Aerodynamics of a loop

[flyingcheesehead]

When coming through the top of a loop, you relax back pressure. Why?

I was listening to a podcast that talked about a plane flipping over when a student pilot held back pressure past the top of the loop. What would cause this? It also reminded me of a comment Chip made the last time we flew the Extra along those same lines - Don't pull when you're upside down.

My original thought when Chip was teaching me loops was that when you come through the top of the loop gravity would be helping you tighten the loop up and you'd relax the back pressure to keep the loop more circular. Now, I'm guessing there's more to it.



Thanks,

 

[gibbons]

When coming through the top of a loop, you relax back pressure. Why?

I was listening to a podcast that talked about a plane flipping over when a student pilot held back pressure past the top of the loop. What would cause this? It also reminded me of a comment Chip made the last time we flew the Extra along those same lines - Don't pull when you're upside down.

My original thought when Chip was teaching me loops was that when you come through the top of the loop gravity would be helping you tighten the loop up and you'd relax the back pressure to keep the loop more circular. Now, I'm guessing there's more to it.



Thanks,

Kent,
You're thinking too hard. You "float" the top of the loop to make the loop round. If you pull 4 g's at the start of the loop when your airspeed is high, then you must pull fewer g's at the top of the loop whe your airspeed is low - otherwise the loop will be egg shaped.

As for not pulling when upside down, I think that was in reference to the rolls we were doing.

And as for the airplane "flipping over" at the top of a loop because of too much pull - I'm not sure about that one.

 

[Greg Bockelman]

What Chip said. He beat me to it.

 

[gismo]

When coming through the top of a loop, you relax back pressure. Why?

As Chip said, a loop wil look more like a lower case script 'L' if you don't. I didn't really like the rest of his explanation though. Even if you had enough power to maintain constant airspeed all the way around, you'd still need to reduce your pull (and g load) over the top because gravity is assisting your change in pitch there vs fighting it at the bottom. In fact, I think you'd need more not less pull (and AOA) at the top if lower speed was the only difference. Now I could be wrong as Chip has way more aerobatic experience than me, but I think I'm right and Chip probably just hasn't given much thought to the issue since he can make round loops all day long.

I was listening to a podcast that talked about a plane flipping over when a student pilot held back pressure past the top of the loop. What would cause this? It also reminded me of a comment Chip made the last time we flew the Extra along those same lines - Don't pull when you're upside down.

My original thought when Chip was teaching me loops was that when you come through the top of the loop gravity would be helping you tighten the loop up and you'd relax the back pressure to keep the loop more circular. Now, I'm guessing there's more to it.

No, that's pretty much it. The caution is likely because in many planes your speed at the top will be getting pretty low and if you were pulling the 3-4 g's (or even 1 g) you had upon entry you'd stall. You might even end up in an inverted spin since it's awfully easy to get a bit uncordinated when you are coming through the top.

 

[gibbons]

Even if you had enough power to maintain constant airspeed all the way around, you'd still need to reduce your pull (and g load) over the top because gravity is assisting your change in pitch there vs fighting it at the bottom.

A loop is, after all, a turn. And we know that the faster the speed the larger the radius of a turn given a specific bank angle (g-load). Same with the loop. We pull harder to keep the radius constant when we are going faster, and pull less when we are going slower. That's why we start an upwind loop with less pull than we do a downwind loop. Of course you're correct, gravity conspires to screw the whole thing up.

In fact, I think you'd need more not less pull (and AOA) at the top if lower speed was the only difference.

I've read this several times and don't follow. Let's say gravity doesn't play a part. If I enter a loop at 150 kts and pull 5 g's to start it, then continue to pull 5 g's over the top at 80 kts, how can the effect be anything other than decreasing the radius of the loop over the top?

...since he can make round loops all day long.

Not according to the judges.

 

[flyingcheesehead]

You're thinking too hard. You "float" the top of the loop to make the loop round. If you pull 4 g's at the start of the loop when your airspeed is high, then you must pull fewer g's at the top of the loop whe your airspeed is low - otherwise the loop will be egg shaped.

Ah, OK. I was thinking more gravitational forces than airspeed. So it's not so much "don't pull as much during the third quarter of the turn" as it is continuously varying the pull throughout the maneuver?

