We were discussing with Kate about possibilities to use tip propellers, in tractor configuration. Tractor configuration would cause a swirling motion to the opposite direction of the tip vortex before the tip wortex forms and it would reduce the tip vortex. Therefore the propeller at the wing tip would give more efficient dynamic thrust for the plane than propeller at some other location in the aircraft. In cruise condition when the tip vortex is low with low span loading wing, this could eliminate significantly the unfavorable tip vortex. With electric motors, additional tip propellers would be fairly easy to arrange.
Some analysis would be needed how much this would help. Of course it would depend on the weight of the plane. The heavier the plane, the more leakage to the tip, the more tip vortex would form. The bigger the benefit of having a opposite direction swirling motion to nullify the tip vortex formation.
John Roncz / OSH11: aspect ratio has nothing to do with induced drag
I was listening to John Roncz's presentation in Oshkosh 2019. We had just talked with few aerodynamics people about the induced drag and that it is actually tangent of downwash angle (except for the tip vortex which forms from leakage of pressure over the tip and due to the movement of the plane, a swirling motion is created).
John Roncz was talking in his presentation about wing span, and finally noted about wing area, that he does not care about wing area, because induced drag has nothing to do with aspect ratio. That's why gliders have not only skinny wings, but also very long wings. That's why Rutan's aircraft have long wings, not only skinny wings. Lots of span is needed for low induced drag.
However, there is a geometrical relation about AR to the drag: The lower the AR, the higher is the wetted area for the given span. Wetted area is bad, it causes drag, you don't want extra wetted area. So the wing becomes skinny by definition. But now the wing is skinny (high AR) but also very long, and not only skinny.
There is another problem: I would think then that I want 20 meters long wing span, but very very narrow chord. The chord can not be infinitely narrow in order it to be structurally any sound, especially in speed. Therefore the higher the AR gets, and the lower the wetted area gets, the heavier the wing becomes. And the heavier it becomes, the worse gets the span loading if this is added to the weight of the plane. Then there is the another consideration, where I could taxi such plane which would have 20 meter span? On our airport even the Diamond's comparatively modest wing span is in some places a bit tricky.
Interesting dilemma. This also answers why there are multiple pods on some Rutan's aircrafts, along the span. The reason is to reduce induced drag, by moving the weight from the center more along the span. Then the lift required on the center where the lift given by the wing is worst does not give that unfavorable dip to lift distribution. And it reduces induced drag. On planes, like Globalflyer, induced drag plays major role in how much range the Brequet's equation gives.
But there is even more to this: the higher the AR gets, the lower the Re gets. The higher the altitude, also the lower the Re is again. The lower the Re, the higher is the profile drag. To get high L/D and thus efficiency one has to get also the profile drag down. And airplane efficiency is all about L/D (lift/drag), no less.
So my basic concept remains and does not need to be revised for another configuration alternatives:
- conventional (to be able to use efficient flaps)
- large span, low span loading (to reduce induced drag)
- high aspect ratio, relatively high wing loading (to avoid extra wetted area and that way to reduce drag and AR also to have steep lift curve slope (in other words, closer to the 2D airfoil simulations of infinite wings)
- larger than minimum size elevator for larger CG allowance - this is for practicality rather than minimum trim drag
- The large AR is also needed for this: cruising with high wing loading causes need for high Cl for cruise, which in turn causes high alpha. To reduce alpha, the steepness of the lift curve slope is your friend. The lift curve slope steepness will make the plane to cruise fairly low angle of attack despite of flying at high Cl at high altitude with high wing loading.
John Roncz was talking in his presentation about wing span, and finally noted about wing area, that he does not care about wing area, because induced drag has nothing to do with aspect ratio. That's why gliders have not only skinny wings, but also very long wings. That's why Rutan's aircraft have long wings, not only skinny wings. Lots of span is needed for low induced drag.
However, there is a geometrical relation about AR to the drag: The lower the AR, the higher is the wetted area for the given span. Wetted area is bad, it causes drag, you don't want extra wetted area. So the wing becomes skinny by definition. But now the wing is skinny (high AR) but also very long, and not only skinny.
There is another problem: I would think then that I want 20 meters long wing span, but very very narrow chord. The chord can not be infinitely narrow in order it to be structurally any sound, especially in speed. Therefore the higher the AR gets, and the lower the wetted area gets, the heavier the wing becomes. And the heavier it becomes, the worse gets the span loading if this is added to the weight of the plane. Then there is the another consideration, where I could taxi such plane which would have 20 meter span? On our airport even the Diamond's comparatively modest wing span is in some places a bit tricky.
Interesting dilemma. This also answers why there are multiple pods on some Rutan's aircrafts, along the span. The reason is to reduce induced drag, by moving the weight from the center more along the span. Then the lift required on the center where the lift given by the wing is worst does not give that unfavorable dip to lift distribution. And it reduces induced drag. On planes, like Globalflyer, induced drag plays major role in how much range the Brequet's equation gives.
