Why are (some) delta wings stable without a horizontal tail?

[xerj]

I remember coming across the answer somewhere, but I've lost it.

Anyone know?

TIA

[Dudley Henriques]

"xerj" > wrote in message
...
>I remember coming across the answer somewhere, but I've lost it.
>
> Anyone know?
>
> TIA

The aerodynamic factors inherent in a tailless delta wing planform are
complex, but basically the trailing edge of the wing has designed upsweep to
compensate for any longitudinal negative pitch moment produced by the
difference between the cg and the aerodynamic center of lift normally
handled by the horizontal tail downforce.
Dudley Henriques

[Darrell S]

The aft edge of a delta wing is further aft than ailerons would be on a
conventional aircraft. The elevons on the trailing edge both work in the
same direction for elevator action and in opposite directions for roll. The
two inputs are combined when executing a bank entry with a pitch change at
the same time. All successful delta winged aircraft are basically stable
if handled with care. Many have pitch dampers to assist in stability. The
B-58 delta I flew had roll, pitch, and triple-redundant yaw dampers. When
things went right it was an extremely stable platform. In flight refueling
was a piece of cake. But when things went wrong it could eat your lunch.

One issue that might confirm your suspicions that a delta is not always
stable is "elevon coupling" during a landing attempt. If you're sinking
towards the runway too fast and try to correct too quickly you find one area
of instability. To raise your nose quickly you must move the stick back
quite a bit. That raises both elevons. Elevons are part of the delta wing.
Raising them temporarily loses lift. So, by moving the stick aft quickly
the aircraft descends due to loss of lift until the nose raises enough to
stop the increased descent. When you have the desired pitch attitude and
move the stick back towards neutral the elevons come down to a relatively
streamlined position further increasing lift. Whoops, ballooning above the
runway. Try to correct by pushing the stick forward to lower the nose
causes the elevons to come down (like adding flaps). So while your nose is
coming down the elevons have increased the lift adding to the balloon.

I've seen guys going down the runway like a jackrabbit due to elevon
coupling. I had it happen to me on the checkout ride in a TB-58 and thought
I wouldn't be cleared to go "solo" (pilot) in the B model which doesn't have
an instructor pilot position. But my following landings were OK and the
instructor (in debriefing) asked me if I had previously thought "elevon
coupling" was an old wives tale. I had to admit he was right. He asked
what I'd learned. I said I learned if I can't control the flare without
making rapid large pitch inputs it's time to go around. He said "Go forth
and fly, Grasshopper, because you have learned a valuable lesson". (a
little poetic input)

--

Darrell R. Schmidt
B-58 Hustler History: http://members.cox.net/dschmidt1/
-

"xerj" > wrote in message
...
>I remember coming across the answer somewhere, but I've lost it.
>
> Anyone know?
>
> TIA
>

[xerj]

Wow, Darrell I can just picture that elevon coupling with the way you've
explained it. Tricky stuff. The plane would be thinking "what the hell do
you want me to do? Make up your mind!" as you oscillate down the glidepath.

Incredible looking plane that you flew!

[xerj]

Thanks Dudley.

Make sense now.

[Dudley Henriques]

"xerj" > wrote in message
...
> Thanks Dudley.
>
> Make sense now.
No sweat. Darrell hit on some of the operational issues for you. Also in
fighters like the 106 and the Mirage, you could grab a yard of stick above
corner, rotate the lift vector, and bleed the Ps down with the drag curve so
fast that overshoot became a real issue for a shooter sitting on a tracking
solution.
It was Lippitsch I think who pioneered the delta without a tail. The idea
was popular for a while, but the extremely high landing speeds made handling
the airplane a real challenge. The whole deal centered around the trailing
edge design. Putting the horizontal stab back on the airplane (Mig 21/blown
flaps etc) helped the low speed/angle of attack situation on landings, but
the delta remains even today as a huge induced drag machine into the left
side of the envelope.
Power control and front side/back side power curve issues on final are still
major areas of concern for the deltas.
Darrell knows......that arrowhead of an airplane he flew was one hell of a
piece of equipment :-))
Dudley Henriques

[Darrell S]