So, tell me if this is roughly correct: Coming into the loop, we give a firm, swift application of back pressure at the bottom. An eighth of the way through the turn, our airspeed is still high and just beginning to drop, so we're still pulling hard. As we come through vertical a quarter of the way through the turn we begin to slowly relax pressure. Three eighths through, gravity is helping us out more and more and airspeed is still falling. At the top of the loop we're at our lowest airspeed and gravity is giving us the most help, so we're at our least amount of pull. For the next quarter turn, we allow gravity to help us around. Gravity itself would give us a parabolic flight path, and our airspeed is increasing, so from 5/8 to 3/4 of the way through we'll be increasing the pull. After passing the downward vertical, gravity is once again working against us and our airspeed continues to increase so we'll pull significantly harder, similar to the beginning.

Sound about right?

As for not pulling when upside down, I think that was in reference to the rolls we were doing.

IIRC it was early on when we were yanking and banking and burning fuel out of the wings. I didn't quite get it but I was having too much fun to care at the time.

And as for the airplane "flipping over" at the top of a loop because of too much pull - I'm not sure about that one.

Well, considering the person who said it is a 29-hour student pilot who happened to get a ride in a T-6 and was on the controls at the time, it may very well have been some sort of stall/spin type of event and he just didn't understand what really happened.

 

[gibbons]

So, tell me if this is roughly correct: Coming into the loop, we give a firm, swift application of back pressure at the bottom...

Sound about right?

Sounds about right except for the initial application of back pressure. I think of it more like "steady" instead of "firm, swift", but that may just be perspective. Now add a 15 kt headwind and the whole thing changes.

 

[flyingcheesehead]

Now add a 15 kt headwind and the whole thing changes.

Awww man... I really wish I had access to a plane I could practice this stuff on. I guess step one is to get that tailwheel endorsement taken care of. No acro instructors that are very close either. And, no more vacation coming until next May.

 

[Steve]

Diana had asked a while back how to do a 2 G loop in another thread. I found a reference describing how to calculate the force vectors involved in performing a loop, albeit in a high performance jet.

Unfortunately, the link

http://flighttest.navair.navy.mil/unrestricted/FTM108/c6.pdf

is now dead.

And I didn't have the foresight to download the document. Ack!

 

[flyingcheesehead]

Diana had asked a while back how to do a 2 G loop in another thread. I found a reference describing how to calculate the force vectors involved in performing a loop, albeit in a high performance jet.

Unfortunately, the link

http://flighttest.navair.navy.mil/unrestricted/FTM108/c6.pdf

is now dead.

And I didn't have the foresight to download the document. Ack!

Steve,

Archive.org saves the day!

http://web.archive.org/web/20030628...st.navair.navy.mil/unrestricted/FTM108/c6.pdf

 

[gismo]

A loop is, after all, a turn. And we know that the faster the speed the larger the radius of a turn given a specific bank angle (g-load). Same with the loop. We pull harder to keep the radius constant when we are going faster, and pull less when we are going slower.

I've read this several times and don't follow. Let's say gravity doesn't play a part. If I enter a loop at 150 kts and pull 5 g's to start it, then continue to pull 5 g's over the top at 80 kts, how can the effect be anything other than decreasing the radius of the loop over the top?

I didn't quite have it right. What I meant to say was that at a lower speed you'd need to pull to a higher AOA in order to maintain the same g loading, but as you point out with the lower speed you want less g force to maintain the circle.

Actually I believe the two cancel almost exactly since centripetal force (g load in a loop w/o gravity) and lift (@constant AOA) are both proportional to velocity squared (ignoring changes in air density), so in a loop without gravity it now looks to me like you'd need a constant AOA. Of course, due to static stability you'd have to increase the backpressure to maintain AOA as the speed decreased over the top, so you are right and I was mistaken (no big surprise there).

Edit: Now I'm thinking that in a loop w/o gravity the plane would fly the whole loop at the same airspeed provided there was enough power added at the initial pullup to balance the additional induced drag. If that's true then in this hypothetical gravityless loop there would be no need for increased AOA or backpressure at the top. Back to you Chip....

That's why we start an upwind loop with less pull than we do a downwind loop. Of course you're correct, gravity conspires to screw the whole thing up.

I suppose aerobatics and flying in general would be considerably different If you could shut off the effects of gravity.

 

[Steve]

Dang!

And all I thought archive.org was good for was downloadeing B- movies like "The Phantom Planet"...

Steve,

Archive.org saves the day!

http://web.archive.org/web/20030628083522/http://flighttest.navair.navy.mil/unrestricted/FTM108/c6.pdf