Don:
Sounds like you are getting there.
I should clarify, I got ahead of you. the partial piece of rubber is valuable for evaluating performance, and for low-ceiling testing prior to a higher ceiling event. This assumes the plane is already trimmed out (decalage, balance). Decalage (difference between stab and wing incidence) and balance are observed during slower portions of the flight (let-down). See Ol' Bill's posts on Hip Pocket. He is adamant that you trim incidences in this phase of flight, then leave it alone! Do not change incidence to change the climb characteristics. For this we do indeed use a full motor, with partial winds (usually 50 cranks on our 10:1 winder). The climb does not matter at this point, you are looking at the let-down, trying to get it to "almost stall". One old piece of rubber will last forever doing this as you are not stressing the rubber.
Generally I have found that on these W.S. planes, about 1.5 to 2 degrees of decalage seems to work, though your mileage may vary based on design. If close to 0, you will find the plane is very sensitive to adjustments, and disturbances (girder strike, air movement) will really upset the plane. If this is the case (based on your description), move the CG forward, adjust decalage again, getting it close to stall. Note that ANY change should be followed by a trim adjustment flight or two, especially things that change the circle size (rudder offset). Another way to do this is to set your decalage to 1.5 or 2 degrees, and then adjust CG to just eliminate stall (move it back until a stall is seen, then move it forward). Once your CG is close, then you may want to explore small changes of CG/decalage against the stopwatch (performance, so using your partial rubber at full fractional winds). The suggested CG is just a starting point. Be sure to CG with rubber installed.
While some have suggested glide testing with clay replacing the prop weight, I have found it less useful than a partial wind on an old motor. Tossing at just the right speed is not easy.
My rubber numbers are in g/in. Yours are probably in inches of width. I believe something like 0.0425g/in will be about 1/16" (0.0625"). So your rubber is quite thinner than mine. That is fine, it matters whether it matches your prop and plane. We use a 10:1 winder (its what they had), and for contest we are looking at 220-250 cranks (2200 to 2500 winds) with probably 60 to 150 unwinds (6 to 15 cranks). Our 1.5g rubber is 44-45cm long. Don't read too much into these numbers, as we are matching the prop we have (Bill Gowen style prop).
I assume you are using the Ikara hanger. Yes, no easy way to adjust. We have left thrust (1.5-2 degrees) and no down thrust. You can effectively change the relative down thrust by changing the incidence of both the wing and the stab in unison.
Now once you have the plane trimmed for later flight portion (let-down), work on the rubber. Using the contest rubber (or partial at relative full winds), look at the plane during cruise and letdown. If no cruise and fast letdown, you need thicker rubber (or if consistently a LOT of leftover winds). If consistently using all the rubber (no winds left), then you need thinner rubber. Or you can adjust the prop pitch. If using all the winds, add more pitch. Notice that it is still the later portions of the flight (cruise and letdown) that are determining whether the prop is matched to the rubber, not the climb.
Finally, work on adjusting how many backoff turns to take to control your climb (or adjust your launch torque, if you have a torque meter. Several simple designs on Hip Pocket, I made a digital one with a Harbor Freight scale, a broken electric plane bearing, and about a half an hour of effort). If everything else is dialed in, you should be able to get a direct correlation between launch torque (or backoff turns) and peak altitude. Note: I previously said the rubber seems shot after a flight. I think with this thin rubber we were just not letting it rest enough between attempts. Using 3 or 4 pieces of rubber let us rotate through and we get more consistent torque vs backoff winds.
Yes, by "relative winds" I mean that winding a 1/4 motor will take 1/4 the winds of a full motor, as well as unwinds. I find at 1/2 motor the kids need to be careful, things can happen quickly. Moreso with 1/4 motor, the backoff will drop the torque in a hurry. with 1/2 motor we were backing off 3-5 cranks, or 30-50 unwinds. That would be 2-3 cranks on yours, or at 1/4 motor could be fractional unwinds. Of course we are using different prop and rubber size, so mileage may vary.
With these (very) small props, we have seen steeper climbs than last year, but the "burst" of torque drops off quickly with the thin rubber. It is taking a lap or two on half rubber to reach our target elevation (15 feet, as Regionals will be in a 30-foot gym).
Chuck
----- Original Message -----
From: "dweigt47_at_gmail.com [Indoor_Construction]" <Indoor_Construction_at_yahoogroups.com>
To: "Indoor Construction" <Indoor_Construction_at_yahoogroups.com>
Sent: Tuesday, January 24, 2017 12:00:37 AM
Subject: RE: [Indoor_Construction] Re: Any A6 tutorial?
Chuck, thanks again for sharing all this info!
We must be doing something wrong, because to prevent stalling at the suggested CG, we had to lower the wing's leading edge until I couldn't be sure the wing and stabilizer were at different angles. It went from stalling in the glide to just descending in a straight line without pulling up, with just a tiny adjustment. So, we raised the wing's leading edge a bit, and moved the CG forward. Then it glided without stalling and with pitch stability.
Is it fair to test glide these models? Seems they probably won't have the prop stopped at any time in a powered flight. A stopped prop can't have the same drag as one at low power near the end of a flight.
Even after it glided OK, we had trouble with stalling under (partial) power until we added a little bit of left rudder to tighten the turn. This was in addition to having some stabilizer tilt already. I ran out of ideas. The thrust line isn't adjustable. The propeller bearing presses into a plastic mount glued into the front of the fuselage stick. If it was adjustable, I'd have tried removing the right thrust and adding a little down.
We know that flying with a partially wound full motor isn't as good for adjustments as shorter motors wound fully to have the proper torque at the beginning of the flight, But, it was the only choice we seemed to have, if we didn't want to end the testing. Jack had only one short motor. I wondered if he'd make another one there, but he didn't do so.
Jack is using a 15:1 winder, backing off five turns of the crank, 75 turns in the rubber, regardless of how many turns he flies with. So, he may be over winding and unwinding too much with shorter motors. A motor one fourth as long probably should be over wound and backed off one fourth as much..
He's using 0.056 rubber, because the 0.052 also supplied wouldn't make it climb on his previous outings. I'm not sure it that's width in inches or grams per inch, or even some other unit. But, if it is the same units, then his plane needs a lot more power to fly than your plane! But, Jack did say his model seemed to climb more steeply early on than those he'd seen on the internet, so maybe the rubber is too thick. His motor is at about 1,950 turns when wound fully. Your must be about 2,600 turns.
We aren't using a torque meter. I haven't asked if Jack has one. I'm pretty sure he'd be using it if he did.
Don Weigt
Received on Tue Jan 24 2017 - 08:04:07 CET
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