High altitude aircraft stability.

[EdwardB3020]

I recently finished building a shuttle that flies perfectly fine in both high and low altitudes. The main problem is that I have to push the center of the lift farther back from the center of mass. Is that needed for high-altitude aircraft or is there something wrong with my design?

[Echo__3]

It is going to be difficult to give you pointers with your design without pictures. But you need to be aware of how your center of mass shifts as fuel is used as well as how the center of aerodynamic pressure changes as you change your angle of attack.

[EdwardB3020]

1 hour ago, Echo__3 said:

It is going to be difficult to give you pointers with your design without pictures. But you need to be aware of how your center of mass shifts as fuel is used as well as how the center of aerodynamic pressure changes as you change your angle of attack.

Generally, I make sure the location of the CoM is at the same place both dry and wet so that’s not the problem. The main issue is figuring out the aerodynamics. It seems for high-altitude planes mainly SSTOs and Shuttles that act like the NASA STS program tend to have that need but if I put the CoL closely behind the CoM then it tends to flip which is an issue for SSTOs more because they need to fly both in thick and thin atmospheres. 

Edited November 17, 2022 by CaptWhitmire
Grammatical fixes.

[Echo__3]

2 minutes ago, CaptWhitmire said:

Generally, I make sure the location of the COM is at the same place both dry and wet so that’s not the problem. The main issue is figuring out the aerodynamics. It seems for high-altitude planes mainly SSTOs and Shuttles that act like the NASA STS program tend to have that need but if I put the COL closely behind the COM then it tends to flip which is an issue for SSTOs more because they need to fly both in thick and thin atmospheres. 

How are you designing your wings? Are you just putting them on flat? Do you change their angle of incidence? Do you use a dihedral angle? What are you using for a vertical stabilizer and rudder? Do you use airbrakes or anything else to increase drag on the back of your craft? What is your angle of attack like during reentry? Placing wing pieces can get rather complicated as can your flight path. It would be very helpful to see some pictures of your aircraft.

[EdwardB3020]

16 minutes ago, Echo__3 said:

How are you designing your wings? Are you just putting them on flat? Do you change their angle of incidence? Do you use a dihedral angle? What are you using for a vertical stabilizer and rudder? Do you use airbrakes or anything else to increase drag on the back of your craft? What is your angle of attack like during reentry? Placing wing pieces can get rather complicated as can your flight path. It would be very helpful to see some pictures of your aircraft.

I put my wings flat most of the time when making aircraft and I’m mainly working with a medium to small vertical stabilizer. On drag, I try to incubus the parts that have large drag into cargo bays and fairings and I like to be around a 30 to 40-degree AOA for reentry.  I don’t have my computer on me at the moment so I can’t provide imagery. 

[DeadJohn]

I'm not sure if you are talking about a space shuttle style glider spaceplane returning from space, or a jet powered high altitude airplane.

Airplanes: For high altitude jet flight I don't recall needing any special stability tricks compared to low altitude flight. My general guidelines for building planes: keep lift a little behind center of mass; arrange fuel drain so mass doesn't shift much during flight; select between rear elevators vs nose canards for pitch control depending on whichever location is further from center of mass for maximum control.

Spaceplanes: Shuttles that are reentering from space are more prone to flipping than purely atmospheric planes.  I think higher speeds rather than altitude are the cause of the flips. I usually add oversized control surfaces, use elevators and canards at the same time, or move center of lift further back. If test flights find vehicle flips at very high altitudes (above 25km) where elevators and canards don't have enough air to work well, then I might add reaction wheel or RCS.

4 minutes ago, CaptWhitmire said:

parts that have large drag into cargo bays and fairings and I like to be around a 30 to 40-degree AOA for reentry

Ok, now I'm positive you are discussing spaceplane reentry. Keep higher drag parts behind center of mass, so the drag tends to push the nose down if you AOA gets too high. High drag noses will flip.

You might need to nose down a little as you hit thicker air.

