# Ducted Fan Pseudo-Jets

### Help Support Homebuilt Aircraft & Kit Plane Forum:

#### Dan Thomas

##### Well-Known Member
Alright, point taken - hydrogen is difficult to store compactly. Meanwhile, methane, propane, butane are significantly easier to store, while having comparatively lower carbon content relative to other hydrocarbons - that means less CO2, and also less engine coking problems. They also offer comparatively higher specific impulse, though not as high as hydrogen.
Methane is essentially natural gas and requires high-pressure storage. Propane and butane will liquify under lower pressures and are easier to store.
I'm no chemist, but I believe that none of those generate the energy per pound that gasoline does, meaning that you'll burn more of them per mile and likely emit just as much CO2 as the gasoline does. No gains there. I had a dual-fuel truck years ago, gasoline and propane, and the thing had considerably less power running on the propane.

#### Vigilant1

##### Well-Known Member
Meanwhile, methane, propane, butane are significantly easier to store, while having comparatively lower carbon content relative to other hydrocarbons - that means less CO2, and also less engine coking problems.
It is technically not difficult to maintain the MANY advantages of liquid (at STP) hydrocarbon fuels (Jet A, gasoline diesel, etc) while contributing zero carbon to the atmosphere. If the fuels are made with carbon from the atmosphere to begin with (biofuels, etc), then burning them adds zero net carbon. >IF< we (collectively) think this is important and are willing to make the many sacrifices needed to achieve it (don't brush that aside, it is huge, measured in reduced prosperity and life-years), this IMO is a much more practical, faster, and less expensive way to achieve it than changing the engine of virtually every powered air vehicle now in use, and rolling out an entirely new energy infrastructure to support it.

Last edited:
PMD

#### BBerson

##### Light Plane Philosopher
Propane and hydrogen have more energy per pound than gasoline, but propane has less energy by volume (like hydrogen) so hence the lower power when using propane vapor. The new propane trucks with liquid propane injection do have more power. A dual fuel truck would be less power on propane without the variable compression propane would need to be optimal.
So, a simple optimized propane truck is almost ideal.

Last edited:

#### nicknack

##### Well-Known Member
If consider cost of power plant and maintenance - this aircraft will have no use for average GA pilot/owner
And for use as a trainer aircraft no one will need it too
No GA or average pilot will fly a ducted aircraft, it has no advantage or appeal whatsoever compared to a cessna or one of the new LSA propeller planes. It doesn't sound like a jet, sounds exactly like a vacuumn cleaner and will likely be banned from most GA airports because of the sound.

Premise for the turboshaft UL39 is as an alternative to the BD-5J or Subsonex JSX-2T. In both these examples the jet engine costs ~ 50k, are aerobatic certified engines and reliable power plants. These two jets have anywhere from 250nm to 400nm range. There's a definable niche market. A two seater will be easier to use as a trainer for the owner to transition to using their experimental jet. Ie the manufacturer can set up a training/transition program with an instructor in one of the other seats. It's harder with a single place seat.

Also the subsonex 1 seat does not cost 1million, closer to 200k to 300k, maintenance should be relatively okish for a simple small jet engine.

If spending a million dollars, get a used turbo-prop instead and then deal with the huge maintenance cost.

#### cheapracer

##### Well-Known Member
Log Member
Jetson is trying to do their electric version with a more stripped down look, like a flying dune-buggy:

But there's no way batteries can beat hydrocarbon fuel.
How do you power 8 or more small engines to give the safety factor that batteries can and do, with fuels ....

Still don't understand why these designers won't put the center of mass way below the blade line or blade plane (not sure what you call it) for stability and control.

#### sanman

##### Well-Known Member
Well then how in the world could you do a jet-like piston-engine design without a ducted fan? Oh, right…. ;-p

View attachment 118504View attachment 118503

I do indeed find the Pseudo-Jet styling of SF-1 Archon to be very appealing - including the clever way it embeds the pusher prop.
However its VNE is listed as 155mph, and if we try to make the prop go faster, then at some point the prop noise will escalate hugely, as tip speeds go transonic. Would scimitar blades help? How much would they help?

Pj-2 Dreamer with its ducted fans lists a VNE as 350kph(217mph) with 310hp engine.
Ducted Fans can help to mitigate the noise problem.

But you all point out the duct drag problem that escalates as airspeeds go significantly over 100mph.

What about reducing duct drag by making the ducts themselves proportionately larger, like on the Mig31 Foxhound? That jet has really huge intakes, and imitating that style seems fine to me for a Pseudo-Jet, since we're not looking for low radar cross-section, blahblah.

I love the look of SF1 Archon, I love the look of PJ2 Dreamer - but I like the Mig31's look too.
They all have nice jet aesthetics.

I don't understand why turbofans don't suffer from similar drag problems when they have ducts.
It seems to me that the only difference between Ducted Fan and Turbofan is the power source and the exhaust temperature & velocity.

If hotter exhaust flows better, what about ionizing the flow through the duct in order to reduce drag?

