Can anyone explain the aerodynamic/physics difference of an outside loop vs. an inside loop? Most planes including trainers can easily do an inside loop, but there are plenty of planes that cannot do the outside loop. Not being a physicist/rocket scientist, I cannot see too much difference in the two maneuvers. I would LOVE a knowledgeable explanation!!!

Regards,

Jeff

I think there are a few factors involved, but airfoil and dihedral stand out for me.

The airfoil can produce very different amounts and type of lift during the outside loop. Mainly the arfoil is at a negative angle-of-attack the whole way through the manouvre and this is worst when pulling up from the bottom (inverted) part of the loop. A symmetric airfoil has no problem with this, but if the bottom of the airfoil is flat or concave you get a lot less lift when it is inverted. The non-symmetric airfoils are also more prone to stall while inverted. Also, the smaller the radius of the leading the worse the inverted stall becomes.

Diheral, on the other hand, will make the plane want to flip over and fly right-side-up. The wing is in an unstable lift state and will want to a move to the normal stable configuration. And bank angle will make the plane flop out of the outside loop in a cork-skrew fashion.

The best configuration for an outside loop would be a wing with a symmetric airfoil and no diheral.

A less important factor is the weight distribution in the model. If the plane has a high wing the weight is below the wing in normal flight. This creates a stable condition due to the pendulum effect. When inverted, the weight is on the inside of the loop and will tend to want to go to the outside due to angular momentum. Any bank while in the outside loop gives the weight a direction in which to flip the wing over so that the weight is on the outside of the loop.

So, the best configuration as far as weight goes is to have the wing mounted centrally,or even at the bottom of the fuselage.

-Q.

It is mainly due to how the airfoils involved produce lift at different angles of attack in the negative angle of attack (lift) range.

A typical aerobatic pattern ship has symmetrical airfoils (whose characteristic is that the lift curve slope is a straight line (linear) from -12 deg to +12 deg) with all surfaces set at 0 degrees with respect to each other. As a result when you do an inside loop with an aerobatic airplane (positive g force toward the canopy and positive angles of attack) or an outside loop (negative g force away from the canopy and negative angles of attack) when the loops are the same diameter the g forces and angles of attack are the same.

Everything is symmetrical and unless you look for a canopy you can't tell the top from the bottom. Well sometimes the rudder and wheels give it away but the general idea is that from an aerodynamic point of view there is no difference and it flies like that is the case. It makes it really easy to fly a contest pattern.

A typical trainer ship has flat bottom airfoils with the wing set about +3 degrees with respect to the horizontal tail. This makes the airplane more of a self correcting airplane, what you need for a trainer.

The flat bottom airfoil has an interesting characteristics, that over a small range of angles of attack (-4 to +10 or thereabouts) the wing lift curve slope is linear ( it acts like the symmetrical wing). But, and it is a big but, as the negative angles of attack increase negatively beyond -4 (or so) the wing no longer acts like the symmetrical wing and for a given angle of attack has less lift.

As a result when you do an inside loop with the trainer airplane (positive g force and positive angle of attack) the airplane works like the aerobatic airplane. But, and it is the same big but, when the airplane tries an outside loop ( needing negative g force and negative angles of attack which are already offset by the incidence angle) it starts having trouble because the wing isn't producing lift as well. To keep trying to do the outside loop you push in more down elevator and eventually might run out of elevator travel before the airplane outside loops.

Bottom line, the symmetrical airfoil can't tell if it is upside or not. The flat bottom airfoil can have a lot of trouble making the negative lift to make an outside loop.

It's late and I might hope this makes sense, if not I plead diminished capacity by vitue of being too sleepy.

thanks, a088008, but I am still a little confused as to your 2nd reason. wouldn't the physics be roughly the same (assuming same airfoil, moments, weight, power, etc) for a high winger doing an inside loop vs. a low winger doing an outside loop?

Astrohog

Thanks to you as well, Ben. I think I am getting the idea about the different airfoils, but what about a bipe? Does this change things, too? The whole reason I started this thread is that I was out at the field today and one member was flying his scratched Smith miniplane bipe. He is an accomplished builder and flyer. it is a large bipe w/ a U.S. 41 for power. It has very good vertical punch and all other flying characteristics are very good. it is an aerobatic ship. I AM NOT SURE WHAT AIRFOIL HE USED (he did cut his own foam cores) BUT THE WINGS APPEAR TO BE FULLY SYMMETRICAL (or darn near). He could NOT get it to outside loop, it kept rolling out of it near the top. Was this a POWER problem, or a BIPE thing, or could it be the airfoil reasons that you discussed in your reply? Thanks again for your help!! ( I LOVE LEARNING MORE ABOUT THIS STUFF!!!!)