"Dudley Henriques" > wrote in message
k.net...
>
> "xerj" > wrote in message
> ...
>> Thanks Dudley.
>>
>> Make sense now.
> No sweat. Darrell hit on some of the operational issues for you. Also in
> fighters like the 106 and the Mirage, you could grab a yard of stick above
> corner, rotate the lift vector, and bleed the Ps down with the drag curve
> so fast that overshoot became a real issue for a shooter sitting on a
> tracking solution.
> It was Lippitsch I think who pioneered the delta without a tail. The idea
> was popular for a while, but the extremely high landing speeds made
> handling the airplane a real challenge. The whole deal centered around the
> trailing edge design. Putting the horizontal stab back on the airplane
> (Mig 21/blown flaps etc) helped the low speed/angle of attack situation on
> landings, but the delta remains even today as a huge induced drag machine
> into the left side of the envelope.
> Power control and front side/back side power curve issues on final are
> still major areas of concern for the deltas.
> Darrell knows......that arrowhead of an airplane he flew was one hell of a
> piece of equipment :-))
> Dudley Henriques

Yeah, Dudley, I was an instructor in the TB-58 towards the end and we
demonstrated high sink to the new pilots. We'd slow well below approach
speed in level flight and raise the nose to stay level. We'd slowly add
power up to 100% but stay out of afterburners. Pretty soon we'd hit high
drag/high sink and I'd have the student note his vertical speed. We
"looked" level but were descending at nearly 4,000 FPM. He could still
change the pitch and roll with apparently normal response but he couldn't
stop sinking until he could increase the speed. We practiced at medium
altitudes where you could lower the nose, accelerate and fly out of the
danger area. The main point of the exercise was to show how important it
was during approach to landing to keep the speed within normal boundaries.
On final approach there's not enough altitude to lower the nose and fly out
of it. If you ever got into high sink on final you'd have to light all
four afterburners and hope they all lit off. If one didn't light at that
very low speed you'd not have enough control to handle it. Especially if
it was an outboard engine that didn't light off.
>
>

[Dudley Henriques]

Hi Darrell;
Don't know if you got to the T38 or went through in the T-Bird, but what you
are describing isn't all that far off from what can happen to you in the 38.
Of course the Talon is a trapezoidal low aspect wing, but on final, if you
weren't careful, you could set up a sink that was unrecoverable. I've seen
6000 FPM on the VSI in the 38.
The Hustler must have come down like a brick with all that hardware hanging
under the wing :-)
Getting one of these high performance birds way back in the coffin corner
killed many an otherwise descent pilot.
I'm sure you remember that kid out at Edwards in the 100 who got into the
backside so deep he ended up in the corner and couldn't power out without
reducing the angle of attack and didn't have the room to generate any nose
rate in pitch at all. I'll bet I've seen that film clip a thousand times.
It's still the best lesson for keeping the final approach speed where it
belongs that I've ever seen.
I guess in the Hustler you went backside and into the left corner mighty
fast
:-))
Dudley


"Darrell S" > wrote in message
news:Cqz_f.186$Oe2.34@fed1read07...
>
> "Dudley Henriques" > wrote in message
> k.net...
>>
>> "xerj" > wrote in message
>> ...
>>> Thanks Dudley.
>>>
>>> Make sense now.
>> No sweat. Darrell hit on some of the operational issues for you. Also in
>> fighters like the 106 and the Mirage, you could grab a yard of stick
>> above corner, rotate the lift vector, and bleed the Ps down with the drag
>> curve so fast that overshoot became a real issue for a shooter sitting on
>> a tracking solution.
>> It was Lippitsch I think who pioneered the delta without a tail. The idea
>> was popular for a while, but the extremely high landing speeds made
>> handling the airplane a real challenge. The whole deal centered around
>> the trailing edge design. Putting the horizontal stab back on the
>> airplane (Mig 21/blown flaps etc) helped the low speed/angle of attack
>> situation on landings, but the delta remains even today as a huge induced
>> drag machine into the left side of the envelope.
>> Power control and front side/back side power curve issues on final are
>> still major areas of concern for the deltas.
>> Darrell knows......that arrowhead of an airplane he flew was one hell of
>> a piece of equipment :-))
>> Dudley Henriques
>
> Yeah, Dudley, I was an instructor in the TB-58 towards the end and we
> demonstrated high sink to the new pilots. We'd slow well below approach
> speed in level flight and raise the nose to stay level. We'd slowly add
> power up to 100% but stay out of afterburners. Pretty soon we'd hit high
> drag/high sink and I'd have the student note his vertical speed. We
> "looked" level but were descending at nearly 4,000 FPM. He could still
> change the pitch and roll with apparently normal response but he couldn't
> stop sinking until he could increase the speed. We practiced at medium
> altitudes where you could lower the nose, accelerate and fly out of the
> danger area. The main point of the exercise was to show how important it
> was during approach to landing to keep the speed within normal boundaries.
> On final approach there's not enough altitude to lower the nose and fly
> out of it. If you ever got into high sink on final you'd have to light
> all four afterburners and hope they all lit off. If one didn't light at
> that very low speed you'd not have enough control to handle it.
> Especially if it was an outboard engine that didn't light off.
>>
>>
>
>

