Piper Cherokee - Page 3 - RC Groups
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Feb 12, 2018, 07:53 AM
Ken's CAD Models
dz1sfb's Avatar
Quote:
Originally Posted by AleRRon
I have a question for the aeronautical engineers out there. On the model I'm building the elevator is not parallel to the wing plane. It's tilted up towards aft. I hope its on purpose but I don't know why. On any other plane I've built so far, the two are parallel.
Since this aircraft has an all flying horizontal stab/elevator you can always adjust the trim for the negative decalage you have described. However, in general, the horizontal stab will fly parallel to the airstream presented to it. This means the aircraft may be designed for a nose up or down appearance in flight depending upon its purpose.

https://en.wikipedia.org/wiki/Decalage

Ken
Latest blog entry: Mini H Quad Project
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Feb 12, 2018, 04:43 PM
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AleRRon's Avatar

Negative decalage


I dunno. Seems weird to me. If the real plane is an all flying horizontal stab but the model is a conventional stab and elevator then I just hope it flies. I'll find out soon enough! I hope I'm not breaking any rules but I posted a zoomed up part of the plan with the stab. The line right above it is the fuse horizontal line. It's in line with the motor shaft and the flat lower wing surface.
Feb 12, 2018, 09:22 PM
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AleRRon's Avatar

Tail


I think I figured it out. Have never seen this design but what I gather is that the elevator is more properly called a trim tab. No wonder it's so small. The balsa model plans call for tin or aluminum hinges and no servo or horn and it says "manual adjust". So what I gather is that you have to test fly it and move the trim tab as needed to achieve level flight and leave it be. I'm guessing you pitch up or down using wing flaps. Anyone ever see this before?

See below, this assumption is wrong
Last edited by AleRRon; Feb 14, 2018 at 08:42 AM. Reason: More info
Feb 12, 2018, 10:22 PM
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AleRRon's Avatar
More info. Had no idea until now! This may require some design changes. The following is an answer on a aeronautical forum to a question about why Piper often designed all- moving “stabilators” on their small aircraft.
The piper selection of the stabilator instead of the conventional stabilizer/elevator configuration was done for several reasons. The stabilator gives a wider range of pitch control over all flight speeds. The stabilator is lighter with lower drag. The use of the anti-servo trim design causes the tab to move with the stabilator but the combination requires more pilot input with any increase in speed or deflection. . The stabilator utilizes an "antiservo" tab that deflects upward on the trailing edge of the stabilator as the controls come back. This antiservo tab generates the necessary control feel and feedback to the pilot to maintain the necessary "stick force per G" to keep a hamfisted pilot from easily breaking the airplane with excessive control movement. This is a safety device improves longitudinal stability while at the same time limiting the pilot ability to cause structural damage.

The "stabilator" affects flight exactly the same as an elevator. However, stability is more difficult to attain with the stabilator because its larger effective surface increases sensitivity. There are two different sizes of stabilators on PA 28 aircraft. One is over three feet less than the other. The control effectiveness of these in landings makes it very important that the pilot be aware of which stabilator is on the aircraft. There are distinctive skills required for proper flying of the older Hershey bar wing with the small stabilator. The older (smaller) stabilator will run out of effectiveness at slower speeds. This is especially critical when the aircraft is loaded toward the aft limits. The stall under these conditions will be unlike any usual Piper stall. It will be abrupt, violent and give a spin all in the same moment. Fuel consumption will cause a gradual rearward movement in the weight and balance envelope. Pipers at gross tend to fly tail low with much greater fuel consumption.
Feb 13, 2018, 07:40 AM
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AleRRon's Avatar

Stabilator


Whew boy, I did a lot of reading. This plane was designed with a stabilator which has good and bad characteristics. It’s smaller and lighter than a conventional stab and elevator. But it lacks feedback resistance to the pilot and that’s why there’s a trim tab that increases resistance and provides the pilot with more feel. It also loses effectiveness at low speeds, which is bad, at least for someone like me. Also, apparently this plane flies nose up and the stabilator is designed with more upward than downward movement. Maybe that’s why this balsa model plan has negative decalage.
I can modify the design for a stabilator, and leave the trim tab off since when RC flying, there is no resistance feedback. The feel is in the transmitter sticks and visual line of sight. The problem may be in getting the throws right since it’ll be very sensitive. And the pitch control may go all to hell at slow speeds especially when landing.
Or I change the design to a conventional stabilizer and elevator and make the whole assembly larger. Any thoughts?
Feb 13, 2018, 07:55 AM
treefinder
springer's Avatar
Make it easily removable so you can try both and give us a report!
Feb 13, 2018, 06:59 PM
Ken's CAD Models
dz1sfb's Avatar
Putting in my own $0.02 worth, the stabilator will work just fine. There are a couple of important consideration;

