Design

[TC-SciOly]

Thank you to all who helped! Unfortunately, when we tried flying with more winds today, the plane just plunged at a faster rate... Not sure what is still wrong. We tested with different attack angle, rubber band sizes, and also the number of winds. Seems like the best times for us are the ones with ~40winds.

[InfiniCuber]

Thank you to all who helped! Unfortunately, when we tried flying with more winds today, the plane just plunged at a faster rate... Not sure what is still wrong. We tested with different attack angle, rubber band sizes, and also the number of winds. Seems like the best times for us are the ones with ~40winds.

Perhaps your horizontal stabilizer is at a weird angle? or you have too much angle on the thrust bearing? That's all I can seem to think of !

[jander14indoor]

That starts making me suspect something fundamental wrong that isn't coming across in chat like this.
Wrong angle of attack for wing or tail or really strong down angle on prop shaft are possibilities. But if you are flying OK on 40 winds that shouldn't be a problem. I'd expect problems with power stalls if anything. Though if your prop angle is REALLY that down it could cause this.

But I'm a suspicious that you have a perhaps more subtle problem. Is your motor stick bowing significantly as you add winds above 40? If yes, you need to fix that. If you fly OK at 40 winds, describe the flight to us. Then describe what happens when you wind to 50. Look at the motor stick and plane as you increase winds. Do you notice the tail drooping? Then try 60. etc.

Jeff Anderson
Livonia, MI

[torqueburner]

. . .Unfortunately, when we tried flying with more winds today, the plane just plunged at a faster rate...

TC-SciOly and Jander: here is something that may help to sort this out. Yesterday, I helped a student with his FF plane. This was my first hands-on experience with this design. Detailed below are my observations, and some speculation. Jeff Anderson and Brian T. have much more flying experience that I, and can no doubt offer you better advice that might help with your problems, some of which sound familiar based on yesterdays flying session.

We started out with some low torque trim flights to adjust the plane for the best cruise and descent, as recommended to me by Bill Gowen some years in the past. When we do this with our own student planes, as we increase decalage, we see times increase up to a point, then start to decrease, usually as the plane starts to stall.

First, I asked the student to trim his plane as he had been doing in the past. When released from shoulder height, the plane cruised poorly, and descended pretty rapidly. We increased the wing incidence, the main trim adjustment available for this design. This helped at first, but soon the times started to decrease, and we just couldn't cause the plane to stall. At this point, the LE of the wing was about 8 mm higher than the TE, so I wonder if this doesn't result in too much down thrust, since the thrust line was parallel to the motorstick?

Upon closer inspection, we noticed that the stabilizer actually had some positive incidence due to the fact that it was glued to the top of the tailboom, which was tapered on the top. So I'm thinking that this would require even more wing incidence to get the plane to climb?

The student had glued carbon fiber to the motor stick, but it was on the bottom. I've never used carbon, but it seems to me that it should be on the top. We saw classic signs of motorstick flex with launch torque as low as 0.3 in-oz. Just flexing the stick by hand showed it to be pretty springy.

Throughout the day we were frustrated by really quirky behavior. The plane would fly a little better for a while, then worse, with the same trim settings. We then discovered that the cg was way back - about 5 cm behind the TE of the wing. If memory serves me, I recall a past message board discussion that suggested if the cg is too far back, that the plane might be extremely sensitive to tiny changes in trim. Again, perhaps Jeff and Brian can comment on this. We tried moving the cg forward, but this required even more wing incidence, which worsened the problems.

[bjt4888]

torqueburner wrote:

TC-SciOly and Jander: here is something that may help to sort this out. Yesterday, I helped a student with his FF plane. This was my first hands-on experience with this design. Detailed below are my observations, and some speculation. Jeff Anderson and Brian T. have much more flying experience that I, and can no doubt offer you better advice that might help with your problems, some of which sound familiar based on yesterdays flying session.

We started out with some low torque trim flights to adjust the plane for the best cruise and descent, as recommended to me by Bill Gowen some years in the past. When we do this with our own student planes, as we increase decalage, we see times increase up to a point, then start to decrease, usually as the plane starts to stall.

First, I asked the student to trim his plane as he had been doing in the past. When released from shoulder height, the plane cruised poorly, and descended pretty rapidly. We increased the wing incidence, the main trim adjustment available for this design. This helped at first, but soon the times started to decrease, and we just couldn't cause the plane to stall. At this point, the LE of the wing was about 8 mm higher than the TE, so I wonder if this doesn't result in too much down thrust, since the thrust line was parallel to the motorstick?

Upon closer inspection, we noticed that the stabilizer actually had some positive incidence due to the fact that it was glued to the top of the tailboom, which was tapered on the top. So I'm thinking that this would require even more wing incidence to get the plane to climb?

The student had glued carbon fiber to the motor stick, but it was on the bottom. I've never used carbon, but it seems to me that it should be on the top. We saw classic signs of motorstick flex with launch torque as low as 0.3 in-oz. Just flexing the stick by hand showed it to be pretty springy.

Throughout the day we were frustrated by really quirky behavior. The plane would fly a little better for a while, then worse, with the same trim settings. We then discovered that the cg was way back - about 5 cm behind the TE of the wing. If memory serves me, I recall a past message board discussion that suggested if the cg is too far back, that the plane might be extremely sensitive to tiny changes in trim. Again, perhaps Jeff and Brian can comment on this. We tried moving the cg forward, but this required even more wing incidence, which worsened the problems.

