# Ducted Fan Pseudo-Jets

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#### EzyBuildWing

##### Well-Known Member
DF flying on improved MSFS

#### nestofdragons

##### Well-Known Member
@Malish

I used to have several of Bob Hovey's books, especially those on structure and aerodynamics that involved the Whing Ding II. At one point I had his phone number, but his wife said he was no longer interested in aircraft and was totally focused on breeding roses. Wish I'd been able to talk to him myself.

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I just came across an RC Ducted Fan of the Martin X-24A Lifting Body... Aerofred has it as the "Mystery Wing" It was designed by the same people that designed the X-24A Lifting Body.

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If i recall right this project was nicknamed Pumpkin Seed due to its shape. It contained gas filled zones.

#### sanman

##### Well-Known Member
What about Forward-Swept Wing design being used with Ducted Fan?
Can that be feasible?

You've all pointed out how a shorter duct is better for reducing duct drag.

With forward-swept wing, the root of the wing is closer to the rear of the aircraft, so that duct intake can be positioned back there too, at the start of the root.
This will mean shorter duct length.

Look at Grumman X-29 or Sukhoi Su-47 Berkut

(Incidentally, the X-29 design is a relative of the F-5 and F-20 designs which I am a fan of, although you can see its ducts are still longer)

These designs all look very cool as well. What's wrong with ducted fan version of Forward Swept Wing fighter?

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#### Woodenwings

##### Well-Known Member
High speed stalls are not what you might expect! You will understand if you made an RC one of those!

#### cluttonfred

##### Well-Known Member
Supporting Member
For the relatively low speeds (compared to actual jets) likely for this type of aircraft, I think unswept wings make much more sense. Look to the early single-engine, subsonic jet fighters of the late 1940s and 1950s: Lockheed Shooting Star, Grumman Panther, Hawker Sea Hawk, de Havilland Vampire/Venom, Mig-15/17/23, Lavochkin I-150, etc.

#### J.L. Frusha

##### Well-Known Member
For the relatively low speeds (compared to actual jets) likely for this type of aircraft, I think unswept wings make much more sense. Look to the early single-engine, subsonic jet fighters of the late 1940s and 1950s: Lockheed Shooting Star, Grumman Panther, Hawker Sea Hawk, de Havilland Vampire/Venom, Mig-15/17/23, Lavochkin I-150, etc.

I forget the name given to the 3D designed twin ducted-fan biplane... Oracle II? The guy may eventually make an RC model. I suggested a few changes. No idea if he ever modified the design. If built, it would never be an ultralight without major changes.

#### Malish

##### Well-Known Member
What about Forward-Swept Wing design being used with Ducted Fan?
Can that be feasible?

You've all pointed out how a shorter duct is better for reducing duct drag.

With forward-swept wing, the root of the wing is closer to the rear of the aircraft, so that duct intake can be positioned back there too, at the start of the root.
This will mean shorter duct length.

Look at Grumman X-29 or Sukhoi Su-47 Berkut

(Incidentally, the X-29 design is a relative of the F-5 and F-20 designs which I am a fan of, although you can see its ducts are still longer)

These designs all look very cool as well. What's wrong with ducted fan version of Forward Swept Wing fighter?

Both of those aircraft have very long air ducts.
Also, it's possible to design just one DF system to work with different type of aircraft, but it would be very expensive to build number of different air frames! Better to come up with one air frame that looks like many different aircraft - that what we did designing our PJ-II "Dreamer"...

#### vhhjr

##### Well-Known Member
Supporting Member
There are three reasons that I can think of to put swept wings on an airplane design:
1. To manage the shock wave starting at trans-sonic speeds. (Not usually important on a homebuilt.)
2. To manage the C.G. (Can be fixed in the design phase to avoid wing sweep.)
3. It just looks cool. (Highly subjective and very important)

Unfortunately, swept wings can introduce some problems:
1. It's more difficult to design and build a torsionally stiff swept wing than a straight one. (This can add weight, our biggest enemy)
2. Forward swept wings have to be very stiff to avoid flutter that can be rapidly divergent. (Wing separation to be avoided as it wipes out the coolness factor)
3. Aircraft with forward swept wings may require active stability control to keep them pointing in the right direction. (Inflight end swapping to be avoided.)
3. Swept wings can suffer from higher spanwise airflow than non-swept ones resulting in flow separation and aileron stall. Some early Russian fighter designs had serious flow fences to deal with this problem.