But there is even more to this: the higher the AR gets, the lower the Re gets. The higher the altitude, also the lower the Re is again. The lower the Re, the higher is the profile drag. To get high L/D and thus efficiency one has to get also the profile drag down. And airplane efficiency is all about L/D (lift/drag), no less.
So my basic concept remains and does not need to be revised for another configuration alternatives:
- conventional (to be able to use efficient flaps)
- large span, low span loading (to reduce induced drag)
- high aspect ratio, relatively high wing loading (to avoid extra wetted area and that way to reduce drag and AR also to have steep lift curve slope (in other words, closer to the 2D airfoil simulations of infinite wings)
- larger than minimum size elevator for larger CG allowance - this is for practicality rather than minimum trim drag
- The large AR is also needed for this: cruising with high wing loading causes need for high Cl for cruise, which in turn causes high alpha. To reduce alpha, the steepness of the lift curve slope is your friend. The lift curve slope steepness will make the plane to cruise fairly low angle of attack despite of flying at high Cl at high altitude with high wing loading.
Approved some comments in old posts
Sorry for not being very active on this blog lately because I have been busy (work, summer vacation (I have been busy (work, current airplane, summer vacation etc.). I noticed that there were plenty of not yet approved comments. Sorry for the delay, I have been busy. Your comments are now approved and after you have been approved once, I think you can comment without prior approval in the future. Thanks for writing comments!
EAA article about "Carplane Developers Criticize BiPod... and Burt Responds"
I was reading http://www.eaa.org/news/2019/2019-08-11_bipod.asp
I also listened Burt Rutan's presentations about Bipod in Oshkosh 2019.
This article on the EAA news tells that some carplane designer thinks Burt's Bipod is "too slippery". I really wonder what is the definition of too slippery. There is a group of misguided people who want airplanes to have lots of drag for them to "not be too slippery", in other words have aerodynamics of brick. I have bumped into Cessna pilots who think like that and they look for example our Diamond that "oh that is too slipperly plane for me". From my standpoint, that is not too clever.
Drag is always unfavorable and waste of resources. There can never be too little drag (except in landing configuration when drag is helpful to land the plane in a meaningful distance). Low drag when plane is cruising, has absolutely nothing to do with the flying qualities of the plane. Having more drag does not make the plane any easier to fly. Having more drag just means you have to burn more fuel, you have to have bigger engine, you have to beef up structure, to compensate, you have to put even bigger engine, and have even more fuel on board. Airplane being slippery is a myth. Some Cessna pilots think our Diamond is "slippery" or "too slippery for them". Yeah right, the truth is that the Diamond has better flying qualities than the C172, is easier to land and especially flare and it also stalls softer.
I wholeheartedly agree with Burt [about his Bipod]: "Gee, he complains that we have too much drag as a car but not enough drag as an airplane!"
I think the US LSA specification is deeply flawed as they have introduced the top speed limit. It will limit the category of LSA planes to such that it is not worth to make efficient planes and high drag has been made a standard. That is not too clever either. Apparently the rules have been set by non-pilots who do not have slightest clue on what makes airplanes safe and what makes them easy to fly [and land]... It is all about stability, stall speed, stall charasteristics and inertia. Europeans have understood that better since there is no top speed limit in Europe but there is a stringent stall speed requirement. Low inertia, low stall speed and gentle handling qualities, and no matter what is the top speed, the plane will be easy to fly.
Read about Burt Rutan's Bipod here:
http://www.airventure.org/news/2019/110727_bipod.html
I also listened Burt Rutan's presentations about Bipod in Oshkosh 2019.
This article on the EAA news tells that some carplane designer thinks Burt's Bipod is "too slippery". I really wonder what is the definition of too slippery. There is a group of misguided people who want airplanes to have lots of drag for them to "not be too slippery", in other words have aerodynamics of brick. I have bumped into Cessna pilots who think like that and they look for example our Diamond that "oh that is too slipperly plane for me". From my standpoint, that is not too clever.
Drag is always unfavorable and waste of resources. There can never be too little drag (except in landing configuration when drag is helpful to land the plane in a meaningful distance). Low drag when plane is cruising, has absolutely nothing to do with the flying qualities of the plane. Having more drag does not make the plane any easier to fly. Having more drag just means you have to burn more fuel, you have to have bigger engine, you have to beef up structure, to compensate, you have to put even bigger engine, and have even more fuel on board. Airplane being slippery is a myth. Some Cessna pilots think our Diamond is "slippery" or "too slippery for them". Yeah right, the truth is that the Diamond has better flying qualities than the C172, is easier to land and especially flare and it also stalls softer.
I wholeheartedly agree with Burt [about his Bipod]: "Gee, he complains that we have too much drag as a car but not enough drag as an airplane!"