Last edited:

#### BJC

##### Well-Known Member
No GA or average pilot will fly a ducted aircraft,
I’ve seen one HBAer fly one. Looked good.
will likely be banned from most GA airports because of the sound.
The sound of the one that I saw fly was not objectionable, but the few rotary engined propeller airplanes were.
If spending a million dollars
I think that I know where one could buy a real L39 for less than that.

BJC

#### Riggerrob

##### Well-Known Member
Dear sanman,
It is relatively easy to increase the diameter of the propeller on an ARCHON style pseudo-jet. Just define prop diameter early and design the rest of the plane around it. Though I suspect that you will make better progress by increasing the number of blades. Companies like Whirlwind will cheerfully supply props with up to 6 blades for homebuilt airplanes or swamp boats. Maintaining a 72 inch diameter propeller/fan and spinning it less than 2500 rpm - the same as certified Continental, Lycoming, Rotax, etc. - will keep prop tips sub-sonic.
The pseudo duct also reduces the noise heard by spectators.
Much of the noise emitted by modern airliners comes from the front face of the turbo-fan. I can hear airliners adjust power and I can see the fan at the same time. When the airliner has past me, I can no longer hear fan noise. Airline manufacturers (e.g. Rohr) devote much effort to noise absorbing materials in the front parts of engine nacelles.

Archon is powered by a stock Rotax engine ... like half the other kitplanes. Rotax come from the factory with propeller speed reduction untis that keep stock prop tips sub-sonic,
I suspect that the primary reason that Archon is limited to 150 ish mph is its market. Archon is aimed at private pilots or light sport pilots who are only good enough to fly simple light planes - not much more than ultra-lights.

#### Vigilant1

##### Well-Known Member
However its VNE is listed as 155mph, and if we try to make the prop go faster, then at some point the prop noise will escalate hugely, as tip speeds go transonic. . .
Pj-2 Dreamer with its ducted fans lists a VNE as 350kph(217mph) with 310hp engine.
Sanman:
VNE has nothing to do with the aircraft's installed HP. Zero.

As a suggestion, perhaps go to a local FBO and take a few hours of instruction in an airplane. See if you like it. If you do, take a ground school course and pass the written exam for a private pilot license. You'll learn about aerodynamics, the national airspace system, a lot of other cool stuff. And you may discover and appreciate that the folks who design airplanes, big and small, are a pretty smart bunch and haven't somehow overlooked the scores of great ideas you keep hatching.
Enthusiasm is great, but it is most useful when accompanied by hard work and humility.

Last edited:

#### Malish

##### Well-Known Member
I think that I know where one could buy a real L39 for less than that.
Yes, real L-39 can cost 500,000 USD, but owner will spend $2000+ per flying hour only for fuel + maintenance! - that what I saying about aircraft with turbo jet engines Our DF aircraft uses about$100 fuel per hour and have little maintenance cost and it's looks and fly like jet aircraft

Last edited:

#### PMD

##### Well-Known Member
Oh, boy....afraid to introduce something else into this conversation....but - fools rush in, as they say.

I have built more than a hundred "ducted fans" in my airboat days. With about an inch of clearance between the prop and duct wall, I doubt we ever had any aerodynamic/thrust improvement, but the real reason was to protect people from the prop and provide some yaw stability with aero surfaces aft of CofG. (convergent yaw stability both aero and hydrodynamically was one of our trademark design standards).

My close encounters of the duct variety and the observed noise from large engine cooling fans takes me to another place where the duct-to-blade tip clearance is zero - i.e. a "ring shroud". These are if not common at least well known in the cooling fan business (I believe in attempt to cut noise from tip turbulence) but does anyone know what the tradeoff of end plate effect vs. axial component of drag from forward velocity comes to equilibrium?

#### BJC

##### Well-Known Member
I think that I know where one could buy a real L39 for less than that.
Should also note that I know where one might buy custom wing tips to replace the tip tanks on the L39 and achieve lower induced drag at high Cl. They look good, too.

BJC

#### henryk

##### Well-Known Member
the duct-to-blade tip clearance is zero -
=NOTCHED duct,
=STEPPED duct...

BTW="to protect people from the prop "

=kevlar composite ring inside notch/step...?

#### Attachments

• 40.9 KB Views: 6

#### PMD

##### Well-Known Member
=NOTCHED duct,
=kevlar composite ring inside notch/step...?
Would have to think about how much deflection of blade in axial plane had to be accommodated. Also allowance for thermal and load related radial distortion of the duct. Interesting to think about, though.

BTW: Why Kevlar? (I tend to lean towards Basalt fiber these days).

