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Performing Loops

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Norman CYYJ

Well Known Member
I was up doing loops and rolls the other day with a average size guy(180 lbs).
At the top of the loop just before easing the back pressure off it felt like and sounded like someone hit the tail of the plane with a two by four. I landed and inspected the tail section, no damage or cracks. Tried again with different entry speeds and still this happened. I have never experienced this before have any of you? OAT was around 65 and alt. was 5500 asl.
 
Were you doing multiple back to back loops. Maybe hitting your own prop wash? In other aircraft I've done it in level turns, but never vertical.
 
No it happened on the first loop and I was already almost completely inverted easing the back pressure off. I have hit my own turbulence before horizontally and vertically and feeling was not the same.
 
You probably have thought of this already.... but did you look for an unknown object floating around in your tail area that came loose?
 
Loose item in baggage compartment? I cannot think of anything except for a sizable piece of FOD that started floating around once you got into zero G territory at the top of the loop.
 
Check for oil canning on the bottom skin of the fuselage. I had a similar sound and that's what it turned out to be. It got my attention the first time or two.
 
When I was training in my old Cherokee 140 I would get a "bang" whenever we did a spin, and the top bulkhead panel separating the cabin from the tail cone would pop out. Happened every time, and definitely seemed to be a little oil canning in the fuse.
 
Norman CYYJ said:
I was up doing loops and rolls the other day with a average size guy(180 lbs).
At the top of the loop just before easing the back pressure off it felt like and sounded like someone hit the tail of the plane with a two by four. I landed and inspected the tail section, no damage or cracks. Tried again with different entry speeds and still this happened. I have never experienced this before have any of you? OAT was around 65 and alt. was 5500 asl.
Click to expand...

I outlined the key, low airspeed wile still loaded, the tail stalled, been there done that. More speed on entry and a little more G at entry will get you to the top before it slows to a stall speed. this happens a lot easier two up then solo.
 
Russ McCutcheon said:
I outlined the key, low airspeed wile still loaded, the tail stalled, been there done that. More speed on entry and a little more G at entry will get you to the top before it slows to a stall speed. this happens a lot easier two up then solo.
Click to expand...

I'm unclear on what you are connecting, or what a "tail stall" is. You can stall, buffet, and snap airplanes every which way and not hear banging in the airframe. Oil canning is a good thought. Though this is definitely not a "typical" thing with RVs and aerobatics, no matter how you choose to fly through loops.
 
Too slow over the top.

It can 'sit down' over the top sometimes - feels really awful.

Look at entry speeds and g loads.

There are many books out there - my favourite is by Eric Muller and Annette Carson. It's called Flight Unlimited '89.

Sometimes difficult to get hold of, but superbly written in an easy to read and understand way.

Helped me with aeros a lot.
 
Thanks Russ, I tried various air speed entries and it still did it. I will pay more attention to the air speed and see what happens. I have being doing loops in this plane now for five years and my previous RVs and never had it happen before. I have been heavy in the past and light with none of this happening. I know what it is like to stall at the top and anywhere in between and have never experience it before. There was no feedback from the stick when it happened either. It was just like someone out there slapping the tail, maybe oil canning but the skins are all tight.
 
There isn't a lot going on back there mechanically. Take off the inspection panels and fairings and give everything a thorough lookin at. If you're satisfied all is as it should be...I would suspect oil canning or tail stall. A discussion with Vans wouldn't hurt either.
 
I have had all the inspection panels off and the intersection fairing and no faults found. I guess it was a tail stall but have never had that happen before, but there appears to a first for everything.
 
Ok well that?s odd, sounds like you?re not new to this stuff, if you?re sure it?s not a stall buffet and it?s a new sound you better start looking close at things, tail attachments, hinges, etc. take out the baggage compartment too and make sure you don?t have a flashlight floating around back there too.
 
Russ McCutcheon said:
Ok well that?s odd, sounds like you?re not new to this stuff, if you?re sure it?s not a stall buffet and it?s a new sound you better start looking close at things, tail attachments, hinges, etc. take out the baggage compartment too and make sure you don?t have a flashlight floating around back there too.
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That's exactly my thoughts. On my Rv6a right behind the baggage bulkhead is where the recommended area is to mount the elt and strobe box. Perhaps some how one of these came loose? Just thinking out load here.

Bill
 
If the shoulder straps were loose, the Hooker buckle near the rear attach point can bang on the top of the fuse of the tandems. You did not say what plane you were flying. SbS models would not demonstrate this.
 
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Another "out there" suggestion: Don't focus entirely on the tail. A few years ago I hit a seagull while flying a Cessna 150... The bird glanced off the top of the leading edge, on the pilot's side, about 4' out. No serious damage, but the "bang" when it hit was deafening.

I was 100% certain that the bang came from the tail of the airplane. After I landed I went over the entire tail looking for anything that would explain the bang, but couldn't find a mark. I didn't find the dent in the wing until I climbed up to fuel it.
 