[Jose]

> Getting one of these high performance birds way back in the coffin corner
> killed many an otherwise descent pilot.

Doesn't a high sink rate sort of =define= a descent pilot?

Jose
--
The price of freedom is... well... freedom.
for Email, make the obvious change in the address.

[Dudley Henriques]

"Jose" > wrote in message
m...
>> Getting one of these high performance birds way back in the coffin corner
>> killed many an otherwise descent pilot.
>
> Doesn't a high sink rate sort of =define= a descent pilot?
>
> Jose

You know Jose, when you make a post like this to someone, you can go one of
two ways. You can stick a smilie on the end of it demonstrating the good
nature intended and make a friend, or you can just be a pedantic bore and
lose one. :-)
Dudley Henriques

[Jose]

> You know Jose, when you make a post like this to someone, you can go one of
> two ways. You can stick a smilie on the end of it demonstrating the good
> nature intended and make a friend, or you can just be a pedantic bore and
> lose one. :-)

Smilie intended.

Jose
--
The price of freedom is... well... freedom.
for Email, make the obvious change in the address.

[Dudley Henriques]

"Jose" > wrote in message
m...
>> You know Jose, when you make a post like this to someone, you can go one
>> of two ways. You can stick a smilie on the end of it demonstrating the
>> good nature intended and make a friend, or you can just be a pedantic
>> bore and lose one. :-)
>
> Smilie intended.
>
> Jose

Fair enough. Take care

DH

[.Blueskies.]

"Dudley Henriques" > wrote in message
ink.net...
>
> "Jose" > wrote in message m...
>>> You know Jose, when you make a post like this to someone, you can go one of two ways. You can stick a smilie on the
>>> end of it demonstrating the good nature intended and make a friend, or you can just be a pedantic bore and lose one.
>>> :-)
>>
>> Smilie intended.
>>
>> Jose
>
> Fair enough. Take care
>
> DH
>

I think Sean Tucker was a descent pilot last week... ;-)

[Dudley Henriques]

".Blueskies." > wrote in message
. ..
>
> "Dudley Henriques" > wrote in message
> ink.net...
>>
>> "Jose" > wrote in message
>> m...
>>>> You know Jose, when you make a post like this to someone, you can go
>>>> one of two ways. You can stick a smilie on the end of it demonstrating
>>>> the good nature intended and make a friend, or you can just be a
>>>> pedantic bore and lose one. :-)
>>>
>>> Smilie intended.
>>>
>>> Jose
>>
>> Fair enough. Take care
>>
>> DH
>>
>
> I think Sean Tucker was a descent pilot last week... ;-)

Sean was very lucky. It's an extremely dangerous business and he's one of
the best in it. I was very happy to learn he made it. I have had a lot of
friends who didn't.
Sean was lucky in that he had time to set up and get out. Usually when
something like this happens in the low altitude demonstration community, you
don't get time. It's over in seconds.
We used to have an inside joke in the business about structural failure in
the Pitts. If you had an upper wing fail on you, or the struts went south,
you didn't have to worry about getting out. The flying wires would hold the
wing on and it would beat you to death before you could open the canopy or
hit the ground.
I'm very happy for Sean. He got that golden second chance that many of us
who work and have worked in the show demonstration venue never get or got
when and if things went wrong.
Dudley Henriques