Simple pusher pseudo-jet

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Aerowerx

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Looks like a BD-5.
Might be easy with electric motor in the back and batteries in the front for balance.
Except....

Having masses distributed like you suggest can greatly affect the stability and controlability. It is best to have the mass as close to the CG as possible.
 

nerobro

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Except....

Having masses distributed like you suggest can greatly affect the stability and controlability. It is best to have the mass as close to the CG as possible.
I'm not sure that's a problem. I think you might just end up with a plane that rides "nice". If I am to pick up what you're putting down, are you referencing the problem with jets, and roll coupling?
 

Vigilant1

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My 2 cents:
- Of the planes that look jet-ish" from the outside, the Vampire looks best and is a practical configuration. Anything with gingerbread tacked on strictly for looks is a non-starter.
- I think eschewing the onlookers and just going for a jet "feel" for the pilot (on a budget) is best. That, to me, means a pusher, the wing nowhere in the pilot's field of view, good climb, and responsive handling (to include roll, so no 25' wingspan). Single seat, or maybe a tandem seat if everything is kept very light. This is a toy, just 1 seat is best. Put the engine in the back of the pod and use a low tailboom. A single VW would give enough thrust for good performance. On a budget, two 25-30 HP industrial engines in the wing roots driving closely spaced pusher fans would get the job done and look fine, but the plane wouldn't be able to climb on just one unless the wing (and span) was increased, this would change the whole character of the plane.
Again, all just my opinion.
 
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Vigilant1

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I'm not sure that's a problem. I think you might just end up with a plane that rides "nice". If I am to pick up what you're putting down, are you referencing the problem with jets, and roll coupling?
Roll coupling could be one issue, but also spin recovery. With the mass way out on the edges, there's a lot more inertia to keep the spin going. Tip tanks can also be problematic for this reason. And with the mass at the front and back of the fuselage, the spin may be flatter.
 

nerobro

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That's a tradeoff, definitely. It also makes the plane less sensitive to trim changes (eg, movement of fuel, or people, in the plane) Becuase of the whole lever arm thing. This is where someone needs to make engineering decisions.
 

Aerowerx

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I'm not sure that's a problem. I think you might just end up with a plane that rides "nice". If I am to pick up what you're putting down, are you referencing the problem with jets, and roll coupling?
It could be a real problem, as others have said.

It has nothing to do with jets and applies to any aircraft. But could affect roll coupling and make it harder to prevent dutch roll. It could be critical in spin recovery.

As an example, if something starts to raise the nose it will be harder to bring it back down because of the inertia of that mass at the nose and tail. This could result in pilot induced oscillations. Very careful attention would have to be paid to stability in the design.

Take a 5 pound weight in your hand. It is easy to move around in any position. Now put the same weight on the end of a stick and try to move it around with your hand at the other end. See what I mean?
 

Victor Bravo

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With all respect to the Sadler Vampire ultralight/LSA back in the 1980's, the mention of the Vampire in Post 463 above makes sense. This is also apparently along the lines of Tin Buzzard's airplane which has been under renewed discussion.

But... an actual, correctly shaped, retractable geared, 3/4 scale DH Vampire with a propeller at the stock location of the jet exhaust would look right and quite possibly fly right. The general configuration worked well. The aerodynamics are not tricky or risky. You'd need a five or six blade propeller of a smaller diameter, but this lends itself to using a higher RPM engine without a reduction (or with a lower ratio).

Imagine a WAR Aircraft Replicas Vampire using one of the hotrod motorcycle engine conversions being talked about these days.

The downside of this configuration the crash safety. A quick deceleration is going to be far less survivable with that engine behind you.
 

Himat

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But... an actual, correctly shaped, retractable geared, 3/4 scale DH Vampire with a propeller at the stock location of the jet exhaust would look right and quite possibly fly right. The general configuration worked well. The aerodynamics are not tricky or risky. You'd need a five or six blade propeller of a smaller diameter, but this lends itself to using a higher RPM engine without a reduction (or with a lower ratio).

Imagine a WAR Aircraft Replicas Vampire using one of the hotrod motorcycle engine conversions being talked about these days.
Well, if sort of scale I would rather emulate the de Haviland Sea Vixen than the Vampire. Ok, with the canopy on the centreline and no man down in the hole over there.
 

Vigilant1

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The downside of this configuration the crash safety. A quick deceleration is going to be far less survivable with that engine behind you.
I share this concern about the engine behind the pilot, but I wonder is it's not overstated. We just need to design for it. Presumably our pilot will have shoulder restraints attached to that same structure/rear firewall, and they need to restrain a larger mass than the engine to the same deceleration level. We hear very little talk about the impracticality of designing for these passenger restraints, but if we add the weight of an engine it then becomes too hard? I don't see why--go to slightly larger tubes, etc and it is done.

