Dihedral wings during reentry

[Wjolcz]

So after a bit of testing I noticed that angling the wings of my aircraft up reduces the turn radius and adds some lift (which is very helpful when landing).

So with this logic I thought: "Wouldn't that also stabilize an SSTO during reentry?"

Let's replace the air with water for a moment here. It seems logical that if something is angled and we push the stream against it the fluid/air/whatever will "slip" off it instead of splashing in all directions, thus reducing the friction. So if we try and pull the nose up it will be easier to expose the belly of the aircraft, and not dive nose-first and horribly die in flames.

Are my logic, assumptions and observations correct?

E: We're talking stock aero here.

E2: Can dihedral affect the top speed?

Edited August 16, 2015 by Veeltch

[match]

Well, sort of. Theory is correct, but conclusion is flawed. Mounting the wings with a slight angle will provide additional stability, but the airflow is still predominantly moving from front to back of the airfoil (re-entry is normally pretty fast).

The stability of the dihedral is derived from opposing forces. That is, they are providing a force (lift) against each other, cancelling each other out and providing stability at the cost of some lift overall.

If you held a 1 kg bar in your open hands and stretched your arms straight out, you probably could keep it relatively stable. your arms are acting in opposition to gravity and can achieve equilibrium by providing a constant force against it. Now introduce some other elements such as doing this while walking down the sidewalk on a windy day. The static situation has turned dynamic, all sorts of other forces are working on that bar now and it may prove difficult to keep level. If you closed your hands around the bar now and added slight pressure inward, towards the other hand, you would find that it is easier to hold the bar level. Some of your strength is devoted horizontally, so you wouldn't be able to lift quite as much, but you would be able to exert more control over the object in question.

To further this thought, would mounting the wings with an anhedral angle also give the same benefit?

[Goddess Bhavani]

I use both anhedral and dihedral in this micro shuttle design to provide stability - such that it is possible to fly a batteries out, no SAS re-entry. Note there is -no- yaw authority at all, but the combination of anhedral and dihedral in this design makes pointing the nose accurately very easy using pitch and roll controls.

[Wolf Baginski]

Dihedral helps with the stability, though there are other ways of doing it. Put the wings at the top of the fuselage, such as with the Hawker Harrier and, to get the lower stability of a fighter aircraft, you need anhedral, the opposite.

But it doesn't do a lot for the early stages of reentry. You will see more drag from the SAS set to radial rather than prograde, and the more speed you can lose over 50km altitude the better, but you won't see much from the stability boost of dihedral. There's just not enough air.

The SAS system can cover up a lot of weaknesses in a design, and this roll stability is one of them. And it needs a lot of dihedral to get a significant loss of total lift. But it needs a lot less lift to roll the plane than to stop it falling out of the air.

It's a bit of a simplification but a 5° dihedral will lose less than 1% of total lift, and for small changes, drag is proportional to lift. But the control surfaces roll the plane by generating lift. So a small dihedral is going to make a very small difference. What can matter more is the pitch control.

In a conventional layout the centre of mass is in front of the centre of lift, and the pitch control, the tailplane is at the back. So, to keep the plane in balance, the tail has to be pushed down. Which means more total weight needing more lift and creating more drag. Canard foreplanes get the same effect by lifting the nose, which means less lift needed for the same weight, and so less drag.

Does it matter? You can make more of a mess by flying the wrong ascent profile.

[Wjolcz]

So to sum this up: Not worth messing with dihedral on conventional SSTOs like this one:

[GoSlash27]

veeltch,

It's a little complicated...

Adding dihedral doesn't automatically add roll stability. It instead creates the tendency to roll away from a slip. If you have proper rudder area and leverage, it creates a tendency to self- level *BUT* If you overdo it or have insufficient vertical stabilizer, it can create a very odd roll coupled oscillation that makes it wallow.

It has to be balanced with the vertical stab to create a critically- damped state, or else it will not only not add roll stability, but will actually throw you out of control.

I like it in very small amounts for spaceplanes, but it's not really necessary.

HTHs,

-Slashy

Edited August 16, 2015 by GoSlash27

[Starhawk]

veeltch,

It's a little complicated...

Adding dihedral doesn't automatically add roll stability. It instead creates the tendency to roll away from a slip. If you have proper rudder area and leverage, it creates a tendency to self- level *BUT* If you overdo it or have insufficient vertical stabilizer, it can create a very odd roll coupled oscillation that makes it wallow.

It has to be balanced with the vertical stab to create a critically- damped state, or else it will not only not add roll stability, but will actually throw you out of control.

I like it in very small amounts for spaceplanes, but it's not really necessary.

Interesting stuff, Slashy. I have a couple of sp designs that exhibit strange roll behaviour and I've been playing around with a dihedral setup which does seem to help.

Seems more stable without any yaw authority, as well, but that could just be my design.

Happy landings!

[GoSlash27]

Interesting stuff, Slashy. I have a couple of sp designs that exhibit strange roll behaviour and I've been playing around with a dihedral setup which does seem to help.

Seems more stable without any yaw authority, as well, but that could just be my design.

Happy landings!

Starhawk,

It matters where you've got the rudder (s) mounted in the vertical axis. If it's mounted high, it creates the same roll response as dihedral. If it's mounted low, it merely yaws into a slip with very little roll response. Dihedral works better with low rudders IME.

Best,

-Slashy

[Pecan]

Alright, time to 'fess up - I think di/anhedral looks cool, so I use it a fair amount.

For the minimal benefit though, the biggest headache is then getting the CoT (axis) from wing-mounted engines back in the right place. Veeltch, for yours the engine nacelles, etc. snap to that position; tilt the wings and you'll have to fiddle about with the offset tool instead. Your call whether it is worth it.