and would find their way into hardware. Ducted fans have existed for at least a century and their principles are reasonably simple and well understood, yet only few specialized applications have emerged.

Yep. Where's our X-22? How many ducted propeller aircraft are in service? Why don't quad-copters have ducts? Hell, ducts could even be marketed as safety features yet none have them. Why not if they're so great?

 

[jsport]

Fact is, as you aerofranz stated, Duct fans produce more static thrust of the same diameter.

repeat as many times as you need.

 

[_Del_]

Sure. At what cost in application?

 

[AeroFranz]

Jsport, if all your vehicle does is hovering on one spot, then yes, ducted fans would be a good choice.
Especially if you are for one reason or another constrained in maximum dimensions, noise, or for safety reasons (you might survive bumping into things, as opposed to open props).
When you have to go somewhere else, things like propulsive efficiency in cruise (which is different from propulsive efficiency in hover), empty weight, and L/D also matter. On the whole aircraft designers have found that open props had a more favorable balance.

 

[jsport]

Jsport, if all your vehicle does is hovering on one spot, then yes, ducted fans would be a good choice.
Especially if you are for one reason or another constrained in maximum dimensions, noise, or for safety reasons (you might survive bumping into things, as opposed to open props).
When you have to go somewhere else, things like propulsive efficiency in cruise (which is different from propulsive efficiency in hover), empty weight, and L/D also matter. On the whole aircraft designers have found that open props had a more favorable balance.

you are well aware mass flow based thrust is what we are discussing otherwise jets would be some sort of open air props w fuel thrown in god knows where. .Props produce horrible large radar returns. "Favorable balance" , (surely you jest) how about lazy aircraft designers who use proven cheap but inefficent designs. The Fury being bought by Lockheed from a hobby ship. The Killerbee from Swift Enginnering (not an aircraft conpany) by Raytheon and also Northrop. Just path of least resistance, least cost, least effort, desperate small shops.. etc. lazy. Give me a break.

 

[quellish]

Props produce horrible large radar returns.

Props not optimized for a low radar cross section, sure. There is such a thing as a low observable propellor.

 

[jsport]

Props produce horrible large radar returns.

Props not optimized for a low radar cross section, sure. There is such a thing as a low observable propellor.

Isn't the large fast motion a pretty good doppler return especially if youre on the right freqs. Hard to cover.

 

[AeroFranz]

you are well aware mass flow based thrust is what we are discussing otherwise jets would be some sort of open air props w fuel thrown in god knows where.[...] "Favorable balance" , (surely you jest) how about lazy aircraft designers who use proven cheap but inefficent designs. The Fury being bought by Lockheed from a hobby ship. The Killerbee from Swift Enginnering (not an aircraft conpany) by Raytheon and also Northrop. Just path of least resistance, least cost, least effort, desperate small shops.. etc. lazy. Give me a break.

Fury was designed by Aeromech Engineering (before it turned into Chandler May, and was subsequently bought by Lockheed), based in San Luis Obispo, California. I know for a fact the designers were Aerospace engineers who graduated from nearby CalPoly State University. So at the very least they knew more than your average aircraft modelers. They're the same people who designed Desert Hawk (admittedly a hand launch), which has hundreds of thousands of operational hours.
KillerBee was designed by Mark Page during his time at Swift. Mark was an aerodynamicist at Boeing under famed Bob Liebeck, he might have even worked at Douglas before the merger. He is the lesser known father of BWBs.
I don't buy the assertion that people in the aerospace business are lazy; with few exceptions government awards are very competitive and if you don't sharpen your pencil you're not going to win, and thus stay in business very long. I do agree that proven technical solutions are often chosen to reduce risk (and cost), but new technologies are eventually embraced if the perceived benefits provide a competitive advantage.

 

[jsport]

you are well aware mass flow based thrust is what we are discussing otherwise jets would be some sort of open air props w fuel thrown in god knows where.[...] "Favorable balance" , (surely you jest) how about lazy aircraft designers who use proven cheap but inefficent designs. The Fury being bought by Lockheed from a hobby ship. The Killerbee from Swift Enginnering (not an aircraft conpany) by Raytheon and also Northrop. Just path of least resistance, least cost, least effort, desperate small shops.. etc. lazy. Give me a break.