I am at a loss for what it was. I have since done a very thorough annual on the RV7. There was nothing amiss except a worn out carpet on the pilots side floor which was replaced. No extra parts except the usual dirt found anywhere. The SB was done on the tail no cracks found. No loose or broken bolts back there. I don't remember the airspeed going over the top, the entry airspeeds that were tried varied from 140kts to 160kts IAS. Went up by myself it could not be duplicated even at slower speeds. The loops just wouldn't be as pretty at the lower entry speeds. I tried stalling it going over the top at various locations and it would stall but not behave with the same noise back at the tail. Some day I will take him up again and see what happens. By the way he use to teach aero and he was at a loss also.
 
Norman,

Do a search here about oilcanning in the -7, it's well covered.

I know I wouldn't like hearing that during a loop either.

Glenn Wilkinson
 
Occurring with only a passenger, I would look at his headset cord. Is it noise cancellation? The control could be hitting something when negative. I'm assuming its a tandem. I now see its a 7, but it still could be the culprit.
 
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Are you actually going negative g's over the top? If so, that might trigger the return of the oil canning. My 7 oil canned when the engine was mounted and the tail came up. It took only a touch to trigger it. Not 7's all are this sensitive. I added some very light stiffeners, but not flying yet. I can post pictures if you are interested.
 
No unfortunately it was not anything banging around in the cabin or baggage area, the plane was empty for doing acros. No I don't go negative, I hate cleaning the oil off of the belly. Learned that years ago. I have been doing acros in RV's now for about 12 years and have never had this happen before. There are no skins lose enough to oil can on the ground and I don't think that was it, it was positive G all the way around each time. Seat belt cables were tight.
 
Any chance it is the pickup weight or end of the flop tube falling aft as you approach the top third or past vertical?
 
No chance it was the pickup weight, both pickups are fixed. I am talking of a loud bang like a brick or large hammer hitting the tail.

Thanks for all of the suggestions. I'll just have to try and simulate it again in the near future. It may have been as Russ suggested just a tail stall, but usually when I have experienced that before it was just a bit of buffeting.
 
Can you mount a GoPro or similar camera underneath the tail and get some video (maybe use the tail tiedown if -A model, or mount it to the tailwheel fixture if it's a taildragger)?
 
You stalled going over the top and felt the buffet. You can stall in any attitude even going straight down. I've done this pulling too tight a loop. Sometimes it feels like an engine miss, sometimes it feels like a bump, sometimes it sounds like a bang, and other times any combo of these.
 
BillL said:
Are you actually going negative g's over the top? If so, that might trigger the return of the oil canning. My 7 oil canned when the engine was mounted and the tail came up. It took only a touch to trigger it. Not 7's all are this sensitive. I added some very light stiffeners, but not flying yet. I can post pictures if you are interested.
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When adding skin stiffeners, be sure to tie them into the bulkheads at each end.
Otherwise you will get skin cracks near the ends of the stiffeners.
 
Yes, Mel,
I read about this here extensively, and decided not to run them long ways. I ran them parallel to the bulkheads. There is one rivet on each end and the bottom is bonded with 3M windshield urethane to the skin. I think most stresses are longitudinal so hope that this will not generate a crack.

I made a perfect contour with card stock, make a 3/4" MDF buck and bent the flange over, fluted it to straighten and it matched perfectly. One in the center still allowed some dimpling, so added two.

IMG_0725.JPG


These are not for all planes. Apparently very few actually dimple, and mine was extremely easy with downward force on the tail end.
 
Russ McCutcheon said:
I outlined the key, low airspeed wile still loaded, the tail stalled, been there done that. More speed on entry and a little more G at entry will get you to the top before it slows to a stall speed. this happens a lot easier two up then solo.
Click to expand...

For this and other oil canning theories, is there another flight configuration and condition that could be attempted that would provide similar dynamics to the "coming over the top" stresses on the airframe?

And for an out-there and maybe unsafe suggestion on testing for oil canning in the rear fuselage: could one temporarily affix arrow stock or similar to the skins with 200mph tape either internal or external (simultaneous to multiple skins) to see if this would eliminate the bang?
 
Potential (small) bird strike?
That doesn?t always leave a mark... just a thought.
Good luck, Jim
 
RV-8?

If it's an RV-8 then there's a good chance it's the canopy. Two up at about 4 g or so doing a loop will cause slight separation at the wing root which grabs at the canopy as you go around. Tries to pull the canopy out from the fuselage at the back then it slams down causing a loud pop and can also cause some air to blow on your passenger.

Was very disconcerting the first couple of times it happened to me. When flying with a passenger, use a higher entry speed and lower g during loops and the problem should disappear. You can also mount strakes on the fuselage in front of the wing to help keep the airflow attached at the root.
 
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