[EdwardB3020]

On 11/17/2022 at 2:18 PM, DeadJohn said:

I'm not sure if you are talking about a space shuttle style glider spaceplane returning from space, or a jet powered high altitude airplane.

Airplanes: For high altitude jet flight I don't recall needing any special stability tricks compared to low altitude flight. My general guidelines for building planes: keep lift a little behind center of mass; arrange fuel drain so mass doesn't shift much during flight; select between rear elevators vs nose canards for pitch control depending on whichever location is further from center of mass for maximum control.

Spaceplanes: Shuttles that are reentering from space are more prone to flipping than purely atmospheric planes.  I think higher speeds rather than altitude are the cause of the flips. I usually add oversized control surfaces, use elevators and canards at the same time, or move center of lift further back. If test flights find vehicle flips at very high altitudes (above 25km) where elevators and canards don't have enough air to work well, then I might add reaction wheel or RCS.

Ok, now I'm positive you are discussing spaceplane reentry. Keep higher drag parts behind center of mass, so the drag tends to push the nose down if you AOA gets too high. High drag noses will flip.

You might need to nose down a little as you hit thicker air.

Ah, this is what is probably causing the issue. Thanks for the tip. I usually put solar panels on the side and that’s it but it’s surprising how much drag Solar Panels and Antennae create. I might have to put them into a cargo bay or a service bay. 

Edited November 18, 2022 by CaptWhitmire
Grammar and clarification.

[FleshJeb]

Try using the CorrectCoL mod to show your stability at various AoAs, speeds, and altitudes. It's saved me a lot of time getting things right the first time.

EDIT: CorrectCoL also accounts for non-wing drag, unlike the stock indicator.

The link for it is in my signature.

Edited November 18, 2022 by FleshJeb

[18Watt]

As others have mentioned, the movement of your COM during fuel consumption plays a big role.  Just because it's good when full and empty does not mean it is going to be in the right location when you have a quarter of your fuel left.

If you are having stability issues during re-entry, it could be due to the high angles of attack that you might be flying.  At high angles of attack, the COL will move, if the COL moves forward that will cause pitch instability.  Here are two tricks I use to manage the aerodynamics during re-entry:

• I generally re-enter with at least a little fuel.  I can then transfer fuel between the tanks in the front of the plane and the tanks in the back of the plane to move my COM if needed.  During re-entry with high angles of attack, I usually transfer some fuel (weight) into the frontmost tank.  That adds pitch stability, and makes it less likely to flip.  Once I get down low and slow (subsonic) I can transfer some fuel to the rearmost tank if I need some more pitch authority.
• Another trick is to add a pair of Airbrakes to the back of the plane.  I mount them as far back as I can get them.  When deployed during re-entry they do two things- they help slow the plane, and also add some pitch stability.

 

[EdwardB3020]

10 hours ago, 18Watt said:

As others have mentioned, the movement of your COM during fuel consumption plays a big role.  Just because it's good when full and empty does not mean it is going to be in the right location when you have a quarter of your fuel left.

If you are having stability issues during re-entry, it could be due to the high angles of attack that you might be flying.  At high angles of attack, the COL will move, if the COL moves forward that will cause pitch instability.  Here are two tricks I use to manage the aerodynamics during re-entry:

• I generally re-enter with at least a little fuel.  I can then transfer fuel between the tanks in the front of the plane and the tanks in the back of the plane to move my COM if needed.  During re-entry with high angles of attack, I usually transfer some fuel (weight) into the frontmost tank.  That adds pitch stability, and makes it less likely to flip.  Once I get down low and slow (subsonic) I can transfer some fuel to the rearmost tank if I need some more pitch authority.
• Another trick is to add a pair of Airbrakes to the back of the plane.  I mount them as far back as I can get them.  When deployed during re-entry they do two things- they help slow the plane, and also add some pitch stability.

 

Thanks for the help. The general reentry path I like to follow is Bradley Whistance’s old 1.0.5 SSTO tutorial.