Design Discussion: Dornier Seastar

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justifidejoe

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The Dornier Seastar is, to me, a very intriguing configuration.
Dornier_Seastar_In-Flight.jpg

Ultimately, my question is: Do you think such a configuration could be made more efficient (in terms of drag) than the Rutan Boomerang configuration? Let's remove the sponsons and streamline the fuselage for a non-amphibian design.

My comments:
  • Like the Boomerang, this configuration has only 2 nacelles (1 for engines, 1 fuselage). A 'conventional' twin has 3. I think this is one of the major contributors to the Boomerang's efficiency.
  • According to this thread, a pylon wing could significantly reduce the induced drag of the fuselage/wing intersection. A mid-wing design (a la Boomerang) has significant induced drag, according to this thesis.
  • Don't have to worry about spar carry-through with regards to fuselage arrangement.
  • A SeaStar-like configuration could have laminar flow over a vast majority of the wing.
  • Furthermore, the fuselage would be out of the prop wash, meaning laminar flow on at least a portion of the fuselage. And much less turbulent flow over the remaining non-laminar portion, right?
  • The tail would still be in the prop wash, beneficial for avoiding deep stall, flight control.
  • Fantastic visibility
  • Probably pretty quiet (rear prop may be noisy)
  • If FIKI is a concern, avoid the need for icing protection on the props, as the rear prop would be in the heat wash from the engines.

Your thoughts?
 
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TFF

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Its a cool plane. It would be a mess to build. It cool that the company is playing on their historical aircraft and as a niche it works.
 

SVSUSteve

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The idea of a low drag seaplane is kind of like the idea of a virginal prostitute. A lot of guys dream of it but I don't think anyone has seen a verifiable one.

If FIKI is a concern, avoid the need for icing protection on the props, as the rear prop would be in the heat wash from the engines.
Further reducing the efficiency of the already inefficient pusher concept and probably increasing noise and drag. Since when is an electrically heated FIKI prop something that is to be avoided due to drag issues?

Laminar flow is hard enough to maintain on wings in practice (that's why only competition gliders really get super anal retentive about it)....good luck doing that on a stepped hull, etc.

My big question about these tractor/pusher configurations is always "How does it perform with the critical engine failed?" and under the least favorable loading conditions. The best example why is the Cessna 310 which- as a friend who owns one put it- "flies like an ice covered turd" in that situation.

The tail would still be in the prop wash, beneficial for avoiding deep stall, flight control.
Deep stall is really only a major concern in airliners and other aircraft with swept wings especially with T tails. Can anyone else think of a case of a GA aircraft (without obvious design issues) crashing because of it? Instead of trying to use prop wash to avoid it, how about just following the basic guidelines for placement of the rudder and horizontal stabilizer?

I fourth what Chris and TFF said....that's a beautiful bird.
 

justifidejoe

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The idea of a low drag seaplane is kind of like the idea of a virginal prostitute. A lot of guys dream of it but I don't think anyone has seen a verifiable one.

Laminar flow is hard enough to maintain on wings in practice (that's why only competition gliders really get super anal retentive about it)....good luck doing that on a stepped hull, etc.
Agreed. Re-read my original post about removing the sponsons for a non-amphibian design.


Further reducing the efficiency of the already inefficient pusher concept and probably increasing noise and drag.
My understanding is that pusher props can be used efficiently if given enough distance between the prop and whatever leading body is ahead of it (e.g. a wing). Of course the tractor prop in front would diminish the efficiency, probably more than a pusher prop with small clearances. I have no idea though.

Since when is an electrically heated FIKI prop something that is to be avoided due to drag issues?
Not related to drag, just a comment. Most of the downsides of a pusher prop are minimized with a SeaStar setup (taxi cooling, runway FOD, pusher prop deice). Of course it adds the problem of being the tractor prop wake.



My big question about these tractor/pusher configurations is always "How does it perform with the critical engine failed?" and under the least favorable loading conditions. The best example why is the Cessna 310 which- as a friend who owns one put it- "flies like an ice covered turd" in that situation.
You mean the 337? My understanding is that the non-ideal engine-out characteristics of the 337 are related to the large fuselage cross section in between the props, and flow separation when only the front prop is operating.

Deep stall is really only a major concern in airliners and other aircraft with swept wings especially with T tails. Can anyone else think of a case of a GA aircraft (without obvious design issues) crashing because of it? Instead of trying to use prop wash to avoid it, how about just following the basic guidelines for placement of the rudder and horizontal stabilizer?
I'm sure you're right, just a thought that came to mind.
 

