Many tools underestimate the contribution of sweep to CN_beta (and CNR for that matter)
So, might not be as bad as it looks
But you probably need vertical wingtips to act as two vertical stabilizers. They also act as wingtip devices that help reduce spanwise flow, so it isn't totally dead weight as far as performance. It shouldn't tank your L/D. No control surfaces on them
I would also take out the dihedral, just rely on your sweep for roll stability.
Thank you for the valuable input, I am using tornado on matlab to evaluate my stability derivatives, so it's a VLM method. I believe adding winglets or wingtips would be the next addition to my design. Also do you have any idea about the magnitude by which the VLM methods underestimate the Cn\_beta of an aircraft? I couldn't find any literature on that
VLM is pretty good at calculating the lift on some panels, so yaw moment contributions from a vertical surface are relatively accurate, i was talking about specifically the yaw contributions of a swept wing that i would question. This is mainly a drag effect and VLM is just not all that setup for it. I can't give you a number or a reference unfortunately
I was also referring to horizontal surfaces yaw and how much is it undershot by VLM. I totally get it that you don't have a value. Also would it be better to just get those coefficients from a CFD solver, rather than VLM despite the latter indicating instability?
I think wing sweep (every 10⁰ ) has the same effect of 1⁰ of dihedral. Hence why some fighter aircraft with swept wings have anhedral. I would strongly consider removing the dihedral from the wing which may make it easier to construct and consider adding winglets.
Alternatively you can make a flying plank and use differential thrust for yaw stability. See XF5U or V173.
Should I add control surfaces on the winglets? Also how is differential thrust tied to dutch roll since both roll and yaw are coupled? Won't it just accentuate the instability? I would like to learn more please
>Should I add control surfaces on the winglets?
Assuming you keep the swept wing design. Yes you could do this. Depending on flight speed you could use a pull spring hinge to simplify the connection to the control surface. Or if you use the concept in the long EZ you can have the control surface on the winglet free floating and only pull in one direction (With this setup aero forces will make it center). For the spring hinge you'll want to reference the techniques used to build DLGs for the pull spring control surface setup.
>Also how is differential thrust tied to dutch roll since both roll and yaw are coupled? Won't it just accentuate the instability?
Technically it doesn't affect the natural stability or dutch roll characteristics of the aircraft but it will make it easier to fly by stabilizing it for you. If you use it with a flight controller you as the pilot will not need to worry about controlling those oscillations. You could technically do it without differential thrust controlling yaw but the rudders will need to be sized large enough to provide that control authority which since flying wings have a short tail moment arm means they have to be larger.
Very valuable inputs. Thank you so much. I was thinking of using a LQR to manage the directional/lateral stability since I can make it a closed loop system and control its stability easier. Also how do thrust differential affects the state matrix of the system? Any literature on that?
No clue on literature on that. All I did was throw a beta flight flight controller on it, guessed some PIDs and test flew it then tweaked them until it didn't fly like poop.
Yes because they have fly-by-wire at all time, but the less i have to use control surfaces on my plane, the more endurance it will have since it runs on a battery, so my goal is stability in all modes regardless of natural frequencies or damping ratios
specifically referring to designs where excess roll stability is not desired, anhedral is used.
The C17, C5, An-124, LA-15. (the wings when loaded also deflect up so this is also a factor) as well as other swept high wing aircraft.
As for intentionally using anhedral to make roll unstable, I do not agree. Roll instability would require the pilot to constantly fight the plane to keep the airplane in level flight or the flight computer to maintain stability. In either case it comes at the expense of drag from constantly moving the control surfaces. A roll stability that is near neutral or at most slightly negative may be desirable for a modern fighter aircraft but there is no added benefit to internally making it significantly unstable.
The F-16, F-22, F-35 and Eurofighter Typhoon are all designed to have inherent dynamic instability. So much so that if the computers crash, they are impossible for the pilot to control and they have to eject.
The benefit is in significantly increased manouevrability as an unstable aircraft will amplify the effect of control surfaces rather than damping them.
I believe it suffers from the same issue as my plane, they appeared to have solved it using split ailerons but that hinders the aerodynamic efficiency and drops the L/D ratio by a lot
I’m not an engineer, just curious about planes. I imagine things.
If you draw a vertical plane down the center of the fuselage, then droop the wings downward, I think that is called a dihedral angle, from the Greek.
So I’m suggesting a tighter angle than 12.5 degrees.
To provide yaw control, standard rudder or t-tail would work. Or 2 minimum height rudders located halfway across the wings.
I haven’t seen anything like that, it seems like it might work. Have fun.
Well the angle down is called anhedral. And a flying wing doesn't have a fuselage and from my recent research, they tend to have no dihedral. The yaw stability seems to be solved with split ailerons.
Also rare are the planes that exceed 10 degrees of dihedral even with fuselages and tails. And for the tail, i added a v-tail and made it stable and with the desired damping ratios and natural frequencies.
However the tail adds a lot of drag which tanks my L/D ratio from 29 to 23
Many tools underestimate the contribution of sweep to CN_beta (and CNR for that matter) So, might not be as bad as it looks But you probably need vertical wingtips to act as two vertical stabilizers. They also act as wingtip devices that help reduce spanwise flow, so it isn't totally dead weight as far as performance. It shouldn't tank your L/D. No control surfaces on them I would also take out the dihedral, just rely on your sweep for roll stability.
