HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Jan 11, 2013, 10:15 AM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,213 Posts
Discussion
Flat-spinning the Hobby Zone Super Cub

(Edit- as you'll see if you read this thread, my understanding of the spin dynamics of this plane evolved over many months. To "cut to the chase", you might want to skip ahead to post #29 and then skim back through some of the earlier posts, reading from newest to oldest. Note also that I've had several different fuselages and wings. Some preferred to spin to right, while most preferred to spin to the left. That's why you'll see descriptions of spinning in different directions in different posts. Steve )

(Edit- -I changed the title of this thread-- I believe that the camera was contributing to the flat spinning tendency, but it wasn't clear that the effect couldn't be replicated with the same weight distribution but a different aerodynamic shape. I'm now making the plane flat-spin consistently even without the camera on the tail. End edit)

I had an interesting experience the other day-- I was flying with one of those keychain video cameras taped to the left horizontal stab of my Hobby Zone Super Cub foamie and I discovered some interesting effects on spins. Spins to the right were rather flat, with the nose only about 30 degrees below the horizon or possibly less, the yaw rotation rate extremely rapid, and the descent rate extremely low. I was holding full pro-spin controls (full up elevator, full right rudder) and had the power at idle. The plane has no ailerons. Moving the rudder to full left followed by full down elevator had essentially no visible effect on the aircraft and did not cause recovery. As I continued to hold full anti-spin controls, a blast of power did cause recovery, presumably because the propwash increased the effectiveness of the tail control surfaces.

(Think about it, in a flat spin the aircraft is spinning like a pinwheel and the airflow is likely almost perpendicular to the vertical tail surface, so it's not surprising that the rudder and elevator don't do much, in the absence of propwash).

Meanwhile, left spins had an almost vertical nose-down pitch attitude, with a much lower rotation rate, appearing to involve much roll as well as yaw, and a dramatically higher sink rate. Again I was holding full pro-spin controls with idle power. Full right rudder broke the spin, even if I continued to hold full back stick.

Then I took off the camera and mounted an equal amount of weight at the same distance aft of CG, on the aircraft centerline. Now left and right spins were basically the same, a little less nose-down than in the left spin described above, nose maybe 60 degrees below horizon, while holding full pro-spin controls with idle power. Full anti-spin rudder always broke the spin even while holding full back stick.

What do you think is going on? Do you think the main factor is the aerodynamic effect of the camera, or the asymmetrical mass distribution?

Things I plan to try-- repeat with the camera on the bottom of the stab instead of the top. Repeat on the right side of the stab rather than the left.

Also try using a more compact weight like 3 quarters taped near the far outboard tip of the stab to get a good asymmetrical mass distribution at the tail without such a large object affecting the airflow.

Also try using a balsa replica of the camera to get the same airflow disturbance but without the off-center mass distribution. Or taping a counterweight to the other side of the stab to keep the weight on the aircraft centerline, while removing some of the weight near the tailwheel to make sure the CG is not shifted further aft.

Any guesses as to results of these various tests?

If the camera is simply spoiling the stab's lift in a way that promotes the flat spin, would cutting off part of the stab have the same effect?

I'll post future results...

It was pretty neat seeing the plane pinwheel down in such a flat spin. The in-flight movie will be interesting too!
aeronaut999 is offline Find More Posts by aeronaut999
Last edited by aeronaut999; May 21, 2013 at 06:35 PM.
Reply With Quote
Sign up now
to remove ads between posts
Old Jan 11, 2013, 10:46 AM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
7,174 Posts
Your results are to be expected
- The weight bieng ,of course , the biggie.
With our old IMAC planes we would set em up tail heavy then practice spins - using varying power and elevator settings - the planes would spin flatter or steeper depending on power and elevator - You will also note that as the plane spins flatter - the rotational speed slows - just like an ice skater extending or pulling arms in .
(conservation of angular momentum some will say).
richard hanson is online now Find More Posts by richard hanson
Reply With Quote
Old Jan 11, 2013, 11:11 AM
Registered User
Canada, AB, Red Deer
Joined Apr 2010
312 Posts
The effects of too far aft CofG during a spin a fairly well known. What you are experiencing with the "flattening out" of the aircraft followed by difficult recovery is exactly what is expected. In fact, this is on of the main reasons for the aft CG limits in many full size aircraft. My full scale Cessna 172 is approved for spins, but only if the aircraft is configured in the "Utility" CG envelope, which is basically the forward end of the range, and not at Gross Weight.