#### Malish

##### Well-Known Member
My close encounters of the duct variety and the observed noise from large engine cooling fans takes me to another place where the duct-to-blade tip clearance is zero - i.e. a "ring shroud". These are if not common at least well known in the cooling fan business (I believe in attempt to cut noise from tip turbulence) but does anyone know what the tradeoff of end plate effect vs. axial component of drag from forward velocity comes to equilibrium?
It's will be difficult to build HI RPM ducted fan with ring over tip blades -structure and balance problems due to heavy centrifugal force. Best to try use "notched" duct(as henryk suggested), but this may have problems with construction too - DF blade tips flexing a lot and notch in the duct must be wider then tip tract. This is too will affect fan efficiency...

#### PMD

##### Well-Known Member
It's will be difficult to build HI RPM ducted fan with ring over tip blades -structure and balance problems due to heavy centrifugal force. Best to try use "notched" duct(as henryk suggested), but this may have problems with construction too - DF blade tips flexing a lot and notch in the duct must be wider then tip tract. This is too will affect fan efficiency...
This is where the question of how many blades comes in. Yes, they need to be EXTREMELY light, but adding blades might allow reducing diameter and bending loads from rotation of the "ring duct"

#### sanman

##### Well-Known Member
Dear sanman,
It is relatively easy to increase the diameter of the propeller on an ARCHON style pseudo-jet. Just define prop diameter early and design the rest of the plane around it. Though I suspect that you will make better progress by increasing the number of blades. Companies like Whirlwind will cheerfully supply props with up to 6 blades for homebuilt airplanes or swamp boats. Maintaining a 72 inch diameter propeller/fan and spinning it less than 2500 rpm - the same as certified Continental, Lycoming, Rotax, etc. - will keep prop tips sub-sonic.
The pseudo duct also reduces the noise heard by spectators.
Much of the noise emitted by modern airliners comes from the front face of the turbo-fan. I can hear airliners adjust power and I can see the fan at the same time. When the airliner has past me, I can no longer hear fan noise. Airline manufacturers (e.g. Rohr) devote much effort to noise absorbing materials in the front parts of engine nacelles.

Archon is powered by a stock Rotax engine ... like half the other kitplanes. Rotax come from the factory with propeller speed reduction untis that keep stock prop tips sub-sonic,
I suspect that the primary reason that Archon is limited to 150 ish mph is its market. Archon is aimed at private pilots or light sport pilots who are only good enough to fly simple light planes - not much more than ultra-lights.
Gee, if they're going to style the aircraft in a certain way, they ought to be able to at least go to 200mph. Perhaps they could come up with a more robust airframe in some enhanced later version.

Sanman:
VNE has nothing to do with the aircraft's installed HP. Zero.
Apologies for confusion - VNE is due to airframe - I guess I didn't express my thoughts properly. I only meant that the Archon as a whole was not designed to fly very fast, as compared to PJ-2 Dreamer with its much sleeker airframe and larger powerplant. If your customer wants to fly something that looks like a jet, then it's also likely they'd want jet-like speed.

As a suggestion, perhaps go to a local FBO and take a few hours of instruction in an airplane. See if you like it. If you do, take a ground school course and pass the written exam for a private pilot license. You'll learn about aerodynamics, the national airspace system, a lot of other cool stuff. And you may discover and appreciate that the folks who design airplanes, big and small, are a pretty smart bunch and haven't somehow overlooked the scores of great ideas you keep hatching.
Enthusiasm is great, but it is most useful when accompanied by hard work and humility.
Haha, fair enough - sorry if my ignorance clouded the discussion. I appreciate the advice of those with more experience.

This is where the question of how many blades comes in. Yes, they need to be EXTREMELY light, but adding blades might allow reducing diameter and bending loads from rotation of the "ring duct"
Wouldn't you have to be mostly concerned about static friction? Couldn't the right holes in the duct around the ring enable air bearings to reduce dynamic friction?

#### Riggerrob

##### Well-Known Member
Kevlar is currently fashionable in jet engine cowlings. They use a Kevlar belt to contain burst turbine discs. Kevlar is available in long fibres. I have packed hundreds of (reserve) parachutes with Kevlar suspension lines.

#### Riggerrob

##### Well-Known Member
Dear Sanman,
Archon is aimed at the low-cost, low-speed corner of the envelope that private pilots or LSA pilots can afford. Many of them also lack the skills to fly 200 mph airplanes.
People who "feel the need for speed" can buy a stock L-39 for a lower price than developing an entirely new kitplane with a ducted fan.

#### Riggerrob

##### Well-Known Member
This is where the question of how many blades comes in. Yes, they need to be EXTREMELY light, but adding blades might allow reducing diameter and bending loads from rotation of the "ring duct"
Reducing diameter will create more problems than it solves.
Think lawyers versus engineers.
The biggest problem with small diameter fans or turbines is the massive tip losses caused by even a tiny gap between fan tips and the inner surface of the duct.
Archon avoids this problem with a stock (say 72 inch) diameter propeller hiding in a pseudo-duct. From the sides, bottom and front, it looks like a jet fighter. The pseudo-duct is mainly for show as tip gaps are far too large to contribute to thrust. The pseudo-duct helps mask spectators from prop noise ... reinforcing the "stealthy" exterior.