SVSUSteve

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You mean the 337? My understanding is that the non-ideal engine-out characteristics of the 337 are related to the large fuselage cross section in between the props, and flow separation when only the front prop is operating.
Yes, the 337. I had been working on a review of a case involving a triple fatality crash in a 310 earlier in the day. Must have just typed it on here from habit Thanks for catching that.

My understanding is that pusher props can be used efficiently if given enough distance between the prop and whatever leading body is ahead of it (e.g. a wing). Of course the tractor prop in front would diminish the efficiency, probably more than a pusher prop with small clearances. I have no idea though.
Yeah, I'm a crash survivability guy. Prop efficiency isn't something that really interests me. When people bring up discussions on here about which prop to use my response is "Whichever one the engine manufacturer says to?". Icing and its prevention and management are about the only other aspect of design that I get really interested in.

I do recall someone on here (Orion?) saying that if you have an adequate spacing that it improves the efficiency but the distance required is impractical because of the forces the spinning prop would place on the shaft extension and the bolts anchoring it to the engine.
 

Riggerrob

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Have you compared Dornier's Seastar with Rutan's latest design: Ski Gull?

The Ski Gull has a similar configuration as the Dornier, but with two tractor propellers, more like the Consolidated PBY Catalina flying boat. Of the dozens of different configurations tested on flying boats, Catalina proved the most successful. Half of all flying boats ever built were Catalina's.
 

SVSUSteve

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Have you compared Dornier's Seastar with Rutan's latest design: Ski Gull?

The Ski Gull has a similar configuration as the Dornier, but with two tractor propellers, more like the Consolidated PBY Catalina flying boat. Of the dozens of different configurations tested on flying boats, Catalina proved the most successful. Half of all flying boats ever built were Catalina's.
Not to mention that for a big utilitarian design the Cat was one sexy plane.
 

DangerZone

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Ultimately, my question is: Do you think such a configuration could be made more efficient (in terms of drag) than the Rutan Boomerang configuration? Let's remove the sponsons and streamline the fuselage for a non-amphibian design.
Have you seen the Beriev aircraft, like the jet engine Be-200, the prop Be-103 or the A-40 and compared drag&efficiency?
 

Himat

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The Dornier Seastar is, to me, a very intriguing configuration.

Ultimately, my question is: Do you think such a configuration could be made more efficient (in terms of drag) than the Rutan Boomerang configuration? Let's remove the sponsons and streamline the fuselage for a non-amphibian design.
If this configuration could be made more efficient than the Rutan Boomerang configuration?
Probably if well executed.

Like the Boomerang, this configuration has only 2 nacelles (1 for engines, 1 fuselage). A 'conventional' twin has 3. I think this is one of the major contributors to the Boomerang's efficiency.
Not sure, it could as well be details in the design that are better done than an "ordinary" twin engine airplane.


According to this thread, a pylon wing could significantly reduce the induced drag of the fuselage/wing intersection. A mid-wing design (a la Boomerang) has significant induced drag, according to this thesis.
But with one engine in front of the wing there is a question if the only reduction is due to the lesser size of the obstruction.

Don't have to worry about spar carry-through with regards to fuselage arrangement.
True, the attachment points for the struts in both wing and fuselage may cause the equal amount of worry, especially if designing for building in composites.

A SeaStar-like configuration could have laminar flow over a vast majority of the wing.
There is a question if the engine nacelle would not upset this.

Furthermore, the fuselage would be out of the prop wash, meaning laminar flow on at least a portion of the fuselage. And much less turbulent flow over the remaining non-laminar portion, right?
Some gain could be possible, but there must then be a lot of attention to detail to not spoil this.

The tail would still be in the prop wash, beneficial for avoiding deep stall, flight control.
Fantastic visibility
Probably pretty quiet (rear prop may be noisy)

If FIKI is a concern, avoid the need for icing protection on the props, as the rear prop would be in the heat wash from the engines.


Your thoughts?
To have the tail in the prop wash in some circumstances is beneficial to flight control. Steve have answered the deep stall bit.

The visibility would for sure be great.

Maybe pretty quiet, at least on the inside. The engine exhaust then better be discharged outside the propeller arch.
 

DangerZone

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If this configuration could be made more efficient than the Rutan Boomerang configuration?
Probably if well executed.
Why not compare the Dornier to an amphibian isntead of the Rutan Boomerang? That is, if we would want to compare apples to apples.