Many people have been killed due to an inadvertent spin with an aft CG going flat and not recovering. Actually, one famous example is airshow great, Art Scholl. He was killed while filming the flat spin scene for the movie Top Gun. They had a camera mounted to the tail of his Pitts Special to film the spinning around part of that scene, but he was unable to recover from the flat spin and crashed into the ocean.

Now, as for the difference in recovery from left spin to right spin on your Super Cub.....that I am not sure about. I would expect that is due to the airflow interference caused by the camera.

Interesting side note regarding mass and spinning........

Ever heard of the "Polar Moment of Inertia", and how it relates to aircraft? This is something aerobatic pilots need to be mindful of.

Here is an example of how the polar moment of inertia works. Take a broomstick, and duct tape two full pop cans in the middle of the broomstick, just wide enough apart to put your hand in between them. Now, hold the broomstick in one hand, between the pop cans, and spin it like a baton. Start and stop it a few times to get the feel for how much force it takes to start it and stop it. Then, take the pop cans off, and tape them to each end of the broomstick, and repeat the testing. Which test was easier to start and stop the spinning, the mass in the middle, or on the ends??? Also note that the total weight of the stick and pop cans is the same, and the CG (The balance point where you are holding the stick) is the same. I think most will know the answer without even trying it.

So, how does this relate to aircraft, especially in regards to spins and other aerobatics. Well, for starters, lets look at a simple roll. In a competition aerobatic routine, there is 4 and 8 point rolls. They need to be precise, so if you had a bunch of mass out on each wingtip, lets say just for fun, the aileron bellcranks and linkage is way out towards the tips. The ailerons need to be more powerful to get that mass rolling quicker, and then you need to stop that mass once you get to your point. Take the same aircraft and move the linkage in a close to the wingroot as the ailerons will allow. Now, the total weight of the aircraft is the same, CG is in the sam spot, but the rolls will be quicker and easier to start and stop....thanks to a lowered polar moment of inertia.

Suppose you are doing a spin, and lets say your airplane has the battery mounted right on the firewall, and the elevator linkage has all the bellcranks and crap right at the tail. Typically, starting a spin is no problem once you get one wing to stall, but now you have the extra mass at extreme ends to try and get stopped. Again, take that same airplane and mount the battery under the intsrument panel, put all the elevator bellcranks right behind the piltos seat so there is only one light pushrod running aft. Again, same total weight, same CG position, but now there is less mass at the extreme ends so it will be easier to stop the spin and recover.

So you can see, there is more too it than simply the aerodynamics, airflow and CG.
stardustertoo is offline Find More Posts by stardustertoo
Last edited by stardustertoo; Jan 11, 2013 at 11:26 AM. Reason: Had an incorrect statement to remove....oops
Reply With Quote
Old Jan 11, 2013, 11:36 AM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
7,174 Posts
I like your examples.
We fly some very light stuff electric power and th inertial probles show up there as well
Getting the vertical CG setup correctly also changes it

The theory guys may have a different take on it but actual experience as you note- is very important.
Side Note - I called a show once for Art Scholl ,when he was flying his Chipmunk. He did stuff I really didn't believe could be safely done . The low pass going to a hover at the end of the field - then tailsliding and recovering inverted - to pick up a hanky on string with the rudder was quite an a eye opener. This was all done at low altitude and he only re gained flying speed through the vertical fall and then adding power.

