# Attention Tailless Nerds!

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#### BJC

##### Well-Known Member
HBA Supporter
Yes, but something is missing ....

BJC

#### Aerowerx

##### Well-Known Member
Something is missing on those airplanes.

BJC
No, most airplanes have unnecessary appendages! Read that as extra weight and drag!

#### jedi

##### Well-Known Member
Or borrow from the library on inter library loan from Embry Riddle or other source. I thought about copying it but did not.

#### Doggzilla

##### Well-Known Member
HBA Supporter
What I would really like to see is new research on belly strakes on tailless aircraft.

Ever wonder why the F-16 only needs a single tail but everything else has twin tails? Its because there is a baby set of tails on the rear belly. At high angles of attack they actually stick out into the airstream instead of being in the poor airflow behind the high AoA fuselage. This makes belly strakes far more useful with far less area than a standard tail.

The leading edge angle is also superior since its being swept forward by the increase in angle of attack instead of being swept backwards like a standard swept tail. This makes its effectiveness vastly better at high angles of attack.

Adding belly strakes to a flying wing is the one experiment I would really love to see, since flying wings have so much potential if their yaw and AoA stability can be improved.

#### Aerowerx

##### Well-Known Member
...since flying wings have so much potential if their yaw and AoA stability can be improved.
First, you should not confuse Stability with Damping. Damping is an indication of how long it takes to return to the previous attitude after a disturbance. Stability is what keeps you out of the cemetery.

Flying wings, properly designed, have no worse longitudinal stability than any other aircraft.

If you are talking about pure flying wings, lateral stability can be difficult to achieve, but you can still get acceptable stability. Remember there is little or no additional drag if a pure wing yaws a little. Unlike a tailed aircraft where the empenage and vertical stabilizer will cause extra drag if not aligned with the air flow.

#### Riggerrob

##### Well-Known Member
Dogzilla,
Also consider the delta strakes under the aft fuselages of later Learjets. They provide righting moments (nose down pitch) at steep angles of attack when the wing wake blankets the (T) high-mounted horizontal stabilizer.

#### Aesquire

##### Well-Known Member
Putting vertical fins on a flying wing to improve yaw control or stability is common. Strakes, skegs, whatever to want to call them, must be behind the center of gravity. Putting them on the fuselage that you compromise the flying wing design with since you don't fit inside until it's big enough, is only useful if the fuselage extends aft the CG/center of pressure as seen from the side.

on many swept wing designs, the fuselage tail isn't far enough back to be a good place for a vertical. Like the side slip panels on very early biplanes, you can get destabilizing effects. Tip fins on swept wings are the logical result. Planks? Then a central fin/rudder can make sense.

Btw, a flying wing can certainly do aerobatics. No computers needed. The method of yaw control does figure in, you need 3 axis control. But given rudders, drag or conventional, the main concern is the low drag and speed control in a dive. Like a RV-x, you have to watch the downhill speed increase you don't get as much of in a Pitts.

Flex wing hang gliders cannot perform several aerobatic maneuvers, because they are 2 axis, no rudder control, and don't do inverted flight worth a darn.

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
Btw, a flying wing can certainly do aerobatics. No computers needed.
And what human is going to be able to duplicate a snap roll in a flying wing - with no vertical surface?!
It's not me.

#### Doggzilla

##### Well-Known Member
HBA Supporter
First, you should not confuse Stability with Damping. Damping is an indication of how long it takes to return to the previous attitude after a disturbance. Stability is what keeps you out of the cemetery.

Flying wings, properly designed, have no worse longitudinal stability than any other aircraft.

If you are talking about pure flying wings, lateral stability can be difficult to achieve, but you can still get acceptable stability. Remember there is little or no additional drag if a pure wing yaws a little. Unlike a tailed aircraft where the empenage and vertical stabilizer will cause extra drag if not aligned with the air flow.
I was referring to static stability, which damping is part of. Damping is a form of static stability.

Here is a NACA paper from early swept wing research so you know Im not just making this up. Its definitely a thing.

