GolfOscarDelta
Topic Author
Posts: 102
Joined: Fri Feb 01, 2008 5:17 pm

Why No Ducted Fans And Props?

Sun Apr 11, 2010 11:08 pm

I've been reading up on ducted fans ever since i came across the recent thread about the Astafan and have had a few questions pop up and was wondering if any of you nice folks could clarify

In the past i had read (mostly on the internet and other forums) and also heard from my professors that ducted fans are generally ineffective above around 100 kts since the duct then creates more drag than the thrust benefit it provides (at lower speeds).

However since i read about the Astafan, i've gone underground and dug up some work done by Hamilton Standard (as a contractor for NASA) on a concept called the Q-Fan in the early 70's which shows that ducted fans are better than or on par with standard props (and better for noise) so based on that i have a few questions:

History is littered with many ducted fan examples but we still don't see any flying around

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1. Why are ducted fans not used on current general aviation (pistons) aircraft?
I'm guessing that the added weight and complexity of the structure needed to support the duct imposes a larger fuel penalty than the gains offered by it, i just want to confirm that this is indeed the case.

So in case that ducted fans are not efficient why aren't we seeing advanced props on GA pistons. What i mean by this is most piston engine GA's i see mostly have nearly "rectangular blades". Now i understand that props with larger "spans" are better but they are limited either by:
  • a. Wave drag on the tips
    or
  • b. Ground Clearance of the props

I've noticed that the new Cirruses(Cirrii?) and the Cessna Corvallis have varying chord props but

2. Why is that we don't see scimitar shaped (a la A400M) or some other fancy props to reduce both wave drag and induced drag (may be raked prop tips since "proplets" are obviously impossible) or something more efficient on GA pistons?

3. What about props on larger jets? Now I understand that fans on turbofans serve to compress air while also pushing it out the back but when talking about open rotors etc. why is that we see no duct? Kinda like this


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I guess what I'm trying to ask is why don't we see ducted turboprops? Is the reason the same as why ducted props aren't seen on pistons (i.e. weight penalty offsets drag benefits)

Thanks,
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Why No Ducted Fans And Props?

Mon Apr 12, 2010 1:04 am

Quoting GolfOscarDelta (Thread starter):
History is littered with many ducted fan examples but we still don't see any flying around

You do, in effect, they're just not usually called ducted fans.

Quoting GolfOscarDelta (Thread starter):
1. Why are ducted fans not used on current general aviation (pistons) aircraft?
I'm guessing that the added weight and complexity of the structure needed to support the duct imposes a larger fuel penalty than the gains offered by it, i just want to confirm that this is indeed the case.

Most GA aircraft are too small for it to pay off.

Quoting GolfOscarDelta (Thread starter):
So in case that ducted fans are not efficient why aren't we seeing advanced props on GA pistons. What i mean by this is most piston engine GA's i see mostly have nearly "rectangular blades".

Part of it is the relatively short blade length...the speed variance between hub and tip gets bigger as the prop diameter goes up. For small props, the simplicity of manufacture (read: cheapness) may overcome the potentially marginal gains. Keep in mind that even a basic Cessna doesn't have truly "rectangular blades", they are twisted and have changing chord.

Quoting GolfOscarDelta (Thread starter):
2. Why is that we don't see scimitar shaped (a la A400M) or some other fancy props to reduce both wave drag and induced drag (may be raked prop tips since "proplets" are obviously impossible) or something more efficient on GA pistons?

Scimitar doesn't do much for induced drag, and I doubt many GA props are going fast enough for wave drag to be a problem.

Quoting GolfOscarDelta (Thread starter):
3. What about props on larger jets? Now I understand that fans on turbofans serve to compress air while also pushing it out the back but when talking about open rotors etc. why is that we see no duct?

A duct on an open rotor makes the already difficult mounting problem even worse. Also, the biggest efficiency benefit of an open rotor is the huge bypass ratio, which means a huge (and heavy!) duct.

Quoting GolfOscarDelta (Thread starter):
I guess what I'm trying to ask is why don't we see ducted turboprops?

You do...it's called a geared turbofan.

Tom.
 
GolfOscarDelta
Topic Author
Posts: 102
Joined: Fri Feb 01, 2008 5:17 pm

RE: Why No Ducted Fans And Props?

Tue Apr 13, 2010 12:33 am

Quoting tdscanuck (Reply 1):
Keep in mind that even a basic Cessna doesn't have truly "rectangular blades", they are twisted and have changing chord

Agreed. By "rectangular" i guess i meant their twist and chord is not as much as the twist and chord you see on advanced turboprop blades or turbofan blades.

Quoting tdscanuck (Reply 1):
the simplicity of manufacture (read: cheapness) may overcome the potentially marginal gains.