The Pitts thing which killed him was a surprise to me - The Pitts is a lot different than the Chipmunk being basically a short aircraft - very maneuverable but not as stable .
I often wondered why he elected to use the camera setup he did on the Pitts.
richard hanson is online now Find More Posts by richard hanson
Reply With Quote
Old Jan 11, 2013, 11:52 AM
Registered User
Canada, AB, Red Deer
Joined Apr 2010
312 Posts
Quote:
We fly some very light stuff electric power and th inertial probles show up there as well
Getting the vertical CG setup correctly also changes it
I suppose you would notice it with those little guys a bunch. I only have one small electric, and it is an ARF, I just set it up the way the instructions said and went flying

Quote:
Side Note - I called a show once for Art Scholl ,when he was flying his Chipmunk. He did stuff I really didn't believe could be safely done . The low pass going to a hover at the end of the field - then tailsliding and recovering inverted - to pick up a hanky on string with the rudder was quite an a eye opener. This was all done at low altitude and he only re gained flying speed through the vertical fall and then adding power.

The Pitts thing which killed him was a surprise to me - The Pitts is a lot different than the Chipmunk being basically a short aircraft - very maneuverable but not as stable .
I often wondered why he elected to use the camera setup he did on the Pitts.
Never got to see Art fly myself My Dad often talks about how much he enjoyed watching him perform though. There is a few decent youtube videos though, pretty incredible performance, and like you say, right down on the deck!!
stardustertoo is offline Find More Posts by stardustertoo
Reply With Quote
Old Jan 11, 2013, 12:16 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,213 Posts
no fore-aft CG change

Guys, the fore-aft location of the CG was the same with and without the camera. See original post, 4th paragraph. I had other counterweights near the tailwheel that I added or removed as needed. So the moment of inertia in the yaw axis was also the same. Only the moment of inertia in the roll axis was changed, and I think that that change would have minimal.

Note also that the flat spin only occurred in one direction not the other.

See original post.


My guess though is that is an aerodynamic effect not an offset CG effect.

Note how part of the camera sticks forward of the leading-edge of the stabilizer.

I bet if I take the same camera, rotate it 90 degrees so that it points spanwise not lengthwise, and reposition it back near the hingeline of the elevator in such a way that the stab "blankets" the camera from the airflow, I won't see the same effect.

All guesses for now. I should know more in a day or two.

Another thing I don't know is if the recovery on adding power was due to propwash hitting tail, or motor downthrust. Maybe the propwash missed the tail due to the rotation of the spin. I can find out by putting some yarn on the tail and filming it.

Richard your experience may have been that flattening a spin slows the rotation, but my case the rotation rate was much faster in the flat spins. In my case I'm not transitioning from nose-down to flat in any one given spin, so there's no conservation of momentum issue to consider.

Steve
aeronaut999 is offline Find More Posts by aeronaut999
Reply With Quote
Old Jan 11, 2013, 12:38 PM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
7,174 Posts
Don't know what you are seeing --
but the slow down / speed up in a spin can be demonstrated over n over in same spin sequence- if you have enough altitude
The spin rate always slows as the model "flattens out".
Very easy to see it change
I have only a couple of models with rudder only (no ailerons) turn/ bank setups but all of my stuff will spin sans aileron input.
Just never saw a spin speed up as the angle decreases
. here are some of my various models -- mostly aerobatic stuff
richard hanson is online now Find More Posts by richard hanson
Last edited by richard hanson; Jan 11, 2013 at 12:46 PM.
Reply With Quote
Old Jan 11, 2013, 01:14 PM
Registered User
Canada, AB, Red Deer
Joined Apr 2010
312 Posts
Quote:
Guys, the fore-aft location of the CG was the same with and without the camera. See original post, 4th paragraph. I had other counterweights near the tailwheel that I added or removed as needed. So the moment of inertia in the yaw axis was also the same. Only the moment of inertia in the roll axis was changed, and I think that that change would have minimal.
I have to disagree. Unless you added weight to the NOSE when you put the camera or the weights the tail, the CG will have shifted aft from the stock configuration. Also, with extra weight on the tail as compared to the "stock" configuration, you are adding inertia to the yaw axis. The way to see that is to look at the aircraft directly from above, right down through the center of the wing. You will see that any weight forward or aft of that centerline will add inertia to the yaw axis.

With the camera mounted out towards the tip of the stab in the picture will also add inertia to the roll axis as you mentioned, as well as the yaw axis due to the fact that it is also aft of the CG. But, the roll part brings to mind a possible answer to your question.