The trim method I use is borrowed/stolen and a combination of methods that I have learned from others. I start with the Bernie Hunt design spreadsheet and use a CG location that gives the desired minimum static stability margin (SSM). For my students, I did not have them construct the fuselage with the positive incidence in the tail boom. Although positive incidence in the stabilizer is not an uncommon trim for long tail moment arm designs like this, I recommended to the students to try a little more conservative trim. We set the initial decalage angle using a little negative incidence in the stabilizer (about 0.65 degrees for the Freedom Flight) and a little more positive in the wing (about 2.3 degrees, or 1/8" longer front wing post than back wing post) for a total initial decalage of 2.95 degrees. With the nose moment arm set at 1.5" (we did not use the adjustable wing mount as it was apparent that the kits would weigh 1.2 grams under minimum and this would allow a good-sized lump of clay that we could move around to adjust the CG if necessary, we selected a SSM of 15% (hopefully to provide a little better recovery from ceiling hits), which is a CG at 2.5" behind the wing TE. The Freedom Flight kit measures a neutral point of 2.99" behind the wing TE and has a recommended CG of 2.75" behind the TE for a calculated SSM of 8.5% (this setting would potentially be very low drag, but maybe not so good recovery from ceiling hits). The recommended incidence settings for the kit are 0.62 degrees positive for the stabilizer and 2.3 degrees positive for the wing for a total decalage angle (difference between the two) of 1.69 degrees. This is a pretty low SSM, which correlates well with the low decalage angle and the very long tail moment arm. We assumed that the 2.95 degrees of total initial decalage we selected (along with the 15% SSM caused by the 2.5" behind TE CG) would result in a stalling flight and we were correct. Every Freedom Flight kit we setup this way stalled slightly during the first low power flight (about 80x15 turns and .30 in oz torque). 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. The other very significant trim factor that we encountered was the need for a minimum of left wing washin. It is typical of a design with a very rearward CG and neutral point to be very sensitive to decalage angle and left wing washin. Washin settings my students ended up with were 3/32" at the most and usually about 1/16".

All this being said, I am thinking that based upon your note that the airplane you were testing had a CG 5 cm behind the wing TE that it is very nose-heavy. 5 cm behind the wing TE or 1.97" (sorry, I am still better with inches than cm) would calculate to a 32% SSM if the nose moment arm was 1.5" (I'm not sure if it was this, but if not, you can use the Bernie Hunt spreadsheet to recalculate SSM). Typical good flying SSM range would be from 10% to 25% (10% should require about 1.5 to 1.8 degrees of decalage and 25% should require 2.0 to 2.5 degrees). 32% is indicative of very nose heavy and would require the extreme wing incidence that you are trying (8mm would calculate a wing incidence of 5.8 degrees, or almost triple what should be required). If the student tries these settings (as long as the wing and stabilizer are flat except for 1/16" of left wing washin and as long as the tailboom is installed correctly with the bottom edge of the tailboom parallel to the bottom of the motor stick), they should hopefully be successful: nose moment arm = 1.5", CG with rubber motor installed = 2.5" behind the wing TE (move the clay ballast around to get this), wing incidence = 2.3 degrees (LE 1/8" higher than the TE as measured from the top of the motor stick as a reference point).

A long tail moment arm, rearward CG design like the Freedom Flight kit is capable of very high performance but is a little sensitive to decalage angle and CG. However, once my students found a good trim setting (one flight did it), they carefully marked their wing posts at the edge of each wing post socket and they have had no trouble getting repeatable performance since.

The motor stick wood for all of the Freedom Flight kits we built was pretty good and most motor sticks weighed about 2 grams or a little less. I recommended to the students that they bend custom rear motor hooks and custom front hooks and use Harlan bearings. We mounted the rear hooks at about 13.5" instead of at the end of the 16" motor stick in order to create less bending force on the stick and to avoid possible creation of unwanted downthrust. We have had no issues with bending motor sticks flying up to about .48 inch ounces torque. We have not tried flying at higher torque as we do not have a high ceiling site for testing. I would think that the carbon attached to the fuselage would not be necessary unless flying at high ceiling sites and using 1.0 inch ounces of torque or more. It is true that the Freedom Flight kit did recommend the carbon be attached to the bottom of the fuselage if applied to only one side. I have never tried gluing carbon to only one side of a structure, but I would think that if Dave Ziegler recommended this, I'm sure that he tested it and it should not be an issue.

Sorry for the very long message. Your questions and observations are very good.

For TCSciOly, I agree with the other recommendations you have received. All trim settings (CG, stab incidence, wing incidence) need to be checked and matched to the excellent Freedom Flight kit instruction book and all flying surfaces need to be verified to see if they are flat and straight except for the 3/32" of left wing washin. Also, let me know where your wing is mounted relative to the front of the motor stick. Is the wing leading edge about 1.0 or 1.5" back from the end of the motor stick?

Good luck to all,

Brian T.

[torqueburner]

. . .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

[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.

[TC-SciOly]

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

[bernard]

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.

[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
Livonia, MI