1. They look cool.
2. In flying wings, wing sweepback causes the ailerons to be more effective.
3. Downward pilot visibility may be improved.
4. They add at least 20% to the flyby airspeed as percieved by ground observers.
5. I'm sure you can think of additional ones.

Vince Homer

#### J.L. Frusha

##### Well-Known Member
There are three reasons that I can think of to put swept wings on an airplane design:
1. To manage the shock wave starting at trans-sonic speeds. (Not usually important on a homebuilt.)
2. To manage the C.G. (Can be fixed in the design phase to avoid wing sweep.)
3. It just looks cool. (Highly subjective and very important)

Unfortunately, swept wings can introduce some problems:
1. It's more difficult to design and build a torsionally stiff swept wing than a straight one. (This can add weight, our biggest enemy)
2. Forward swept wings have to be very stiff to avoid flutter that can be rapidly divergent. (Wing separation to be avoided as it wipes out the coolness factor)
3. Aircraft with forward swept wings may require active stability control to keep them pointing in the right direction. (Inflight end swapping to be avoided.)
3. Swept wings can suffer from higher spanwise airflow than non-swept ones resulting in flow separation and aileron stall. Some early Russian fighter designs had serious flow fences to deal with this problem.

1. They look cool.
2. In flying wings, wing sweepback causes the ailerons to be more effective.
3. Downward pilot visibility may be improved.
4. They add at least 20% to the flyby airspeed as percieved by ground observers.
5. I'm sure you can think of additional ones.

Vince Homer

Delayed Stall is usually the 1st reason.

#### Malish

##### Well-Known Member
Unfortunately, swept wings can introduce some problems:

a) Higher TO and landing speed.
b) Higher AOA at low(TO and landing) speed.
That is why modern aircraft have less swept wings:

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#### J.L. Frusha

##### Well-Known Member
a) Higher TO and landing speed.
b) Higher AOA at low(TO and landing) speed.

Depends on the plane and reasoning. I'm planning a FSW Ultralight with LE Slats and junkers style control surfaces. Why? STOL and higher angles of TO and landing is an advantage, flying from a small farm.

#### Woodenwings

##### Well-Known Member
Lifting Line theory Prandtl wings would be nice to see more of!
I made the 3D model below just to see what all the fuss was about.
if you have space for a high aspect ratio machine - do it!

Gooooogle Prandtl and Bowers, spend a few hours getting your head around it! Brilliant stuff!
i was thinking of making a longrange FPV drone but spent the money on a real plane!

It would be a nice ducted-fan model!

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#### Riggerrob

##### Well-Known Member
What about Forward-Swept Wing design being used with Ducted Fan?
Can that be feasible?

You've all pointed out how a shorter duct is better for reducing duct drag.

With forward-swept wing, the root of the wing is closer to the rear of the aircraft, so that duct intake can be positioned back there too, at the start of the root.
This will mean shorter duct length.

Look at Grumman X-29 or Sukhoi Su-47 Berkut

(Incidentally, the X-29 design is a relative of the F-5 and F-20 designs which I am a fan of, although you can see its ducts are still longer)

These designs all look very cool as well. What's wrong with ducted fan version of Forward Swept Wing fighter?

Dear sanman,

Duct length is not tied to wing sweep.

The Grumman X-29 pictured above has forward-swept wings and engine intake ducts as long as a Northrup F-50 or F-20.
IOW its intake ducts are 2/3 the length of the fuselage. If it did not need to fly supersonic, then it could use engine ducts as short as an A-10.
Incidentally, the X-29 uses an F-5 forward fuselage to shorten the design process.