Astrohog

Dihedral doesn't make any difference except if the dihedral angle gets too high it is difficult to keep the airplane level thoughout the outside loop. Dihedral is a flying qualities thing and not the capability of doing the loop.

Mass distribution isn't really that much of a factor if you can keep the wings level.

As for a Bipe. The top wing is above the fuselage and will behave somewhat like a high wing design (a BIG "somewhat" as some Bipes behave quite well and don't have a problem with outside loops).

The point I'm trying to make is that the wing is above the center of mass of the plane. To illustrate, try and balance a broom upside down on the palm of your hand (left or right, which ever is easier ). The center of mass is WAY above your hand and as a result tried to get below your hand so that it can hang like a pendulum. This is why you need to keep moving your hand in the direction the broom starts to fall towards. This is to try and get the center of mass directly above your hand. If it is not, it start to fall over.

The same thing might be happening with the Bipe, if the top wing is quite high above the fuselage. If you look at a Pitts Special Bipe you will see that the wings are close together and the the plane does a wonderful job in aerobatic manouvers, but I'm sure it's less stable in an outside loop than a fully aerobatics "pattern ship".

There is also a change that the difference in incidence of the two wings is the problem. The bottom wing might be stalling before the top wing due to lower incidence and the effects of the top wing.

Having said all of this, is the plane very heavy? Also, is the top wing very forward compared to the bottom?

I find it surprising that it does not do an outside loop, as most planes can be MADE to do one if the pilot fights with it a bit.

-Q.

I have flown a plane with a good amount of wing-tip dihedral. It was a nightmare to outside loop. It would flip over unless I was perfectly level when starting the loop. Even then it would require some serious corrections when inverted to stop it from flipping over at the bottom part of the loop. But the again, other factors of the plane might conteract the flipping tendancy (like a large fin/rudder). The plane I flew did not.

Anybody else find this?

-Q.

.
That's running out of airspeed. The inverted climb in a draggy airplane to start with needs power.
What does the same airplane with a "pull-up" from inverted flight?
I have many planes that loop normally but I won't do a down-going outside loop with.. the ground gets too close. Some of these will pull up thru to horizontal from inverted though.

It's amazing that none of the learned aero engineers and fliers posting in this thread mentioned the primary difference between inside and outside loops.

It's GRAVITY!

There are aerodynamics involved, as Ben pointed out. When a plane "rolls" out of an inverted maneuver, it's actually a high-speed stall, or "tip stall," caused by too great a load (G) and too high an angle of attack.

My M2-E would do that at the bottom of an outside loop, and I was able to eliminate it by mixing flaps with elevator, so that when down elevator is applied the flaps go up. When the plane is at the bottom of an outside loop, the flaps are increasing the camber in the right direction, and sufficient lift is created to complete the loop.

Here's a quiz: The record for simultaneous outside loops was held for a long time by a gentleman in a Great Lakes biplane. For the grand prize, name the pilot, and the number of loops he did.

Was the pilot Ernst Udet? I couldn't guess as to how many loops held the record.

But then later, didn't Joann Osterud break and hold the record for outside loops in her Hiperbipe?

Hi John; No, it wasn't Udet.

Good try, tho'

And I heard the record had been broken more recently. I was told it was a woman, but not Patty Wagstaff, so I'm glad to be better informed on the subject.

I wouldn't fly a plane that can't complete an outside loop. Even my Aeronca Champ would do it, given enough altitude and speed at the entrance, and it has a flat-bottom wing!

AstroHog; Try a larger circle (you'll need lots of altitude). The larger circle will allow you to build up enough speed on the downside to climb back up. Also, the larger circle will keep the angle of attack small, and avoid the "roll-out."

On the otherhand, I suspect the records were set in airplanes that could do very small loops at slow speed, using lots of power to complete, but not exposing the pilot to as much negative Gs as large, fast ones.

Would you believe the old record was over 100? Oh, my aching eardrums!

The quickest way solve the problem: Slap on a bigger engine. With enough power, just about any airplane will do an inverted loop. Having plenty of altitude can't hurt either.

"Remember, if you're in trouble, hang on to your altitude. No one has ever collided with the sky."