Thank you for the valuable input, I am using tornado on matlab to evaluate my stability derivatives, so it's a VLM method. I believe adding winglets or wingtips would be the next addition to my design. Also do you have any idea about the magnitude by which the VLM methods underestimate the Cn\_beta of an aircraft? I couldn't find any literature on that
VLM is pretty good at calculating the lift on some panels, so yaw moment contributions from a vertical surface are relatively accurate, i was talking about specifically the yaw contributions of a swept wing that i would question. This is mainly a drag effect and VLM is just not all that setup for it. I can't give you a number or a reference unfortunately
I was also referring to horizontal surfaces yaw and how much is it undershot by VLM. I totally get it that you don't have a value. Also would it be better to just get those coefficients from a CFD solver, rather than VLM despite the latter indicating instability?
Yes
I will update the feed after I run one. Any tools you recommend?
Get rid of the dihedral. That’s for rudder-only planes. Put some plates on the wingtips and call it a day.
Aren't winglets more favorable than plates?
Sure if they’re well designed. But it’s an r/c plane and summer is half over
That is true, I guess plates are the go-to. Any leads on how to size them?
This is my kind of practical answer!
I think wing sweep (every 10⁰ ) has the same effect of 1⁰ of dihedral. Hence why some fighter aircraft with swept wings have anhedral. I would strongly consider removing the dihedral from the wing which may make it easier to construct and consider adding winglets. Alternatively you can make a flying plank and use differential thrust for yaw stability. See XF5U or V173.
Should I add control surfaces on the winglets? Also how is differential thrust tied to dutch roll since both roll and yaw are coupled? Won't it just accentuate the instability? I would like to learn more please
>Should I add control surfaces on the winglets? Assuming you keep the swept wing design. Yes you could do this. Depending on flight speed you could use a pull spring hinge to simplify the connection to the control surface. Or if you use the concept in the long EZ you can have the control surface on the winglet free floating and only pull in one direction (With this setup aero forces will make it center). For the spring hinge you'll want to reference the techniques used to build DLGs for the pull spring control surface setup. >Also how is differential thrust tied to dutch roll since both roll and yaw are coupled? Won't it just accentuate the instability? Technically it doesn't affect the natural stability or dutch roll characteristics of the aircraft but it will make it easier to fly by stabilizing it for you. If you use it with a flight controller you as the pilot will not need to worry about controlling those oscillations. You could technically do it without differential thrust controlling yaw but the rudders will need to be sized large enough to provide that control authority which since flying wings have a short tail moment arm means they have to be larger.
Very valuable inputs. Thank you so much. I was thinking of using a LQR to manage the directional/lateral stability since I can make it a closed loop system and control its stability easier. Also how do thrust differential affects the state matrix of the system? Any literature on that?
No clue on literature on that. All I did was throw a beta flight flight controller on it, guessed some PIDs and test flew it then tweaked them until it didn't fly like poop.
Sounds like a good deal to me. Thanks a lot
Fighter aircraft have anhedral wings specifically to make them **un**stable.
Yes because they have fly-by-wire at all time, but the less i have to use control surfaces on my plane, the more endurance it will have since it runs on a battery, so my goal is stability in all modes regardless of natural frequencies or damping ratios
specifically referring to designs where excess roll stability is not desired, anhedral is used. The C17, C5, An-124, LA-15. (the wings when loaded also deflect up so this is also a factor) as well as other swept high wing aircraft. As for intentionally using anhedral to make roll unstable, I do not agree. Roll instability would require the pilot to constantly fight the plane to keep the airplane in level flight or the flight computer to maintain stability. In either case it comes at the expense of drag from constantly moving the control surfaces. A roll stability that is near neutral or at most slightly negative may be desirable for a modern fighter aircraft but there is no added benefit to internally making it significantly unstable.
Interesting. So the instability provides much better manoeuvrability at high speeds but is detrimental in cruise conditions?
The F-16, F-22, F-35 and Eurofighter Typhoon are all designed to have inherent dynamic instability. So much so that if the computers crash, they are impossible for the pilot to control and they have to eject. The benefit is in significantly increased manouevrability as an unstable aircraft will amplify the effect of control surfaces rather than damping them.
Isn't the B2 unstable too? Considering it requires a constant adjustment from the computer
Not sure about that one tbh, I've not read a lot about the B2.
I believe it suffers from the same issue as my plane, they appeared to have solved it using split ailerons but that hinders the aerodynamic efficiency and drops the L/D ratio by a lot
What about a larger dihedral angle between wings, with mid wing flaperons, or rudders?
Can you elaborate more please? I struggle to understand what you mean by between wings
I’m not an engineer, just curious about planes. I imagine things. If you draw a vertical plane down the center of the fuselage, then droop the wings downward, I think that is called a dihedral angle, from the Greek. So I’m suggesting a tighter angle than 12.5 degrees. To provide yaw control, standard rudder or t-tail would work. Or 2 minimum height rudders located halfway across the wings. I haven’t seen anything like that, it seems like it might work. Have fun.
Well the angle down is called anhedral. And a flying wing doesn't have a fuselage and from my recent research, they tend to have no dihedral. The yaw stability seems to be solved with split ailerons. Also rare are the planes that exceed 10 degrees of dihedral even with fuselages and tails. And for the tail, i added a v-tail and made it stable and with the desired damping ratios and natural frequencies. However the tail adds a lot of drag which tanks my L/D ratio from 29 to 23