Or the Equator prototype:


A twin engine was also considered, look at: EQUATOR* AIRCRAFT
A design with a high wing and podded engine might be as efficient as parasol wing aircraft with the engine mounted to the wing.
In such a comparison, the Explorer 2 of Stephane also comes to mind.
[video=youtube;RLKNPOSpMpY]https://www.youtube.com/watch?v=RLKNPOSpMpY[/video]
 
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Himat

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Why not compare the Dornier to an amphibian isntead of the Rutan Boomerang? That is, if we would want to compare apples to apples.

In such a comparison, the Explorer 2 of Stephane comes to mind.
Because as I understood the OP he considered a landplane twin configured like the Dornier Seastar.
I and he could probably not find an example of a landplane with the Dornier Seastar configuration, even if there probably is some obscure landplane with twing engines mounted to a parasol wing.
 

justifidejoe

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The Ski Gull has a similar configuration as the Dornier, but with two tractor propellers...
I believe the Ski Gull has a single tractor prop?

0722_Rutan_SkiGull.jpg

...more like the Consolidated PBY Catalina flying boat. Of the dozens of different configurations tested on flying boats, Catalina proved the most successful. Half of all flying boats ever built were Catalina's.
That's really interesting. Thanks for the info!
 

justifidejoe

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Or the Equator prototype...


A twin engine was also considered, look at: EQUATOR* AIRCRAFT
A design with a high wing and podded engine might be as efficient as parasol wing aircraft with the engine mounted to the wing.
I hadn't heard of the Pöschel Equator before. Wikipedia has a little information on the various iterations in the development of the plane. The Equator explored shaft drives (through two 90deg turns, no less), turboprop power, full-span flaps + spoilerons, and more. I wish I could have been an engineer on that project!

Does anyone believe the performance figures quoted on Wikipedia? These numbers are for the prop on the tail, driven through a shaft. Compare to the Mooney M22, which uses the same engine:
comparo 2.jpg
 
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Himat

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IDoes anyone believe the performance figures quoted on Wikipedia? These numbers are for the prop on the tail, driven through a shaft. Compare to the Mooney M22, which used the same engine:
View attachment 44371
A correction to your table, the low stall speed should be for the Equator and the high for the Mooney.

If the performance figures are to be believed?
The numbers may be true, but the question is the conditions for the numbers.
Given that the stated weight is correct the stall speed of the Equator could be ok. (have not recalculated the number.)
The Equator cruise speed seems optimistic, on the other side with a large wing and low weight, flying high the ground speed can be impressive.
 

DangerZone

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I hadn't heard of the Pöschel Equator before. Wikipedia has a little information on the various iterations in the development of the plane. The Equator explored shaft drives (through two 90deg turns, no less), turboprop power, full-span flaps + spoilerons, and more. I wish I could have been an engineer on that project!

Does anyone believe the performance figures quoted on Wikipedia? These numbers are for the prop on the tail, driven through a shaft. Compare to the Mooney M22, which used the same engine:
View attachment 44371
The Mooney M22 is quite an aerodynamic aircraft and a well optimized Piper Malibu with a similar 310HP turbocharhed engine cruises at around 200kts at 55% power. With the figures presented, the mass, the aerodynamics, it is hard to believe any of the three Poschel 300 might have had such performance. The more powerful one with a lighter turboprop 420HP engine might have been planned to come close to a cruise speed of 243kts at high power settings, but this is mere speculation. I believe these performance figures on wikipedia are pretty optimistic.

We gotta note that this was the era of sharp looking aircraft which wanted to discover some revolutionary shape to impress aerodynamics and laws of physics. Hence there were projects like the Colani Cormoran which had a great design, great idea, great looks, but failed at the promise of scaring aerodynamics away and flying faster than the laws of physics allow. It looks cool, even today beyond the 80s. I think I saw some revival of the idea at some airshow expo, it seems they found some investors who put money in this thing again. This is the Colani Cormoran mock up at the Paris airshow expo, notice the lines comforming to 80s coolness rather than aerodynamics:

ColaniCormoran1.jpg

The idea would be worth considering if some serious engineers would try to finish the project to finally make it an amphibian. This was the idea, to make an aircraft which could be capable of landing on water if needed. However, this would mean a complete redesign of the fuselage and wings, so it makes no sense since designing a new aircraft from scratch costs less in the long run.

Finally, notice that these amphibians are all underpowered. That's what usually gets them all in the end, need for more power to get off the water surface efficiently. A 2 ton aircraft can't use only 300HP and fly well, it needs much more than that even if the surface is mirror flat.
 
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