Design

torqueburner
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Re: Design

Post by torqueburner »

bjt4888 wrote: . . .I start with the Bernie Hunt design spreadsheet . . . Almost all of the airplanes my students built needed about 1 degree less wing incidence for a total decalage angle of about 1.9 degrees. . . Sorry for the very long message.

Brian T.
Thanks Brian, for your thorough and detailed answer. I appreciate the opportunity to learn more about indoor flying from you and Jeff A. And I always figure that a long answer is more likely to include more useful information:)

It is apparent that I need to "hit the books". I know of the stability vs. efficiency tradeoff as you move the cg, but I never looked at the numbers as you describe. Where can I find the Bernie Hunt spreadsheet? I saw your previous post saying that you were unable to post that large a file, something like that, but I've looked online without success.

I guess that some of my confusion is due to the fact that your account of trimming the FF planes is so different from our experience. My students' planes are similar in length, have a flat wing with endplates angled about 15 degrees from vertical. The stab, however, has similar endplates, and is mounted with tissue tubes below the tailboom. The cg is currently about 1.3" (3.3 cm) behind the TE of the wing. We originally had the cg farther back, about 2" behind the TE, but the recovery from ceiling hits was poor; the plane would lose 6-8 feet of altitude before recovering. With the new location, recovery is MUCH better, and the flight times seem about the same.

Here is where things are really different from your account. We have the thrust line parallel to the motorstick, and the wing has LE 1mm higher than TE. About 0.7 degrees positive incidence. Our best results have been had with the LE of the stab 7 mm below the TE. About 6.7 degrees negative, so our total decalage appears to be more than 7 degrees. Maybe a little more or less, if the boom is not exactly parallel to the motorstick. But still, this is way, way more than your students are using with their FF planes. I would think that this would greatly increase drag and adversely affect flying times. Maybe it does, but our best no-touch flight is currently 3:39 under a 19' ceiling, which seems pretty decent.

So I'd like to take a look at Hunt's spreadsheet, if you can point me to it. If you have any ideas as to how these two plane designs can trim so differently, but still get similar times, I'd be most interested. And once again, thanks for bringing your expertise to these discussions!

Dave Drummer
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Re: Design

Post by bjt4888 »

Dave,
The Bernie Hunt spreadsheet in located in the files section of the Yahoo Group named "Indoor_Construction". Although the trim your students are using sounds a little different than standard, the results are pretty solid. My students haven't started to experiment with propellers and are not achieving the full potential of their airplanes, so it may be a bit difficult to compare results. Their best competition flight so far (with the Freedom Flight airplane and stock propeller) is 3:27 one touch in a 27 ft. gym.
The main reason that I use the Bernie Hunt spreadsheet is to create a starting point so that trimming goes smoothly and quickly (hopefully). Playing with the spreadsheet helps my discussion with the students regarding trim changes. For example, with most of the Regional wing Freedom Flight airplanes, the ballast clay needed to be located about 1/2" behind the selected CG (CG set at 2.5" behind the LE to get 15% SSM). If we decided to move the clay forward, we knew that a 1/2 inch movement forward caused a 1/8" change forward in the CG which is a 4% increase in SSM (for a total 19% SSM). Once the students had good consistent performance in this trim range, my recommendation to them was to begin testing increased pitch propellers and definitely testing flaring propellers. It looks like they want to start testing flaring propellers by modifying an Ikara later this week.
Just to clarify, I am not a famous indoor flyer. But, as I'm sure is true for you and other SO coaches, I am avid reader of this and many other subjects and i am lucky enough to get good info from other forums from the pros like Bill Gowen, Kang Lee, Fred Rash, Leo P. Stan B. and many others. I have flown outdoor rubber and gas free flight casually since 1968. I also am lucky to have regular access to our local middle-school gym to test various indoor flying ideas.

Your team is doing great!

Brian T.
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Re: Design

Post by TC-SciOly »

Does anyone know if plastic food wrap can be used as a substitute for mylar?
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bernard
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Re: Design

Post by bernard »

TC-SciOly wrote:Does anyone know if plastic food wrap can be used as a substitute for mylar?
I've made two planes, one with plastic food wrap and the other with mylar. I tried crinkling the plastic wrap (as recommended by the Freedom Flight instructions, but for Freedom Flight they use mylar). If I recall correctly, the food wrap didn't really crinkle but it didn't matter much since I ended up stretching it a bit for it to be perfectly smooth. Plastic wrap seems to be heavier and thicker than mylar by just a hair. I stretched the plastic wrap so it would be a smooth surface but I don't recommend that since the tension in the stretched plastic wrap easily causes unwanted warps that can't be easily fixed.

If anyone is curious how I stretched the plastic wrap and got a tight smooth covering, here's what I did. I stretched (just a bit) the plastic wrap over a box that would function the say way as a covering cradle in the Freedom Flight kits (this was before I was using Freedom Flight). I used a hairdryer on the plastic wrap which caused it to naturally stretch a bit. With glue from a glue stick applied to the wing spars and leading/trailing edges, I pushed it into the plastic wrap and held it there for a second. As the plastic wrap cooled, it shrunk and left a tight covering. However, this also led to unwanted warps in both the wing and the horizontal stabilizer. If I were doing this again, I wouldn't stretch the plastic wrap so much or even at all.
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Re: Design

Post by jander14indoor »

MOST plastic food wraps are SIGNIFICANTLY heavier than indoor mylar. As suggested many times, you are generally better using those very thin and light produce bags, and some store product bags can be very light. I've had good success with Home Depot, Quizno's, Canadian Tire. You want the ones everyone hates because they tear too easily. Because I don't want teams to obsess about the covering I never use indoor mylar on my SO planes. Too easy to hit the min weight without it if you spend just a little time on wood selection.

However, these bags aren't sold to minimize weight, you'll need to check the actual weight to make sure you are using a light covering material. Grab a bunch of bags to sample. Cut out 10 in by 10 in samples from each (exact size doesn't really matter, just a convenient size to weigh, and be consistent). Pick the lightest covering you can find.

Jeff Anderson
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Re: Design

Post by bernard »

If my winglets extend back a bit so that the tip of the winglet is farther back than the trailing edge of the wing (kind of like this), would you include the winglets in the wing chord measurement?
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Re: Design

Post by bjt4888 »

Bernard wrote:
by bernard » Tue Mar 10, 2015 8:38 pm

If my winglets extend back a bit so that the tip of the winglet is farther back than the trailing edge of the wing (kind of like this), would you include the winglets in the wing chord measurement?
Bernard,
Of course, this is just my opinion and is not official. Any official rule clarification would have to be via the SO official FAQ page. As the applicable section of rule 3.d. reads, "The maximum wing chord (straight line distance from leading edge of wing to trailing edge, parallel to the fuselage) of the wing must be 8.0 cm or less.", I would think that if your winglet chord is less than 8.0 cm as measured per this rule, you would be fine as the winglet is part of the wing. If I were you, I would submit an official FAQ question. I have submitted an number of questions (the two on the webpage are two that I wrote verbatim) and several that I asked were replied to in the fashion below. This is a question that I sent regarding wingposts that those in charge of the FAQ determined did not need to be posted on the FAQ, as it is already clear in the rules. However, I very much appreciated the evaluation and carefully considered response.

quoted email reply from "[email protected]"

"The definition of the maximum wing chord is very clear, it is a straight line
distance from leading edge of wing to trailing edge, parallel to the
fuselage. The method of attachment to the rest of the airplane can or cannot
be included in the measurement, depending on the configuration. Specifically,
if the attaching component provides lift, it will count as part of the wing
and be included in the chord measurement. If it does not, it will not count
and be ignored in the measurement.

The original question is included for your records; there is no need to
respond unless you feel there is an error."
=========================================================
ORIGINAL MESSAGE:

Please clarify that the maximum wing chord measurement, which is stated in
the rules as, "...(straight line distance from the leading edge of the wing
to trailing edge, parallel to the fuselage.)..." does not include wing posts
attached to the outside edge of either the leading edge or trailing edge.

Wright Stuff

(section: 3. / paragraph: d. / sub-paragraph: / line: 2)

Good luck and, by-the-way, very attractive fin configuration on the model.

Brian T.
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Re: Design

Post by jander14indoor »

Also not official, etc...

If those tiplets are completely vertical you shouldn't have a problem at all as referenced above

Even if not and they are part of the lifting span, you shouldn't. Chord is measured along a line parallel to the fuselage or direction of flight as you can find references to in any good aero engineering source. Those would just be swept back wing tips. Looking at it only visually, since the leading edge retreats faster than the trailing edge, the distance parallel to the fuselage will be less than the max chord of the main wing. Simple geometry.

Regards,

Jeff Anderson
Livonia, MI
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Re: Design

Post by bernard »

jander14indoor wrote:Also not official, etc...

If those tiplets are completely vertical you shouldn't have a problem at all as referenced above

Even if not and they are part of the lifting span, you shouldn't. Chord is measured along a line parallel to the fuselage or direction of flight as you can find references to in any good aero engineering source. Those would just be swept back wing tips. Looking at it only visually, since the leading edge retreats faster than the trailing edge, the distance parallel to the fuselage will be less than the max chord of the main wing. Simple geometry.

Regards,

Jeff Anderson
Livonia, MI
I don't quite understand your descriptions because I'm not good with visualizing some things. I attached a diagram with a side view. Do you think this would be allowed, and would it benefit flight?
Attachments
winglet diagram.pptx
(45.81 KiB) Downloaded 216 times
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Re: Design

Post by bernard »

If you're using the Freedom Flight kit or using plastic tubing/straws to attach your wing posts, how are you attaching your plastic tubing/straws to your motor stick? I've tried using CA and of course it doesn't bond well since its plastic. I've used tape and its worked well but I would like a more secure and permanent solution, possibly one I could do without buying cement.
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