There are also some counterbalancing safety advantages to a pusher setup. Getting that fuel, electrics, hot exhaust, etc out of the likely crush zone surely is a post-crash plus. And, with no (incompressible) engine in front of the pilot, there's an opportunity to design a very effective crush space where it can do the most good.
Beefing up that "rear firewall-to-pilot feet" structure seems fairly straightforward compared to some other things we do.
 

cluttonfred

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I love the idea of a semi-scale DH Vampire and, since one of the inspirations for this thread was Yves Gardan's SIPA S.200 Minijet, we've now come full circle! ;-) To be honest, I'd prefer a simplified Minijet: fixed gear, constant-chord wing, much lighter, but otherwise very similar to the original.



EDIT--And since it's always fun to watch this clip...

 
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BBerson

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Except....

Having masses distributed like you suggest can greatly affect the stability and controlability. It is best to have the mass as close to the CG as possible.
Right. But an electric motor is a small mass. The batteries would be very close to the CG (slightly forward).
Modern aircraft can choose spins prohibited. Without or without a chute.
 

Riggerrob

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If you build a SIPA/dH Vampire pseudo-scale, pseudo-jet, mount the propeller high above the wing root. Moving the prop shaft away from the wing wash will reduce noise and vibration. While you are at it, mount the horizontal tail well above or well below the prop shaft. It will also improve ground clearance to the point that you can use really short landing gear … like SIPA.
 

TiPi

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A DH-100 scale replica is my bucket list project:)
I'm looking at a 2/3 scale to fit in our "regulation free" category for ultralights (10m2 wing area, single seat, MTOW 300kg). Construction would be similar to the Spacek SD-1 (wood, composite, carbon fibre, ply and foam). It will be tight to get the empty weight down to around 180kg (SD-1s are from 120-135kg).
A 70-75kg (750N) jet would give roughly the same power/thrust ratio as the original, with a 20min endurance:(
I'll be looking at a ducted fan for aboout that level of thrust. That will be another challenge as the scale air intakes and the exhaust pipe will be too small for an effective ducted fan.
upload_2020-2-19_20-51-48.png
upload_2020-2-19_20-52-37.png
 

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Vigilant1

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A DH-100 scale replica is my bucket list project:)
I'm looking at a 2/3 scale to fit in our "regulation free" category for ultralights (10m2 wing area, single seat, MTOW 300kg). Construction would be similar to the Spacek SD-1 (wood, composite, carbon fibre, ply and foam). It will be tight to get the empty weight down to around 180kg (SD-1s are from 120-135kg).
A 70-75kg (750N) jet would give roughly the same power/thrust ratio as the original, with a 20min endurance:(
Great looking design. Would it be possible to provide a plan view as well?
These little jets really call out for retractable gear, but it is hard to make the case based on aerodynamics or weight. But, sometimes we need to do things just because...
TiPi, that little B&S 30 HP you've been sweating over might just get the job done, if you can accept a pusher prop back there. As you surely know, the history of homebuilt ducted fans is not a happy tale. They can make thrust, but not efficiently (compared to an open prop).
 

TiPi

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with great appreciation to the bloke who rendered the DH-100 in Sketchup.
The propulsion is going to be difficult. I would prefer an internal unit (ducted fan or maybe a small jet) over the Sadler Vampire rear prop setup. The ducted fan will have a lower efficiency due to the higher pressure ratio needed with the small air in- and outlets. Looking at some of the ducted fans, I might need 60-75hp to achieve the desired 75kg of thrust. The original Vampy does take off like a pregnant duck and only comes alive on the downwind leg.
upload_2020-2-19_22-31-25.png
 

Vigilant1

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with great appreciation to the bloke who rendered the DH-100 in Sketchup.
The propulsion is going to be difficult.
Well, plenty of wing area, that'll help. As you say, I think propulsion will be the biggest challenge. A ducted fan just might not be do-able (or result in good flying qualities) given your challenging weight criteria unless you can find a very weight efficient powerplant. This might be a place for a two stroke, fan, no gearbox. Let 'er rip.
 

cluttonfred

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If we are talking about actual replicas of early jets, I have to put in a plug for the Hawker Sea Hawk. The twin intakes and "pen nib" exhausts actually lead to a single Rolls Royce Nene centrifugal turbojet (49.5 in diameter). 75% scale would drop the 39' wing span down to 29' 3" and the 278 sq ft wing area down to 156.4 sq ft and allow for a about a 36" fan in place of the turbojet with the short inlets and outlets (both a little larger than scale) reducing friction losses. Of course, you'd need an interesting structure to route the longerons around the fan, but I think geodetic construction could work well here.
sea hawk 3-view.png sea hawk in flight.jpg sea hawk cutaway.jpg
 

Vigilant1

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Hey, who posted this?;)
Perhaps this ducted fan conversation including rim drive should go to its own thread since it's getting pretty far removed from "a single pusher piston engine and propeller to simulate the look of a jet"?
Herding cats is a thankless job!
 
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