-Dan Nelson

PS: The way to tell if you have too much power? If you give it full throttle and the engine mount rips out of the plane.

a088008 - The thing about the relative position of the CG to the wing is that my pattern airplanes have the CG above the wing and will do a inside loop with no problems. Difficulty in flying an outside loop with a cabin airplane high dihedral wing are not primarily due to the ability to loop but the ability to keep the wings level in the loop. They are two separate aero things but both combine to make a loop more difficult.


Ralph - I just checked and gravity remained the same when I help the airplane upright and upsidedown (heh, heh, heh)

You are thinking that the outside loop entered from level flight with a heavy doggy airplane will hit the bottom of the loop at 4 to 5 times the speed compared to what it will attain when at the top of an inside loop. To pull the g's at the high speed could cause the snap as you described. Especially if the wing loading was high.

Of course to verify this just do a 180 roll and do the outside up and inside down.

Since Astrohog said the bipe rolled out of the loop at the top of the outside loop it sounds like he did the 180 roll and pushed outside loop upward. But then how could it roll out of it since he would already be upright!

Astrohog, which way was he looping again???

Ben; You're missing the point. Doing an inverted inside loop is not the same as an outside loop.

When you enter an inside loop, you're climbing against G. With a little dive to pick up speed, almost any aircraft will do an inside loop, because when you get to the top, you've given up a lot of speed, and it's easy to complete the loop, even with no power, because G is helping you.

An outside loop, on the other hand, is entered by diving, and G makes you go real fast. Then you apply more down elevator and full power at the bottom, so you can climb up the back side, against G.

Non-symmetrical airfoils often stall in this high-speed, high-load, inverted maneuver. Mine did, and I found that it could be eliminated two ways:

1, A larger loop reduces the wing loading, or

2. Mixing flaperons with elevator.

"Be mindful of gravity, lest the earth rise up and smite thy airplane." - Guru Maricrashi

Ralph the direction that the outside loop is intered into can give results to determine if the problem is entirely aero related (airfoil, tail power, etc) or if it is just too heavy (wing stalling) , bad CG or bad technique.

Just
1. start in level flight, push over into an outside and see what happens or
2. roll 180 and push up into an outside.

Then repeat the same doing an inside loop.

Hi Ben;

I've done the field tests. No problem entering an outside loop inverted. A problem was only encountered at the bottom of an ordinary outside loop, when my low-wing, low-dihedral sport-aerobatic plane would stall and roll out of the maneuver, when I tried too tight a loop. No problem on big, easy outside loops.

The solution (suggested by a pylon-racer friend) was to mix in some flaps with elevator, so that up elevator causes down flaps and down elevator, "up" flaps, or spoilers. This worked great! Eliminated the problem entirely. Here is my analysis:

The highest loads encountered in an outside loop is when the aircraft is inverted at the bottom of the loop, since the loads resulting from the angular velocity are added to G. Also, the airfoil on my plane is only semi-symmetrical, and while it flies inverted quite well, it will stall at angles of attack that would be acceptable right-side-up.

So, when inverted, the down-elevator required to complete the loop causes the flaps (flaperons, in this case) to go "up," which is down, when inverted.

The extra curvature allows the inverted wing to accept the high angle of attack without stalling.

In the case of high winged planes and high positioned upper wings there's the drag couple to address too. Dive the cabin model trainer with limited elevator control and the extra speed piles up the wing drag in a hurry. Add in the extra drag of the negatively loaded lifting airfoil and you get a LOT of high centered drag moment holding the nose from tucking under properley. And with soft responding trainer types there may not be enough down elevator force to totally counter that drag along with the rest of the factors.

Add enough down elevator area and deflection and it'll overcome that but it will never be happy.

I have a Fleet'ish Barnstormer that is marginally powered and it will NOT do an outside from a dive entry. But flip it over to inverted and push down and it'll go around just fine for the first half but the second half is a different story. I did it once by cutting the power to idle and the big 4 stroke prop held it back enough to just squeak around. All the rest were 3/4 outsides to a dive and positive pull recovery.

Semi symetrical airfoil here (low camber and thick in other words ).

Astrohog. Your buddy with the miniplane may have been fighting a couple of factors. One his airfoil may have been like mine. Two, he may not have had enough down elevator. Three, his model setup may have had quite a lot of incidence in the wings and or tail. To do decent ousides you need to be fairly close to 0-0. And finally all the rest of the setup may not have been enough to counter the speed induced high drag couple or at least it may have been just enough to make the elevator's job too hard.

But it's certainly not a "bipe" thing. Ever seen an Ultimate fly?

At least that's my theory and I'm sticking to it....