As others have pointed out forward - or aft - swept wings introduce a whole other raft of structural and aerodynamic problems that homebuilders would be wise to avoid.
Did we mention "deep stall?"
Stalls start with turbulent airflow detaching from the leading edge. If the wing is swept, that turbulence will migrate with the sweep. That means that turbulent flow (aka. localized stall) will migrate towards the wing tips of an aft-swept wing ...
OTOH If wing tips stall first - on a forward-swept wing - that turbulence will migrate towards the center trailing edge.
Either way, you lose lift at the rear-most part of the wing, creating a deep stall that is difficult to recover from. Too often it results in a flat spin. Curtiss Ascender was a WW2-vintage canard. They lost two Ascender prototypes during flat spins. The fix was adding additional wing area at the wing tips. The additional wing area moved the center-of-lift aft. This additional area was outboard of (almost) tip rudders. Those fixed fins sort of acted like stall fences, but they were not enough to tame Ascender's nasty spin characteristics. Burt Rutan's Long Eze canard suffered similar deep stall problems which were fixed with small stall fences that looked like the engine pylons on airliners.

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#### J.L. Frusha

##### Well-Known Member
*Tip Stall...

From wikipedia -

Stall characteristics
Any swept wing tends to be unstable in the stall, since the wing tips stalls first causing a pitch-up force worsening the stall and making recovery difficult. This effect is less significant with forward sweep because the rearward end carries greater lift and provides stability.

However, if the aeroelastic bending is sufficient, it can counteract this tendency by increasing the angle of attack at the wing tips to such an extent that the tips stall first and one of the main characteristics of the design is lost, on a conventional wing the tips always stall first. Such a tip stall can be unpredictable, especially where one tip stalls before the other.

Composite materials allow aeroelastic tailoring, so that as the wing approaches the stall it twists as it bends, so as to reduce the angle of attack at the tips. This ensures that the stall occurs at the wing root, making it more predictable and allowing the ailerons to retain full control.

##### Well-Known Member
For the relatively low speeds (compared to actual jets) likely for this type of aircraft, I think unswept wings make much more sense. Look to the early single-engine, subsonic jet fighters of the late 1940s and 1950s: Lockheed Shooting Star, Grumman Panther, Hawker Sea Hawk, de Havilland Vampire/Venom, Mig-15/17/23, Lavochkin I-150, etc.

Ooh I'd love to see a ducted fan DH vampire replica.
It's tiny for a jet fighter, so you wouldn't need to scale it down much.

#### J.L. Frusha

##### Well-Known Member
Ooh I'd love to see a ducted fan DH vampire replica.
It's tiny for a jet fighter, so you wouldn't need to scale it down much.

#### TiPi

##### Well-Known Member
Supporting Member
Ooh I'd love to see a ducted fan DH vampire replica.
It's tiny for a jet fighter, so you wouldn't need to scale it down much.
A 2/3 (67%) version is on my bucket list, to fit in the 95.10 category (300kg MTOW, 10m2 wing area). At that scale, the Vampy has 10m2 wing area and could come in at the 300kg if built very light. DF with 60-70hp engine or 75kg (750N) jet The challenge for the DF will be the limited air intake area, might have to cheat a bit there.

##### Well-Known Member
A 2/3 (67%) version is on my bucket list, to fit in the 95.10 category (300kg MTOW, 10m2 wing area). At that scale, the Vampy has 10m2 wing area and could come in at the 300kg if built very light. DF with 60-70hp engine or 75kg (750N) jet The challenge for the DF will be the limited air intake area, might have to cheat a bit there.

The Vampire started out with 9.3 kN (~900 kg) thrust, according to Wiki.
MiniJets are up to 100 kg now...

#### TiPi

##### Well-Known Member
Supporting Member
The Vampire started out with 9.3 kN (~900 kg) thrust, according to Wiki.
MiniJets are up to 100 kg now...
The Goblin 3 had about 14.9kN (1,500kg) thrust, MTOW was 5,600kg so a thrust/weight ratio of 0.27 (FB.6)
The RAAF F.30/FB.31 with the Rolls Royce Nene engine had 2,260kg thrust and MTOW of 5,940kg, giving a thrust/weight ratio of 0.38
A 300kg scale model needs 80-115kg thrust for the same ratio. \$ go up exponentially with those small jet engines
The Swiss Air Force was still using the DH-115 and the FB.6 as the primary jet trainer when I went through in the early 80s, have seen many close-up at our airfield. I was trying to master the Aluette II at that time (and failed miserably).