Having the CG aft will of course contribute to the flat spin, whether it is the camera on the stab, or the weights on the tailwheel. BUT, perhaps on such a light aircraft, the weight of the camera off center is why the difference from left to right. That combined with the "blanked" airflow from the camera could by the difference.
stardustertoo is offline Find More Posts by stardustertoo
Reply With Quote
Old Jan 11, 2013, 02:06 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,639 Posts
If you want to stick a camera on the tail without it affecting the planes flight, pick a bigger plane.

On top of the vertical stab -
eflightray is offline Find More Posts by eflightray
Reply With Quote
Old Jan 12, 2013, 09:34 AM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,213 Posts
Quote:
Originally Posted by richard hanson View Post
Your results are to be expected
- The weight bieng ,of course , the biggie.
With our old IMAC planes we would set em up tail heavy then practice spins - using varying power and elevator settings - the planes would spin flatter or steeper depending on power and elevator - You will also note that as the plane spins flatter - the rotational speed slows - just like an ice skater extending or pulling arms in .
(conservation of angular momentum some will say).
We're getting a little off topic from my original question, but Richard would you mind saying exactly what throttle and elevator changes you made to go from steeper spin to flatter spin or vice versa? Hold full up elevator, add power to go flatter? Or--??

Thanks

Steve
aeronaut999 is offline Find More Posts by aeronaut999
Reply With Quote
Old Jan 12, 2013, 09:36 AM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,213 Posts
Quote:
Originally Posted by eflightray View Post
If you want to stick a camera on the tail without it affecting the planes flight, pick a bigger plane.

On top of the vertical stab -
Nice pic; actually I'm pretty eager to fly with stab mount and do some more flat spins. Glad I discovered it in a power plane though as I usually fly gliders. If the same thing is happening there it will be unrecoverable; with the Cub I could only recover by adding power (noted in original post).
aeronaut999 is offline Find More Posts by aeronaut999
Reply With Quote
Old Jan 12, 2013, 09:48 AM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,213 Posts
Starduster, the CG is aft of the stock location but it is not changing (except for the slight displacement to the left, and a slighter displacement upwards) when I add and remove the camera. My comparisons are not camera versus stock, but rather camera versus no-camera, with the no-camera case having some weight added near the tailwheel.

In the stock CG location, I can't get the plane to spin at all.

Speaking of the upwards displacement of the CG, would the plane flat-spin even more readily (perhaps flat-spinning in both directions now) if I put the camera on top of the fin instead of on the stab? I don't think so but I will try it. I think we are seeing an aerodynamic disturbance that is not going to happen with the camera on top of the fin.

Will the results be more dramatic if I put the camera on the outboard tip of the stab? How about if I use compact weights like quarter or lead that doesn't disturb the airflow so much?

How about if I put a balsa replica of the camera on the left side of the stab, and a heavy compact weight like some quarters on the right side of the stab? Will the plane now flat-spin toward the left, right, or both?

If the leftwards CG displacement is really significant, I should be able to get a vastly larger effect if I remove the camera and put the same amount of weight on the wingtip (while adjusting the tailwheel weight to keep the CG the same distance aft and the yaw rotational moment of inertia at least as large or larger.) This would also constitute an upward shift of the CG. But my intuition tells me that the plane isn't going to flat spin as readily in this configuration. Something else to try...

It's been too windy for nice flying the last 2 days so no more results to share....

Steve

Quote:
Originally Posted by stardustertoo View Post
I have to disagree. Unless you added weight to the NOSE when you put the camera or the weights the tail, the CG will have shifted aft from the stock configuration. Also, with extra weight on the tail as compared to the "stock" configuration, you are adding inertia to the yaw axis. The way to see that is to look at the aircraft directly from above, right down through the center of the wing. You will see that any weight forward or aft of that centerline will add inertia to the yaw axis.

With the camera mounted out towards the tip of the stab in the picture will also add inertia to the roll axis as you mentioned, as well as the yaw axis due to the fact that it is also aft of the CG. But, the roll part brings to mind a possible answer to your question.

