karoliina.t.salminen
Well-Known Member
I was asking earlier about the flying wing. I would like to understand throughly the case of tandem wing as well as I want to rule out not good configurations that do not meet the criteria for my target:
- In conventional aircraft you do not have the extra wetted area from the extra wing area that you need for the tandem wing aircraft because of the low Clmax of the rear wing.
- However, in conventional aircraft you might not want the main wing airfoil to have very high pitching moment as for one of the reasons high pitching moment would cause trim drag or would require a very long tail arm. On the other hand, there are examples of conventional aircraft flying with high pitching moment airfoils, one of the most common is Diamond DA40 with FX63-137. I have looked the bottom side though, and I am quite sure it is shortened FX63-137, I think it is cut earlier than the FX63-137 trailing edge would end as the Diamonds wing is not that extremely cambered, it is just heavily cambered.
- With a high pitching moment airfoil it is possible to get very high L/D. In tandem wing aircraft the forward wing is highly loaded, operates at high Cl where such airfoil gives its best L/D and pitching moment does not have moment arm to turn the nose down on the aircraft. So this kind of "super airfoils" could be utilized on the forward wing. It possibly could operate nearer to section optimum than a conventional configuration - for the forward wing case.
So I think the conclusion is that (what Burt Rutan has also said himself) that traditional canard (like Long-Ez or Cozy) should not have that configuration if the goal was ultimate efficiency and that he invented the configuration just for stall prevention.
However: Global flyer is conventional layout, but Proteus is a tandem wing. Why the Proteus would not be conventional if the conventional plane could have possibly gained a little more altitude if it was so much worse than the conventional plane? Has anybody done analysis on how a very optimized tandem wing which takes all the advantages available from aerodynamics of the forward wing compares to a conventional configuration?
I would like to know all the aspects before ruling out a configuration. Daniel Raymer warns about being too much in law with a specific configuration. However, I don't think the configurations can be ruled out with generalizations from literature "yes, it is [usually] worse" or "that's [usually] better" as it could turn out that in some corner case some other configuration than the generalization might be a better compromise.
I know one explanation for Proteus configuration, exchangeable middle part. However, how often does Burt Rutan do a thing for one reason only? I think in most cases if he designs something, it has neat multiple purposes built into one. What would be your take on this? Is the Proteus tandem just for practical reasons or is there something on that or is that one of the "lessons learned" what have made Burt himself to believe that canard type configuration should not be considered if high performance was the goal.
How high the L/D and Clmax of the forward wing should be to compensate for the low Clmax of the rear wing when compared to a highly optimized conventional layout (e.g. Globalflyer)? The forward wing on the Proteus is of substantial size and span. Common sense says, it would be getting more benefits than a canard aircraft from the aerodynamics of the forward wing, and on the other hand, larger portion of the main wing would suffer from the downwash of the forward wing. The altitude where Proteus can fly is not easy to achieve, and that speaks for that even if the common rule is the canard is worse, Proteus can not be so bad at all from aerodynamics standpoint. Also I think the Proteus was designed years after Burt Rutan had concluded that canard is not best configuration for high performance. There must be something else than practical, if it is not aerodynamics, it must be then structural. He usually seems to design synergetic benefits into his planes.
There is one thing above aerodynamics on tandem wing - a tandem wing with high aspect ratio, looks very beautiful. I think Proteus is the most beautiful aircraft Burt Rutan / Scaled Composites has designed and built.
And yes, I know the drawback of Proteus; fuel management because fuel is stored off-CG and in prototype the fuel needs to be manually managed by the pilot. I have heard it was going to be automatic in production version if that would have happened. And of course in production there is a downside for needing molds for two set of wings rather than one. But we can ignore that in this analysis.
Of course there is this one problem:
- induced drag depends on span
- with two surfaces, you are going to have less span for same aspect ratio than with one
- therefore for the same wing area, tandem might be therefore worse unless something else substitutes for the difference
- In conventional aircraft you do not have the extra wetted area from the extra wing area that you need for the tandem wing aircraft because of the low Clmax of the rear wing.
- However, in conventional aircraft you might not want the main wing airfoil to have very high pitching moment as for one of the reasons high pitching moment would cause trim drag or would require a very long tail arm. On the other hand, there are examples of conventional aircraft flying with high pitching moment airfoils, one of the most common is Diamond DA40 with FX63-137. I have looked the bottom side though, and I am quite sure it is shortened FX63-137, I think it is cut earlier than the FX63-137 trailing edge would end as the Diamonds wing is not that extremely cambered, it is just heavily cambered.
- With a high pitching moment airfoil it is possible to get very high L/D. In tandem wing aircraft the forward wing is highly loaded, operates at high Cl where such airfoil gives its best L/D and pitching moment does not have moment arm to turn the nose down on the aircraft. So this kind of "super airfoils" could be utilized on the forward wing. It possibly could operate nearer to section optimum than a conventional configuration - for the forward wing case.
So I think the conclusion is that (what Burt Rutan has also said himself) that traditional canard (like Long-Ez or Cozy) should not have that configuration if the goal was ultimate efficiency and that he invented the configuration just for stall prevention.
However: Global flyer is conventional layout, but Proteus is a tandem wing. Why the Proteus would not be conventional if the conventional plane could have possibly gained a little more altitude if it was so much worse than the conventional plane? Has anybody done analysis on how a very optimized tandem wing which takes all the advantages available from aerodynamics of the forward wing compares to a conventional configuration?
I would like to know all the aspects before ruling out a configuration. Daniel Raymer warns about being too much in law with a specific configuration. However, I don't think the configurations can be ruled out with generalizations from literature "yes, it is [usually] worse" or "that's [usually] better" as it could turn out that in some corner case some other configuration than the generalization might be a better compromise.
I know one explanation for Proteus configuration, exchangeable middle part. However, how often does Burt Rutan do a thing for one reason only? I think in most cases if he designs something, it has neat multiple purposes built into one. What would be your take on this? Is the Proteus tandem just for practical reasons or is there something on that or is that one of the "lessons learned" what have made Burt himself to believe that canard type configuration should not be considered if high performance was the goal.
How high the L/D and Clmax of the forward wing should be to compensate for the low Clmax of the rear wing when compared to a highly optimized conventional layout (e.g. Globalflyer)? The forward wing on the Proteus is of substantial size and span. Common sense says, it would be getting more benefits than a canard aircraft from the aerodynamics of the forward wing, and on the other hand, larger portion of the main wing would suffer from the downwash of the forward wing. The altitude where Proteus can fly is not easy to achieve, and that speaks for that even if the common rule is the canard is worse, Proteus can not be so bad at all from aerodynamics standpoint. Also I think the Proteus was designed years after Burt Rutan had concluded that canard is not best configuration for high performance. There must be something else than practical, if it is not aerodynamics, it must be then structural. He usually seems to design synergetic benefits into his planes.
There is one thing above aerodynamics on tandem wing - a tandem wing with high aspect ratio, looks very beautiful. I think Proteus is the most beautiful aircraft Burt Rutan / Scaled Composites has designed and built.
And yes, I know the drawback of Proteus; fuel management because fuel is stored off-CG and in prototype the fuel needs to be manually managed by the pilot. I have heard it was going to be automatic in production version if that would have happened. And of course in production there is a downside for needing molds for two set of wings rather than one. But we can ignore that in this analysis.
Of course there is this one problem:
- induced drag depends on span
- with two surfaces, you are going to have less span for same aspect ratio than with one
- therefore for the same wing area, tandem might be therefore worse unless something else substitutes for the difference
Last edited:
