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Author Topic: Polecat X wing incidence vs down thrust  (Read 1602 times)
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bentodd
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« on: January 15, 2015, 10:46:31 PM »

I am building DeLoach's Polecat Mk X.  The plans specify a wing incidence of +1 deg and a downthrust of 8 deg.  Not sure why I would want to mess with it, I definitely don't have the expertise of DeLoach.  But I wonder if splitting up the difference more equally, say +5 deg for the wing and 4 deg downthrust for the prop.  This would provide the same angular difference.  It might reduce the friction on the prop shaft since rubber would not be pulling on the shaft at such an angle.  The plans do not specify a specific stab angle, but I would set that to provide the best glide.

Any comments would be appreciated.
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Hepcat
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« Reply #1 on: January 16, 2015, 06:38:11 AM »

I don't recollect ever seeing the 'Polecat' plan but DD is certainly a competent flyer and I am sure what he recommends will work.  However I do understand your puzzlement, 8 degrees is certainly a large amount of downthrust for a modern aeroplane.  Normally downthrust angle has no special relation to angle of incidence and is more usually related to cg position.  Has the 'Polecat' possibly got a high pylon, raising the weight of the wing and consequently the cg?

John
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danberry
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« Reply #2 on: January 16, 2015, 09:53:49 AM »

Well, I would wait until Don learns what he is doing and then copy his alterations.
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BG
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« Reply #3 on: January 16, 2015, 12:39:51 PM »

I always try to set my locked down models (p30, old-timers etc.) up using a PGI trim. This means running the thrust line through the CG (this means you find the vertical and horizontal position of the CG; note that the CG on the plan is not located for the correct vertical position which should be under the wing in the pylon, probably closer to the fuselage than the wing). Then I set the wing incidence so that it is +2deg. relative to the thrust line. The stab is set to obtain between 3 and 5 deg of decalage and is then adjusted for optimal glide. Also add 2 deg of right thrust and tilt the stab port tip up for a left glide circle. If needed a bit of left rudder too.

Set her up this way and you will have a good stable ship.

Bernard
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« Reply #4 on: January 16, 2015, 12:42:23 PM »

Here is a low res plan.
Attached files Thumbnail(s):
Re: Polecat X wing incidence vs down thrust
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« Reply #5 on: January 16, 2015, 12:50:06 PM »

Here is a diagram of how I would set it up
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Re: Polecat X wing incidence vs down thrust
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dosco
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« Reply #6 on: January 16, 2015, 01:18:13 PM »

Well, I would wait until Don learns what he is doing and then copy his alterations.

??
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« Reply #7 on: January 16, 2015, 03:36:17 PM »

I wouldn't mess with what Don recommends.  He is one of the best flyers in the U.S., and the Polecat is a winner.  In his article for the Free Flight Quarterly P-30 issue, he says that the downthrust seems like a lot but that the model needs it.

My guess is that danberry's comment was extremely sarcastic, since Don is one of the best flyers in the U.S.  There is no waiting until he learns what he is doing; he KNOWS what he is doing.  He taught me about P-30 motors, which helped me win a number of contests against up to 11 competitors.

Also, I don't think PGI trim is appropriate for a model with much of a pylon; you just can't move the CG and center of drag sufficiently to offset the high wing. Although with this much downthrust, the thrustline may go through the vertical CG; I haven't done the trigonometry to check.

Stick with Don's recommendations, and you should have a great flying P-30 if you can build it close to min weight.

--Bill
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Tmat
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« Reply #8 on: January 16, 2015, 03:37:58 PM »

Dosco,
Dan Berry is providing the comic relief. What he means is that Don is an expert P30 flyer (won the US Nats with the Polecat just last summer) who knows how to trim his model. If I was going to build Don's design, I'd set it up the way he suggests, unless Don can suggest a better method.
Now personally, I've never used anywhere near that much downthrust. As Bernard mentions, with locked down models I also prefer a PGI trim. Which would place the thrustline through the CG yada yada yada.
If I needed 8 degrees of downthrust, I'd normally move the CG back, reducing the decalage accordingly until a normal amount of downthrust is needed. But I've got to assume that Don knows this and his model prefers to be flown the way that he shows.

