Self correcting platform????

[Tophinater]

I was just thinking about how everyone uses these complex INUs to keep there planes/craft level. Is there a design out there that is self correcting? As in if a gust of wind comes and blows it sideways it will correct its position by the shear mechanics of it. I have an idea of a design that is someone what like the idea used in the original torpedos. Too bad its 2:00AM and my mind is starting to shut down. :confused:

[ManTin361]

Isn't what your explaining kindof of like a trainer? All aircraft with a high wing has some amount of self stablization, thats why they use them for trainers. As the distance between the wing and the CG increases the more stable it becomes. If my understanding is correct a powered paraglider is very stable!

Just my $.02

Matt

[clolson]

I don't know this from personal experience but I've been told this and it makes some intuitive sense: the more self stable an airplane is (i.e. high wing/dihedral) the more it will react to little bumps and turbulence and gusts and wind changes. If you are building a camera platform, that is a trade off you may need to consider. A faster, heavier, more neutrally stable aircraft may provide a bit more stable platform.

[Unterhausen]

I think trainer and low wing loading tend to go together, but probably for stability as much as anything, so I think you are right. I don't think that dihedral matters much with high wing loading. There aren't too many high performance aircraft with a high wing, F111 comes to mind.

When I flew in the back of an F-16, which is as unstable as you'd want to get, it was the smoothest flight I've ever been on. It was a cold winter's day in the mountains with bad weather too. The pilot apologized to me for the rough ride, but I'd take that ride any day over a commercial plane. Even if it wasn't tons more fun. From the turbulence we were going through, it was clear a small private airplane would have been bouncing up and down like a styrofoam cup in the ocean during a hurricane. I'd say it was the only flight I've ever taken where there wasn't an altitude excursion due to turbulence.

[ElectroLawndart]

There aren't too many high performance aircraft with a high wing, F111 comes to mind.


Citabria and Decathalon come to mind.


When I flew in the back of an F-16, which is as unstable as you'd want to get, it was the smoothest flight I've ever been on.

The smooth ride is most likely due to flight control computer. Correcting for departures from the flight path before it can really be felt. I had read somewhere that the computer updates at something like 30 time a second. Waaay faster than a human can give control inputs.

Dart

[Unterhausen]

Yeah, I'll have to hitch a ride on an F-4, and I'll tell you if it's the flight control or the wing loading.

[ElectroLawndart]

Yeah, I'll have to hitch a ride on an F-4, and I'll tell you if it's the flight control or the wing loading.

Hitching rides in F-16's and F-4's!?! Man, am I envious. I got to ride in the back of a L-39 once. Pulled a sustained 4G's. That was enough for me. How many G's did you get in the F-16?

Dart

[JettPilot]

Smoothness has everything to do with wing loading and angle of attack (speed) of the airplane. There are other things that affect it also, but dihedral is not one of them. Dihedral will help it return to level after being upset from level, but it will not create more turbulence...

Having no dihedral will not stop the effects of turbulance either. The airlines like to make flights as smooth as possible for passengers. With modern fly by wire airliners, if it were just a matter of making wings with no dihedral to make an airliners ride smoother, they would have done it long ago.... Its just not that simple.

[ElectroLawndart]

Hi All,

I would think that dihederal would have an adverse effect on smoothness. Some kind of side gust would impart a roll component to the flight path which would then impart a yaw component until it dampens. With this assumption <grin> a stable aircraft should weathercock into the side gust and a 'neutral' stable aircraft should just get a bump to the side. Am I correct in this thinking or am I totally nuts? :D

Has anybody designed and built a fly-by-wire model with 'relaxed' stablility?

Maybe this should be continued in the Modeling Science forum.

Dart

[JettPilot]

I just cannot agreee with that. When considering smoothness, I have been in all kinds of turbulance in many different types of aircraft, and turbulence is felt in the form as jolts and bumps... Very different from banking and yawing which is a much smoother type of motion... An airplane with dihedral trys to return to level by itself, while you need to correct it with ailerons with no dihedral. But the bumps of turbulance are felt just as badly either way.

[ElectroLawndart]

HA! I thought I was being silly. Thinking about it more, I guess it would be more dependent on area loading. As to the dihedral question, I would expect that while an airplane with dihedral will return to level flight while an airplane with no dihedral wouldn't have changed it's roll angle, straight or banked unless the turbulance 'bubble' hits only one wing half.

The worst turbulance that I've experienced is on a flight from Vegas to Van Nuys in a Piper Warrior in the heat of a summer day. The plane bounced, pitched, rolled and yawed all over the place. It had a fair amount of dihederal.

Dart <===Not an aeronautical engineer by a long shot.

[kd7ost]

IMO it's still not that easy. The center of drag on an aircraft needs to be considered as well as the center of lift. (As do all the forces) Somewhere around the fuselage is generally where the center of mass as well as the center of gravity and the center of drag components all come together. (Usually) If you have a low wing, the center of lift is below the center of mass. The plane if left unchecked would want to roll upside down. This is a little simplistic but you get the idea. Like a pendulum. Dihedral raises the center of lift and helps it get closer to or above the mass.

