phugoid1982
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How do elevons work differentially to generate BOTH roll and pitch simultaneously?

Sat Mar 14, 2020 12:07 am

I understand the principle behind elevons on tailless delta wing aircraft and how they can be used as elevators or ailerons but how would they accomplish both say in a level turn where you would need need to pitch up to a higher alpha due to the reduced vertical component of Lift to balance weight. For example in a right roll, the left elevon would generate a pitch down incremental moment and the right elevon would generate an incremental pitch up moment though of a smaller magnitude than the left aileron unless it is a symmetric airfoil due to positive camber. The net effect would be a pitch down moment. However, if you deflect only one elevon in the direction of roll always up, that would create a slight positive pitch up moment and increased profile drag would cause proverse yaw/roll. However, due to not using the elevon on the contralateral side, I assume the roll and pitch response would be sluggish. Therefore, I'm trying to understand how these maneuvers can be accomplished smoothly with differential deflection. Thanks.
 
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Starlionblue
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Re: How do elevons work differentially to generate BOTH roll and pitch simultaneously?

Sat Mar 14, 2020 2:24 am

In your right roll example, the deflection would not be equal between right and left elevons. You'd have to make deflection different up and down anyway because even with a symmetrical airfoil you'd have a positive angle of attack, meaning the airflow on the upper and lower side of the elevon is different.

In practice, all that stuff is achieved in FBW nowadays.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
phugoid1982
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Re: How do elevons work differentially to generate BOTH roll and pitch simultaneously?

Sat Mar 14, 2020 3:23 am

True. I assumed cruise conditions where angle of attack could be assumed to be less than 5 degs where small angle approximations applied. Probably not true for many commercial A/C. Yeah, deflections arent symmetric even for a symmetric airfoil. In that case I'm guessing the downward elevon will be deflected more because it has a positive alpha and it can attain a higher lift coefficient before stall. The upward moving elevon at positive alpha will have a decreasing angle of attack due to the change in camber line and at some point will venture into zero alpha territory where cl=0.

I really have to review Aerodynamics. It's been a decade
 
hitower3
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Re: How do elevons work differentially to generate BOTH roll and pitch simultaneously?

Tue Mar 17, 2020 9:30 am

Interesting thread indeed.
While I could imagine that the task of programming a digital FBW is already highly complex - but how the heck did they manage to implement this complex logic in an analog aircraft like the Concorde?
 
flipdewaf
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Re: How do elevons work differentially to generate BOTH roll and pitch simultaneously?

Tue Mar 17, 2020 10:35 am

hitower3 wrote:
Interesting thread indeed.
While I could imagine that the task of programming a digital FBW is already highly complex - but how the heck did they manage to implement this complex logic in an analog aircraft like the Concorde?


I do have a habit of over simplifying but I don't think the control logic for standard flight in a tailless aircraft would be any more difficult than for conventional its just different, the difficulty comes int he transition between subsonic and supersonic flight and the changing center of pressure. My understanding of the control of a tailless aircraft (I'm 13 years past so a bit rusty) is that the elevon is effectively a hinged flap and you use it to control the moment generation from the wing, the big issue from a control perspective is that in order to change the moment to get an increase in AoA you need to reduce the camber and drop lift, hence the cool and rather difficult landing characterisitcs of delta winged jets, somewhat alleviated by the addition of canards.

Fred
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gloom
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Re: How do elevons work differentially to generate BOTH roll and pitch simultaneously?

Tue Mar 17, 2020 10:37 am

hitower3 wrote:
Interesting thread indeed.
While I could imagine that the task of programming a digital FBW is already highly complex - but how the heck did they manage to implement this complex logic in an analog aircraft like the Concorde?


There are some analogue systems that have similar characteristic - both electric and hydraulic. See negative feedback systems, or similar systems.

Cheers,
Adam
 
Legs
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Re: How do elevons work differentially to generate BOTH roll and pitch simultaneously?

Tue Mar 17, 2020 10:37 am

hitower3 wrote:
Interesting thread indeed.
While I could imagine that the task of programming a digital FBW is already highly complex - but how the heck did they manage to implement this complex logic in an analog aircraft like the Concorde?


Of a similar era, the F-111 did it purely mechanically. Pitch and roll stick inputs came into the mixer as control rods, then a system of idlers, bellcranks and linkages output a single, but often different push/pull command to each stabilator. The autoflight control system dampers (2 of, for pitch and roll) also weighed into the mix here, biasing or adding their own inputs. Very hard to find an online photo of it, however, it was a great bit of engineering.

Edit: Found a link to an F-111 flight manual with a schematic view. Light on detail, but it shows how the separate pitch and roll inputs are mixed into one command output to each stab servo.

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