#### Doggzilla

##### Well-Known Member
HBA Supporter
Putting vertical fins on a flying wing to improve yaw control or stability is common. Strakes, skegs, whatever to want to call them, must be behind the center of gravity. Putting them on the fuselage that you compromise the flying wing design with since you don't fit inside until it's big enough, is only useful if the fuselage extends aft the CG/center of pressure as seen from the side.

on many swept wing designs, the fuselage tail isn't far enough back to be a good place for a vertical. Like the side slip panels on very early biplanes, you can get destabilizing effects. Tip fins on swept wings are the logical result. Planks? Then a central fin/rudder can make sense.

Btw, a flying wing can certainly do aerobatics. No computers needed. The method of yaw control does figure in, you need 3 axis control. But given rudders, drag or conventional, the main concern is the low drag and speed control in a dive. Like a RV-x, you have to watch the downhill speed increase you don't get as much of in a Pitts.

Flex wing hang gliders cannot perform several aerobatic maneuvers, because they are 2 axis, no rudder control, and don't do inverted flight worth a darn.
To clarify, the reason for strakes is for stalls and high angle of attack. As you have pointed out, having more side area ahead of the CG makes it statically unstable, and flying wings have far more side area ahead of the CG if you look at them from the side. This makes them unstable if they stall.

As long as the flying wing isnt swept very heavily like a B2 it should be possible to put strakes far enough rearward to cancel out this effect.

In fact, they can even be protruding backward off the fuselage if needed (think like a pair of stingers) because the angle of attack improves during high AoA and they would stick down into the incoming air.

#### Sockmonkey

##### Well-Known Member
Adding belly strakes to a flying wing is the one experiment I would really love to see, since flying wings have so much potential if their yaw and AoA stability can be improved.
When people say "flying wing" I think of the Horten version with sweep. With sweep and washout they can be made nearly stall-proof as long as the CG is far enough forward. For a Fauvel type some type of belly fin makes sense, but that's best done by having the vertical rudder/s extend below the wing a bit.

#### RPM314

##### Well-Known Member
Adding belly strakes to a flying wing is the one experiment I would really love to see, since flying wings have so much potential if their yaw and AoA stability can be improved.
I can confirm this anecdotally. I've been refining a series of RC models in the ~170g range that I use for flying in small fields, so they do a lot of low speed turns. The first version was a delta with a dorsal fin at the rear, and pretty sloppy through slow turns, I imagine due to dirty air coming off the root of the wing at high alpha. Next version still had large delta strakes and similar amount of vertical area but all surfaces were ventral, which tracked a turn much better.

I was referring to static stability, which damping is part of. Damping is a form of static stability.
This is not correct. Static stability is defined as when disturbing the system produces a net force to push it back to where it started. This can be measured while the aircraft is stationary, hence the name. The paper you linked to does this in a wind tunnel with the model fixed, measuring the forces and moments at several positions to characterize the stability derivatives of the model.
Dynamic stability is defined as when forces are developed as the system oscillates such that energy is removed from the oscillation, which is the case if the damping ratio is positive. Measuring this would require letting the model rotate freely and record its position over time.
Mathematically (and in a dynamics 101 world where everything is linear and cows are spheres), static stability forces are proportional to displacement from the original position, and dynamic stability forces are proportional to velocity. Graphically, static stability is whether that orange line accelerates back down to 0 when it's pulled away from equilibrium instead of exploding to infinity, and dynamic stability is whether the dashed line bounding the amplitude of the oscillation decays to 0 or explodes to infinity. Note that a system can be statically stable but dynamically unstable, or vice versa.

As you have pointed out, having more side area ahead of the CG makes it statically unstable, and flying wings have far more side area ahead of the CG if you look at them from the side. This makes them unstable if they stall.
Are you referring to the side projected area of the wing's airfoil as a vertical surface? I'm not sure if that analysis is the right way to go, since a wing won't generate lateral force if you yaw it in the same manner as a vertical fin. In all the wings I've built and flown they either stall straight ahead an nose down, or drop one wing like any conventional plane.