Tmat
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dosco
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« Reply #9 on: January 16, 2015, 05:09:48 PM »

Dan Berry is providing the comic relief.

lol, thanks.

Hard to tell over the intarwebz. Someone needs to invent a "sarcastic" emoticon.

-Dave
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danberry
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« Reply #10 on: January 16, 2015, 05:28:11 PM »

Dosco,
Dan Berry is providing the comic relief. What he means is that Don is an expert P30 flyer (won the US Nats with the Polecat just last summer) who knows how to trim his model. If I was going to build Don's design, I'd set it up the way he suggests, unless Don can suggest a better method.
Now personally, I've never used anywhere near that much downthrust. As Bernard mentions, with locked down models I also prefer a PGI trim. Which would place the thrustline through the CG yada yada yada.
If I needed 8 degrees of downthrust, I'd normally move the CG back, reducing the decalage accordingly until a normal amount of downthrust is needed. But I've got to assume that Don knows this and his model prefers to be flown the way that he shows.

Tmat

Don's plane is pretty much a THREE MINUTE P30. He treats it like an indoor plane. He flys four flights before he worries about looking for lift.
Yeah, it seems like a lot of downthrust. When you beat Don flying P30 you have accomplished something. The plane he won the '14 Nats with might be ten years old.
If somebody makes a trim change that improves the plane's performance I will be in favor of a rule change.
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dosco
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« Reply #11 on: January 17, 2015, 09:30:10 AM »

Don's plane is pretty much a THREE MINUTE P30.

Interesting.

I can't tell from the plan ... does it use a long or short rubber motor?

-Dave
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danberry
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« Reply #12 on: January 17, 2015, 10:09:35 AM »

Long. It might be five strands 3/32.
The plane is at minimum weight.
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Dave Andreski
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« Reply #13 on: January 17, 2015, 12:18:10 PM »

The Polecat X plan calls for a 6 strand motor of 3/32" TAN II or TAN SS, 9.8 grams dry, for about 2200-2500 turns.
Hook to peg length is 28" with the peg being at the extreme end of the fuselage tube.
Dave
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« Reply #14 on: January 17, 2015, 05:21:44 PM »

The plans I have say the same as Dave mentioned for motor and turns.  However, with recent SuperSport I can't see getting 2200 turns, much less 2500.  I wonder if that applied to typical 6 strands of 3/32 motors, or a motor of 4 strands of thick 1/8?  Per Don and logic, motors should be made up by weight, and not by length.  Thicker batches of 1/8 may be OK, but the motor should be less than 29 inches long after being made up into 6 strands of 3/32 or 4 strands of 1/8.  If the model is above min weight, you should make certain your motors end up slightly shorter, indicating slightly thicker rubber with a bit more torque to make up for the model weight.

With recent batches of SuperSport, I get 1800 to 1950 turns typically.  You may be able to get somewhat more, but since this is 70 to 85% of max torque, you probably won't get much more.  One caveat--I haven't done any break tests in a couple of years, but this is for motors that end up about 28 to 28.5 inches, so this should be close.

--Bill
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Dave Andreski
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« Reply #15 on: January 17, 2015, 06:25:54 PM »

OFF TOPIC-
The plan calls for a 3/4" mandrel for the rolled fuse but Don uses a 5/8" mandrel.
Dave
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« Reply #16 on: January 17, 2015, 07:10:59 PM »

Don really packs in the turns - I think he has a secret method, but I bet he gets more turns/torque in rubber than most people.  I watched his final flight at the 2014 AMA Nats - and that thing went straight up for a long time - probably why he needs so much down thrust.

--george
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« Reply #17 on: January 17, 2015, 08:04:08 PM »

This an older version of the "Polecat." Copied drawing dated 2006. This may be the design he flew to win his first Nats P-30 event.

OTF'er....
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« Reply #18 on: January 17, 2015, 08:14:58 PM »

The way to get the steep vertical climb is to wind to 95% of maximum or more, not to 70 to 85% of maximum.  For F1G, which also uses a 10gram motor, the motor is only used once and normally has at least one broken strand after the flight. If you are not breaking a few motors winding, you are not getting all you can out of the motor.