Having the CG aft will of course contribute to the flat spin, whether it is the camera on the stab, or the weights on the tailwheel. BUT, perhaps on such a light aircraft, the weight of the camera off center is why the difference from left to right. That combined with the "blanked" airflow from the camera could by the difference.
aeronaut999 is offline Find More Posts by aeronaut999
Last edited by aeronaut999; Jan 12, 2013 at 09:56 AM.
Reply With Quote
Old Jan 12, 2013, 12:33 PM
Registered User
richard hanson's Avatar
United States, UT, Salt Lake City
Joined Oct 2007
7,174 Posts
Quote:
Originally Posted by aeronaut999 View Post
We're getting a little off topic from my original question, but Richard would you mind saying exactly what throttle and elevator changes you made to go from steeper spin to flatter spin or vice versa? Hold full up elevator, add power to go flatter? Or--??

Thanks

Steve
Typically add power and elevator - UP if upright DOWN if doing inverted spin These models are balanced such that power can easily add to the desired effect
Some would call em tailheavy or talk about static margins etc.,
Just normal aerobatic setups- about 30 % MAC balance point
richard hanson is online now Find More Posts by richard hanson
Reply With Quote
Old Jan 13, 2013, 07:10 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,213 Posts
New data-- spins with CG shifted left/ right off centerline

New data--

3 quarters taped to left wing makes it tough to enter a right spin-- because plane wants to drop left wing-- but once entered, the spin is very flat and full anti-spin controls do not effect recovery without the addition of power. Left spins are more or less normal, and can be broken with rudder alone while continuing to hold full up elevator.

3 quarters taped to right wing makes it tough to enter a left spin-- because plane wants to drop right wing-- but once entered, the spin is very flat and full anti-spin controls do not effect recovery without the addition of power. Right spins are more or less normal, and can be broken with rudder alone while continuing to hold full up elevator.

Question-- what would be the effect of 3 quarters taped to each wing? I think perhaps a slight tendency for the spin to be flatter than usual, but not hugely so, and rudder alone (or full anti-spin controls) will effect recovery, even without power. I'll try it soon.

All tests were without the camera on the stab. I haven't figured what % of the plane's weight the 3 quarters constitute or how much I am shifting the CG in the left-right direction, but the plane is stock except for 7 quarters taped near the tailwheel, a camera taped near the tailwheel with weight equivalent to 3 more quarters, and the 3 quarters on the wingtip. I'll weigh the plane soon to figure that out.

Still haven't decided whether or not in the earlier tests, the camera was also having a significant aerodynamic tendency to flatten the spin. The left/right displacement of the CG due to the camera would have been dramatically less than the displacement due to the wingtip quarters, because the weights are equal but the camera is so much less outboard than the wingtip quarters. So I am not sure the camera's effects were mainly due to left-right CG shift rather than aerodynamics. Next I'll use a balsa substitute for the camera and carefully counterweight it so that there is no left-right CG shift, as well as no fore-aft CG shift and see if I see any remaining tendency for the spins in the direction opposite to the camera, to go flat. That would suggest that the camera is having an aerodynamic effect as well as a left-right CG-shift effect.

Steve
aeronaut999 is offline Find More Posts by aeronaut999
Reply With Quote
Old Jan 15, 2013, 04:56 AM
Aka: Tom Jenkins
ApexAero's Avatar
Palm Beach County, Fl.
Joined Aug 2008
4,501 Posts
what about P' factor as I insert palm under arm and push elbow down
ApexAero is online now Find More Posts by ApexAero
RCG Plus Member
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Can all props spin the same direction on a v-tail? signal15 Multirotor Talk 1 Jul 04, 2012 04:41 PM
Question ClearView - Spins and Flat Spins? Rifraf Simulators 2 Jun 28, 2012 03:51 PM
Discussion Transition from KE spin to inverted flat spin? Depronicus 3D Flying 10 May 27, 2012 05:05 PM
Video Flat spin to the ground then re-launch - twice! jtprouty Electric Plane Talk 0 Mar 14, 2012 10:04 PM
Discussion vid: low flat spins, and possible flat spin landings, please. jriley1974 3D / Fun Fly Fuel Planes 2 Jun 15, 2010 03:02 PM