I think the US LSA specification is deeply flawed as they have introduced the top speed limit. It will limit the category of LSA planes to such that it is not worth to make efficient planes and high drag has been made a standard. That is not too clever either. Apparently the rules have been set by non-pilots who do not have slightest clue on what makes airplanes safe and what makes them easy to fly [and land]... It is all about stability, stall speed, stall charasteristics and inertia. Europeans have understood that better since there is no top speed limit in Europe but there is a stringent stall speed requirement. Low inertia, low stall speed and gentle handling qualities, and no matter what is the top speed, the plane will be easy to fly.
Read about Burt Rutan's Bipod here:
http://www.airventure.org/news/2019/110727_bipod.html
70% laminar airfoil KS-70pLaminar.DAT
I arrived back from Oshkosh and got lots of new ideas. I had a privilege to talk to many aerodynamics people and also aircraft designers. I met professors, homebuilders etc. It was awesome. I was listening to John Roncz's presentation about how high L/D he achieved in this and that airfoil and attempted the same. I did not get yet 75% laminar flow, but quite close - 70% with thickness 15.72%. I think there could be opportunity for even higher L/D by reducing the thickness but I wanted it to be as thick as I could make it as possible for structural reasons. Thick airfoil also has more volume for storing e.g. fuel.
I created this new airfoil which has 70% laminar flow according to the simulation (please note, this is not tested in wind tunnel). It has a little larger pitching moment than the other airfoils I have done, but the L/D at low angle of attack (zero degrees angle of attack is Cl 0.35) reaches L/D over 100 at Re 5000000. The minimum drag count is 30 (Cd = 0.0030) at Re 7000000. At 5000000 the drag count increases to 31.
The airfoil can be downloaded here: KS-70PLAMINAR.dat.
Simulation results at Re 500 000, 1 000 000, 5 000 000, 7 000 000, 10 000 000 (Cl-Cd polar):
Simulation results at 500 000, 1 000 000, 5 000 000, 7 000 000, 10 000 000 (L/D polar):
Airfoil shape
Pitching moment polar. NACA 2412 and NACA 4412 included for comparison. The pitching moment is between NACA 2412 and 4412 airfoils. Not as good as NACA 23-series airfoils. This airfoil requires aircraft configuration with two surfaces and is not suitable for flying wing.
I will build RC scale model of this airfoil and test it with RC plane. At RC scale it will be a bit worse than best thin turbulent airfoils, but according to simulations, the polars are smooth to low Re which is desirable of course and this airfoil reaches at least the same Cd at the low Re than NACA 2415 unlike some other laminar airfoils.
Thinking: Curtis Channel Wing VSTOL considerations
I was just thinking about Curtis channel wing. I have a concern that this kind of design will result in tip stall in addition to the other problems there could be if something would fail.
However, what if you use electric motors instead and place that channel as a C on the tip of a wing? There is this 3D effect that flow tends to want to slip towards the tip and it causes wake turbulence and reduces Clmax. However, what if there is this C and then there is a prop inside the C. The flow comes to the prop and the prop sends it away and causes even bigger pressure differential between lower side of the wing and the upper side of the wing. With brushless DC electric motor it could be technically doable - one could not think about putting a Lycosaurus to the wing tip. You could even add redundancy by adding two motors in cascade. Should one fail, the another one would still be operational.
I think there are two kinds of aircraft that would be needed to cover the needs of personal air transportation: super stol/vtol for flying to airport from home to the pressurized long range plane that can cover large distances. I think today's general aviation falls in the middle of these, but I think it could be obsolete with these new two categories. I think the today's GA is not popular exactly because it falls between these two categories and does not fit in either purpose properly. And they are neither good toys nor good tools. This first VSTOL would cover the toy part and day to day short distance travel, and the HALE the serious transportation case.
I will write another blog entry about this split of concepts later because I believe I have - as a GA customer - found what's wrong with it. What are the needs and what is the gap. I think I have the answer.
However, what if you use electric motors instead and place that channel as a C on the tip of a wing? There is this 3D effect that flow tends to want to slip towards the tip and it causes wake turbulence and reduces Clmax. However, what if there is this C and then there is a prop inside the C. The flow comes to the prop and the prop sends it away and causes even bigger pressure differential between lower side of the wing and the upper side of the wing. With brushless DC electric motor it could be technically doable - one could not think about putting a Lycosaurus to the wing tip. You could even add redundancy by adding two motors in cascade. Should one fail, the another one would still be operational.
I think there are two kinds of aircraft that would be needed to cover the needs of personal air transportation: super stol/vtol for flying to airport from home to the pressurized long range plane that can cover large distances. I think today's general aviation falls in the middle of these, but I think it could be obsolete with these new two categories. I think the today's GA is not popular exactly because it falls between these two categories and does not fit in either purpose properly. And they are neither good toys nor good tools. This first VSTOL would cover the toy part and day to day short distance travel, and the HALE the serious transportation case.
I will write another blog entry about this split of concepts later because I believe I have - as a GA customer - found what's wrong with it. What are the needs and what is the gap. I think I have the answer.
Pipistrel Panthera videos
I found some interesting videos from Youtube describing the new Pipistrel Panthera four seat aircraft that has incredible performance and economy compared to competition in the similar class:
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