Louis
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« Reply #19 on: January 17, 2015, 09:06:00 PM »

Winding to 95% of maximum, you need to define whether you are winding to torque or turns. I specifically stated torque.  Going from 80% torque to 95% torque is nowhere near a proportional number of turns.  That was my point.  I can easily make my first 4 maxes the way I wind, and I wind to 90-95% for my fifth, depending on how the rubber feels, if the air is still neutral...which it almost never is by then.  I don't believe anybody I've flown against the past 2 years has outclimbed my P30, and the 2 times I've flown head-to-head flyoff flights I know that to be true.  I know I'm not one of the top flyers--Don is--but my objective in posting is to not have people misled into how many turns are possible.

My point was to not have somebody reading this expect they would be able to get 2200 turns.  I'm willing to be wrong, but I'd like to hear from somebody that regularly gets more than 2200 turns from recent SS rubber with 6 strands of 3/32 SS in a motor that weighs <=10 grams lubed.  Untill then, I stand by my statement to not expect to get at least 2200 turns.

 When I get a chance, I'll do some more testing to destruction and see if I can ever get 2200 turns.  As I said, I last tested a couple of years ago.
Speaking of which, I'm off to watch some basketball--and make up some P30 motors for the Ike.
 
--Bill
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« Reply #20 on: January 17, 2015, 09:49:19 PM »

This an older version of the "Polecat." Copied drawing dated 2006. This may be the design he flew to win his first Nats P-30 event.

OTF'er....

This is an older design and I would say at 28" nose to peg length used the longer style motor. Older S.S rubber would take more turns than the current batches. Older SS rubber + 28" motor length might just make 2200 turns. You can forget any rocket-like climb using this trim scheme.

OTF'er.....   




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bentodd
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« Reply #21 on: January 18, 2015, 12:12:20 AM »

Thanks for the replies.  I will let you know how things turn out.
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« Reply #22 on: January 18, 2015, 06:08:46 AM »

Regarding typical turns from a 6 strand 3/32" SS motor, on the last test to destruction I did on some Jan 2012 3/32" SS, the motor broke at circa 72 turns per inch (I do tests on a 6" motor). A 9.8g lubed but unrun-in motor measures around 27 inches, translating into 1944 max turns, which is close to results Bill is achieving. Running in will add a few more turns, but not 10%. As rubber cross section can vary from batch to batch, I would assume that some motors will get close to the magic 2200 figure, others not. The moral is to get testing!

This in no way casts doubt on Don's excellent model and contest record.
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« Reply #23 on: January 18, 2015, 07:38:22 AM »

Bill:

As you point out, torque should be the indicator when winding, rather then the number of turns. If a model is trimmed to handle a specific torque level, exceeding that level could cause erratic trim---not what you want on a contest flight. Since rubber cross-section can vary from batch to batch and even between splices in a box, motors should be made to a standard length, rather then a standard number of strands. Using smaller widths of rubber (1/16 or 3/32) allows motors to to be made up closer to the standard length that you have determined as optimum. It is important to test fly with maximum turns, using a new motor for each flight.

Although my winders are equipped with turn counters and torque meters, I only use the turn counter to evaluate torque build-up at intermediate turn counts. For example I usually check torque at 100 and 200 turns on F1G or F1B motors that are typically wound to 400-425 turns. After that I watch torque build-up and use the feel of the winder to determine when to stop.  Winding outside the model using a half-tube system does help, since there is no danger of damaging the model or getting a blast tube jammed inside the model. For me at least, it does encourage winding to maximum torque. I feel that if you are not breaking an occasional motor winding, you are not getting the most out of the rubber.

Louis
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danberry
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« Reply #24 on: January 18, 2015, 10:49:04 AM »

Don really packs in the turns - I think he has a secret method, but I bet he gets more turns/torque in rubber than most people.  I watched his final flight at the 2014 AMA Nats - and that thing went straight up for a long time - probably why he needs so much down thrust.

--george
It doesn't go straight up at 8 AM.
The thermal he was in might have maxed a ballcap.
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