1. Surface area - the slower you fly the more stabilizer area you need as well as control throw. Also the smaller the model the more area is required from scale ( I have not needed to increase a scale surface more than 10%) because air molecules remain the same distance apart. This is true for a conventional stabilizer/elevator setup as well.

2. Hinge line - the closer the hinge line is to the aerodynamic center of the stabilator, the more twitchy it is likely to be. About 30-35% of chord will give a nice dampened feel without loading the servo to heavily.

I had not thought about the anti-servo tab on a stabilator before. My usual thinking is a servo tab on a conventional tail, which moves in the opposite direction, thereby reducing the feedback pressure. The anti-servo name makes sense that it increases feedback pressure.

Ken
Last edited by dz1sfb; Feb 13, 2018 at 07:06 PM.
Feb 14, 2018, 06:39 PM
Retired CAD guy
birdofplay's Avatar
My flying elevator mechanism worked fairly well as shown in 3 pix in my previous post.

Just had to trim it neutral .

Plenty of Expo to reduce over deflections by "numb thumbs".
Feb 15, 2018, 07:03 AM
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AleRRon's Avatar

All flying elevator


Thanks Bob. To tell you the truth, I had no idea what the heck that was the first time you posted it and I looked at it. I thought it was your unique design and I didn't realize that the Cherokee had that kind of tail. Yours looks like it should work. I gather the servo is forward, and the horn is inside the fuse tail, not noutside it?
Feb 15, 2018, 09:37 AM
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AleRRon's Avatar

Mechanism


Maybe something like this?
Feb 15, 2018, 11:19 AM
Retired CAD guy
birdofplay's Avatar
The Pivot of the flying stab( elevator) is held fast by two pieces of thin lpy
laminated to the insides of the Fuse sheeting at the rear.

The Nose of the Flying stab(elevator) is moved Up/Down by the action of the rotating slotted part.
There is NO HORN as such.
All parts are INSIDE and invisible to the outside.

I can create a 3D ISO drawing if this is still not clear.

It worked just fine as I stated in the original post above.
My particular model had more problems with the wing design that I'd chosen.

I found an extremely poor vid of the mechanism operating from 2011 ( 6 yrs ago )
I think it was using a small HK low obviously rez camera ( likely the $18 one )
I've since used a Mobius with better rez and with some success.
My latest is an Eken H9

Here's the link
https://www.rcgroups.com/forums/show...52&postcount=1
Last edited by birdofplay; Feb 15, 2018 at 11:57 AM.
Feb 15, 2018, 11:39 AM
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AleRRon's Avatar
Yeah, I see what you mean. I may do that (your design) too. My drawing was another idea; maybe simpler. Just a horn attached to the pivot. Not sure how it would work compared to your setup. Also, I would have to make the assembly (tail with pivot rod, horn, and stabilator) separate from the fuse, and then slide it in from the back. I have an idea as to how to do that, pics to come soon. It'll be a little complicated but not too bad.

thanks for the video, I could see it working just fine. Looks great!
Last edited by AleRRon; Feb 15, 2018 at 01:30 PM. Reason: addendum
Feb 15, 2018, 01:40 PM
Ken's CAD Models
dz1sfb's Avatar
K.I.S.S. is often the best.

Ken
Latest blog entry: Mini H Quad Project
Feb 16, 2018, 02:19 PM
Retired CAD guy
birdofplay's Avatar
Your Horn only idea may be very sensitive to input control .

My "ahead of the Pivot point" is more complicated but Solid although NOT KISSable

Then again milling the whole fuse is like monoblock on steroids. ;>}

See pic
Last edited by birdofplay; Feb 16, 2018 at 02:27 PM.
Feb 16, 2018, 03:33 PM
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AleRRon's Avatar
yes, I thought it might be sensitive too. I'll have to experiment with horn and servo arm lengths. I hope to do that this weekend. This scratchbuilding is very interesting work/play but also very slow going when one has to figure it all out-although your and everyone else's advice helps for sure.


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