Fury was designed by Aeromech Engineering (before it turned into Chandler May, and was subsequently bought by Lockheed), based in San Luis Obispo, California. I know for a fact the designers were Aerospace engineers who graduated from nearby CalPoly State University. So at the very least they knew more than your average aircraft modelers. They're the same people who designed Desert Hawk (admittedly a hand launch), which has hundreds of thousands of operational hours.
KillerBee was designed by Mark Page during his time at Swift. Mark was an aerodynamicist at Boeing under famed Bob Liebeck, he might have even worked at Douglas before the merger. He is the lesser known father of BWBs.
I don't buy the assertion that people in the aerospace business are lazy; with few exceptions government awards are very competitive and if you don't sharpen your pencil you're not going to win, and thus stay in business very long. I do agree that proven technical solutions are often chosen to reduce risk (and cost), but new technologies are eventually embraced if the perceived benefits provide a competitive advantage.

Neither design seem to government favorites. Our source on the Killerbee and the Fury airfoils expressed the CFD on these convex underside BWBs is more suitable for jet speeds than a prop speed craft.
Our source was a senior F-35 engineer and a DARPA PM who is now a Professor at a pretty good school so will disagree on even the shape of KB or Fury. Not to mention both required vertical surfaces to control. That may be fixed by now but there are better prop driven BWB designs.

 

[AeroFranz]

Jsport, the beauty about aeronautical sciences is that you can derive results from known basic physics. For example, most of propeller and ducted fan theory can be derived from simple conservation of momentum equations, in less than five steps.
Can you point me to a text book, or provide me with the equations that show that at high speeds ducted fans are more efficient?

BTW, does that DARPA PM teach at UMD by any chance?

 

[jsport]

Jsport, the beauty about aeronautical sciences is that you can derive results from known basic physics. For example, most of propeller and ducted fan theory can be derived from simple conservation of momentum equations, in less than five steps.
Can you point me to a text book, or provide me with the equations that show that at high speeds ducted fans are more efficient?

BTW, does that DARPA PM teach at UMD by any chance?

Open props throw air perpendicular as well as in the direction of intended mass flow ie there is loss. Duct fans limit that phenomenon. Why does that need a text book.

Why would the preference for Duct fans exist in the RC community if open props were just as good and alot cheaper.
He works at Va Tech.

 

[_Del_]

I started having flashbacks that we've already done this circus of repeatedly highlighting one aspect and ignoring all context, but it was over tractor vs pusher.

 

[jsport]

Props are pressure differential w/ loss off the tips while Duct Fans are mass flow no loss off the tips. Duct fans can have near jet military performance and can even being deployed from jets able harvest jet flow for flight. Open props have little military utility for future UAV military performance. props are non stealth for radar and generally IR.


http://164.100.133.129:81/econtent/Uploads/09-%20Ducted%20Fans%20and%20Propellers%20%5BCompatibility%20Mode%5D.pdf

 

[AeroFranz]

yeah, i had dejavu and couldn't remember what it was about...

Open props throw air perpendicular as well as in the direction of intended mass flow ie there is loss. Duct fans limit that phenomenon. Why does that need a text book.
Why would the preference for Duct fans exist in the RC community if open props were just as good and alot cheaper.

Because you are glossying over a bunch of things and using language that, forgive me for saying that, does not suggest full understanding of what's going on. For one, what you call "perpendicular flow", most likely refers to "swirl" in aero speak. You point out correctly (without actually explaining how and why) that it is mostly taken out by stators in ducted fans. In open props it's not a whole lot of loss, certainly less than what you incur for the weight and drag of the duct. Speaking of which, there's also internal friction losses inside the duct where air is scrubbing the centerbody and the inner walls.
At low speeds or static thrust, you want a duct exhaust that's (ideally) diverging, to get maximum thrust for a given power. As you increase speed, the exhaust wants to decrease in cross-section, which can be done but requires variable geometry.
At any rate, please find attached a page from Kohlman "Introduction to V/STOL airplanes" that qualitatively explains the issues of ducts at forward speeds.

The reason i'm asking for equations is because handwaving does not a proof make. Equations derived from basic physics do.

In the RC world, most people don't fly models because they're cheap or perform well; they fly them because they're cool. If your model depicts an F-14, it looks very stupid with a prop in the nose, but it looks and sounds cool with a ducted fan. There's also the fact that it's harder to cut your fingers or injure people on the ground with ducted fans.

Is it just me or the link to the reference of the previous post does not load? [edit: never mind, it finally opened]

 

[quellish]

Isn't the large fast motion a pretty good doppler return especially if youre on the right freqs. Hard to cover.

No, not "hard to cover" at all. Why would it be?
The bigger problem is interaction with the rest of the structure, but again this is solvable. Very low observable propellors have been around for a long time.

 

[jsport]

Isn't the large fast motion a pretty good doppler return especially if youre on the right freqs. Hard to cover.

No, not "hard to cover" at all. Why would it be?
The bigger problem is interaction with the rest of the structure, but again this is solvable. Very low observable propellors have been around for a long time.

dont buy it.

 

[jsport]

yeah, i had dejavu and couldn't remember what it was about...

Open props throw air perpendicular as well as in the direction of intended mass flow ie there is loss. Duct fans limit that phenomenon. Why does that need a text book.
Why would the preference for Duct fans exist in the RC community if open props were just as good and alot cheaper.

Because you are glossying over a bunch of things and using language that, forgive me for saying that, does not suggest full understanding of what's going on. For one, what you call "perpendicular flow", most likely refers to "swirl" in aero speak. You point out correctly (without actually explaining how and why) that it is mostly taken out by stators in ducted fans. In open props it's not a whole lot of loss, certainly less than what you incur for the weight and drag of the duct. Speaking of which, there's also internal friction losses inside the duct where air is scrubbing the centerbody and the inner walls.
At low speeds or static thrust, you want a duct exhaust that's (ideally) diverging, to get maximum thrust for a given power. As you increase speed, the exhaust wants to decrease in cross-section, which can be done but requires variable geometry.

Thank you for the reference. ...may not know much for that is for sure but.. starting from the bottom up though. Duct fans are for near jet speeds not low speed. As for the weight, friction, drag of a duct being so large as to equal your "swirl" loss at high speed open prop.. just dont buy it at all. Your just wrongly speculating. Depends on the build. rendering this an academic exercise of marginal value.

 

[_Del_]

Show me the math so I can "buy it.".

 

[jsport]

Show me the math so I can "buy it.".

and one would do that in an open forum why exactly?

 

[sferrin]

Thank you for the reference. ...may not know much for that is for sure but.. starting from the bottom up though. Duct fans are for near jet speeds not low speed. As for the weight, friction, drag of a duct being so large as to equal your "swirl" loss at high speed open prop.. just dont buy it at all. Your just wrongly speculating. Depends on the build. rendering this an academic exercise of marginal value.

What is your explanation for the distinct lack of ducted propellers in service?

 

[sferrin]

Show me the math so I can "buy it.".

and one would do that in an open forum why exactly?

To support your claims. If not the math at least show evidence that would support your position. So far all we're seeing is denials and assertions with nothing to back it up.

 

[_Del_]

If you're going to accuse the entire industry of speculating instead of doing the math on the tradeoffs involved regarding pusher ducted-fans, I'd like to know that you've done the math to show them instead of speculating.
I'm a big believer in math over speculation, so by all means, let's topple the lazy status quo with some numbers.

 

[kagemusha]

Props are pressure differential w/ loss off the tips while Duct Fans are mass flow no loss off the tips.

You could integrate a sort of ring duct with the propeller.

https://patents.google.com/patent/US20160159458

https://patents.google.com/patent/US20170029091

 

[Sundog]

The reason for the lack of ducted fans in service is;

1) Cost. You're building more structure, which costs more money.
2) Weight. More structure also means more weight.
3) Drag. The gains in propeller efficiency doesn't necessarily outweigh the added drag of the duct.
4) The duct is usually airfoil shaped, which also means it can stall just like a wing and lead to rather odd control effects at the edges of the flight envelope.
5) The reason you haven't seen it on production aircraft is all of the trade studies show the majority of designs are better without them than with them.