In a high wing plane it's already there. The center of lift or pivot point where everything hanging below wants to rotate under is already high. Adding more dihedral is like putting the wing up on a pod. It makes it more inherently or aerodynamically more stable. That's OK but only to a point. The more stable something is, the more it wants to fly itself. Good for a trainer. But if you want to make it do things and do so without a lot of effort, the stability of the airframe can start to work against your efforts. We build models that generally handle what we need.

If you built a low wing plane with symmetrical airfoil and flew it while inducing a yaw component, you want the dihedral to be such that it neither rolls into or out of that turn. (That’s coupling through aerodynamics) Obviously when you build a high wing with dihedral or polyhedral you do want rudder (yaw) to induce a roll. But just enough. Too much roll is not good and neither is not enough. I don’t use dihedral in my high wing aileron ships. They’re stable.

Dan

[ElectroLawndart]

Thanks All,

Forgive me if I seem somewhat dense. Aerodynamics is a very complex subject. Trying to anticipate the actions of billions of little air molecules, each with a mind of their own. :p My education is in electronics where things seem more black and white.

Dart <==Searching the Web for someone whos built a computer stabilized model.

[Unterhausen]

Hitching rides in F-16's and F-4's!?! Man, am I envious. I got to ride in the back of a L-39 once. Pulled a sustained 4G's. That was enough for me. How many G's did you get in the F-16?
Dart
I'm out of that business now, no more hitching rides for me. As far as I know, we never went over 3.5 g's, which felt like the hands of God on my shoulders. The fun part is, it was during a right hand descending turn at 350 ft going 400kts. It was part of a flight test for a terrain following/ground collision avoidance system. Right after that, I took a picture of myself that looked like I was on the ground, we were so close. Most fun I've ever had while losing my lunch.

[TMorita]

Isn't what your explaining kindof of like a trainer? All aircraft with a high wing has some amount of self stablization, thats why they use them for trainers. As the distance between the wing and the CG increases the more stable it becomes. If my understanding is correct a powered paraglider is very stable!

Just my $.02

Matt

Well...paragliders are stable in some ways, and not very stable in other ways.

It does have a low CG, but there are some problems which are unique to paragliders:

1. Stalls. On a fixed wing aircraft, stalls aren't too bad. However, a paraglider is dependent on airspeed to keep the wing inflated. When the wing stalls, there's not enough air pressure to keep the wing inflated, and so it crumples up and it falls straight down. You can recover from this condition, but the timing and technique is tricky. If you do it wrong, you will be thrown into the wing and "gift-wrapped" and tangled in the lines, and recovery is extremely difficult from this condition.

2. Collapses. There are a variety of wing collapses which can occur in turbulent conditions. There's asymmetric collapses where one side of the wing collapses - this will cause the wing to turn abruptly, and is usually not serious unless you're flying close to a hillside because you're "scratching" for lift. You can also have frontal collapses where the entire front of the wing collapses (very scary). You can have cravats where one side collapses and the wingtip gets stuck in the support lines, etc.

There's also the issue that paragliders tend to swing a lot. If you're riding in one, you can weight-shift or use brakes to control the swing, but if you're flying an R/C paraglider, that probably isn't possible.

So there's a whole host of problems which are unique the paragliders.

Toshi (yes, I paraglide)

[cobaltdan]

UAV paragliders are far more dificult to design than rigid wing UAV's.

You are faced with several unique problems.

i.e. your inertial sensors are not rigidly coupled to the wing, they are suspended far below on a semi rigid pendulum. you have to cope with dramatic inu noise caused by yaw oscillations, pitch oscillations and spring constants of the lines.

in a rigid wing uav your inu position is dead coupled to your wing and aircraft position this is not the case in a uav parafoil.

filtering to make sense of your inu data in autonomous paragliders it is a very complex task and the subject of several papers and pending patents by us.

we manufacture fully autonomous UAV powered paragliders for the u.s. military ranging from 50 pounds payload to 3000 pounds on our newest model.

sincerely,

daniel preston
atair aerospace
www.atairaerospace.com

[ElectroLawndart]

i.e. your inertial sensors are not rigidly coupled to the wing, they are suspended far below on a semi rigid pendulum. you have to cope with dramatic inu noise caused by yaw oscillations, pitch oscillations and spring constants of the lines.



Hi Dan,

Cool things are happening in your company.

Is the low frequency yaw oscillations a problem to filter out and still have good update rates for heading?

[cobaltdan]

yes it has definately been challenging. We have submitted a paper to aiaa on our developed solutions. i will post once it is published.

daniel preston
atairaerospace.com

[Unterhausen]

iRobot put a mockup of one of their robots on a parasail. The video is pretty amusing. I can see that the issues would be pretty difficult to deal with.

[ElectroLawndart]

Hi Dan,

Can you say what kind of update rates you were getting on all three axis? I understand if you can't divulge any information until after publishing.

Dart

[cobaltdan]

Hi Dan,

Can you say what kind of update rates you were getting on all three axis? I understand if you can't divulge any information until after publishing.

Dart




20hz

[ElectroLawndart]

Wow cobaltdan!

20Hz with filtering. If anything I would have thought that it might be in the .5 - 1 Hz area with all of the low frequency noise/oscillilations going on. I'm looking forward to reading your paper when it's published. When will it be published.

Congrats in advance,
Dart