Excellent point, I should have thought of that. I've been reading quite a bit about ducted fans and their performance for the last few days and I was thinking only in terms of performance gains on props, completely forgot about manufacturing and cost.

Quoting tdscanuck (Reply 1):
You do, in effect, they're just not usually called ducted fans

I guess technically turbofans can be called ducted fans. I should have stated in my previous post that I meant only piston engines geared to fans in a shroud/duct. Are there any such examples flying around currently? The only one i came across was a duct conversion on a Long-EZ.

Thanks for the great reply Tom
 
DiamondFlyer
Posts: 2165
Joined: Wed Oct 29, 2008 11:50 pm

RE: Why No Ducted Fans And Props?

Tue Apr 13, 2010 4:00 am

Quoting GolfOscarDelta (Thread starter):
2. Why is that we don't see scimitar shaped (a la A400M) or some other fancy props to reduce both wave drag and induced drag (may be raked prop tips since "proplets" are obviously impossible) or something more efficient on GA pistons?

Oh, there are lots of planes out there with a scimitar shaped prop. Take a look at the various prop manufactures, and you'll see that quite a few planes have an STC for them. But honestly, the costs of certifying new equipment and idea's for GA is often too expensive to make it worth the risk. There simply isn't the volume to make up the costs of development, which is why we are still flying around on essentially 1950's engine technology.

-DiamondFlyer
From my cold, dead hands
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Why No Ducted Fans And Props?

Tue Apr 13, 2010 5:09 am

Quoting GolfOscarDelta (Reply 2):
I guess technically turbofans can be called ducted fans.

Basically, yes. The only difference between a ducted fan and a turbofan, in usual parlance, is how you built the gizmo driving the fan. A turbofan typically has much higher solidity (disk loading) than a ducted fan though, mostly due to the much higher power density of turbine engines compared to pretty much anything else.

Quoting GolfOscarDelta (Reply 2):
I should have stated in my previous post that I meant only piston engines geared to fans in a shroud/duct. Are there any such examples flying around currently?

I can think of lots of industrial examples (hovercraft love these), but the first one to pop into my head was this critter:
http://cdn-www.airliners.net/aviation-photos/photos/3/7/9/0612973.jpg

Tom.
 
Fly2HMO
Posts: 7207
Joined: Sat Jan 24, 2004 12:14 pm

RE: Why No Ducted Fans And Props?

Tue Apr 13, 2010 6:33 am

Quoting GolfOscarDelta (Thread starter):

2. Why is that we don't see scimitar shaped (a la A400M) or some other fancy props

You sure about that?

http://www.studentpilotjournal.com/uploaded_images/6-18-2008=CirrusFlight-003-760753.jpg



And those are all stock props, no STCs. Hardly your typical old school props like you'll find in a C172, and these three planes will be having supersonic tip speeds easily at full power. That's why these three planes are pretty damn loud from the outside.

Quoting GolfOscarDelta (Thread starter):

2. (may be raked prop tips since "proplets" are obviously impossible)

Wrong again.

http://www.piperpa30.com/images/props/pa30-speedspinners3.jpg

Nope. That's not a prop strike. Hartzell has taken that "proplet" concept to good use and has been making Q-tip props (squared of tip, like a winglet) for decades.
 
GolfOscarDelta
Topic Author
Posts: 102
Joined: Fri Feb 01, 2008 5:17 pm

RE: Why No Ducted Fans And Props?

Tue Apr 13, 2010 1:06 pm

Quoting Fly2HMO (Reply 5):
You sure about that?
.
.
.
And those are all stock props
Quoting GolfOscarDelta (Thread starter):
I've noticed that the new Cirruses(Cirrii?) and the Cessna Corvallis have varying chord props

Hahah yeah I've seen the Cirrii and the Corvallis forgot about the Ovation though.

Quoting Fly2HMO (Reply 5):
Hartzell has taken that "proplet" concept to good use and has been making Q-tip props (squared of tip, like a winglet) for decades

Hmmm didn't know these existed.

Quoting GolfOscarDelta (Thread starter):
since "proplets" are obviously impossible

I'll be darned, I only thought of forward facing proplets and imagined they would interfere with the airflow, didn't think about rearward facing proplets.
 
Fly2HMO
Posts: 7207
Joined: Sat Jan 24, 2004 12:14 pm

RE: Why No Ducted Fans And Props?

Tue Apr 13, 2010 4:34 pm

Quoting GolfOscarDelta (Reply 6):
I'll be darned, I only thought of forward facing proplets and imagined they would interfere with the airflow, didn't think about rearward facing proplets.

Actually if you look closely at the Mooney's prop I posted, the tips are raked back ever so slightly.

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