Trimming

[DoctaDave]

It's a bit smaller than 1.5 inches, maybe 1.4

[bjt4888]

DoctaDave,
I am retracting my suggestion to move the CG forward. Your CG is already quite far forward. I needed the nose moment arm in order to calculate the static stability margin (ssm). Your plane ssm is over 44% and this is something you could consider reducing. A forward CG resulting in a high ssm requires a high decalage angle (difference between the stab incidence and the wing incidence). Your current decalage angle of 2.0 degrees could possibly be reduced to 1.7 degrees by pushing in the front wing post 1/32". After doing this, move the CG back to about 2" behind the rear wing post. This overall reduction of decalage might make the increase in wing washin have less of a detrimental effect during cruise and let down. You might want to double-check your stab tilt to see if it matches the kit setting of 3/8" measured from left stab tip to stab center. With the wing on 2"x4" blocks and the motor stick propped and the whole airplane on a big table you can measure the two stab tips distance from the table and calculate. If you are already at the kit setting, which equates to 3.74 degrees, you might not want to increase per my previous post recommendation. If you double-check and you are less than 3.74 degrees, go ahead and increase to the full 3.74.
Reduced overall decalage and rearward shift of CG, along with more stab tilt and the increased left wing washin might get you the climb height you want for Nationals and reduce the outward drift during the cruise and let down.
Of course, you will want to introduce these changes one at a time (except maybe the wing incidence reduction and the CG shift can be done at the same time). You are finding that it can be challenging to get an 8.0 gram airplane to the ceiling using rubber as thin as .081. If you have time to test this, I would be inclined to use a 16.5" - 17" motor of the .087 rubber and a prop pitch of 14" (kit prop at 37 degrees at 3" radius). The thicker rubber and lower rpm might not roll the airplane as much and might not need the additional washin and might get you the duration you're looking for.

Brian T.

[bjt4888]

DoctaDave,
The other thing that you could consider is to not change anything and to fly your .65 in oz launch torque flight. If you are getting over 25 ft. of climb with this winding and if you don't need the increased washin at this torque level, this might be your best flight. If you are getting 2:10 when launching at .60 in oz and a flight altitude of 20 ft. it seems likely that you will get 2:30 or more with the .65 in oz launch torque. This would translate to over 5 minutes at Nationals.

The other suggestions i wrote will take a little time and may or may not work.

I am thinking that the very best possible flight at Nationals will require a flaring propeller and a little thicker rubber than .081". As you know, a 65 ft. ceiling is not considered "high ceiling" and is not "full torque" flying. Whenever you are flying under a ceiling that requires backoff turns, a flaring propeller will usually be the best bet for highest duration. the trick to flaring props is how soft the blade is and how much it flares, combined with the static (unflared) pitch performance during the end of cruise and let down and determining the rubber cross section (or density) that best matches the prop (equals lots of testing).

All this being said, your work is very commendable and if you don't have time to develop the flaring prop (and it's a little tricky if you don't have a rubber stripper to create the match of rubber to prop), if you put up a 5:00 plus flight at .65 in oz, I think you will place very well.

Brian T.

[DoctaDave]

During the earlier portion of the season, I was using the flaring prop sanded to the specifications based on Chris Goin's double trouble plans, but when I started doing partial motor tests for nationals, I was not able to get the plane high enough. I was only able to achieve about 40 feet with that (half motor launched at .65 in oz which reached 20 feet). Also, I found that when I was using the flaring prop, the descent was a little unstable. I'm not sure if this has to do with the propeller, but the plane liked to slow down, almost to a stop, but the nose didn't stick up so I'm not sure if it was a stall.

I do have access to a rubber stripper, but at this time since nationals is so close, I think I will either attempt to fly with a higher prop pitch and thicker rubber as you suggested, or just go with what I have now since I dont want to mess with stab tilt etc. with so little time left.

Thanks again Brian!

[bjt4888]

DoctaDave,
Congrats on your thorough preparation. It's good that you built the flaring prop and tested it. Your results using a category I (ceiling ht less than 8 meters) flaring prop to test fly for a low category III ceiling (15-30 meters) are not unusual. A slightly stiffer flaring prop (stiffer blade or less blade area in front of the spar) would most likely work pretty well for the higher 65' Nationals ceiling. I would agree with you that, as time is short before Nationals, a good course of action is to fly the.081" rubber .65 in oz torque setup that you have already tested (or possibly compare to a slightly higher pitch prop and slightly thicker rubber setup).

One of the teams I coached had a similar experience testing a softer bladed flaring prop. After test with the softer blades, they made a different slightly stiffer bladed flaring prop (from the wide bladed "flaring" Ikara) that performed very well for ceilings less than 30 ft. and using .055g/in TSS rubber (this is the team that won the Michigan state champs with a 3:46 under a 21'4" slightly scrubbable ceiling). Remember that the Goins airplane has a lower wing loading and used a smaller diameter prop and a thinner, lighter motor (1.5g and .077" rubber) than this year's rules. As this configuration was putting less power into the prop than with this year's rules, a softer flaring style was suitable (blade thickness progressing from .006" at the edges to .009" at the spar). For this year's rules (higher wing loading, 2g motor and typically .087" rubber), a stiffer flaring blade worked pretty well for my team (blade thickness .007" to ".010"). We measured blade flaring stiffness using the method Bill Gowen demonstrates with his scale in a picture in the Hip Pocket Aero "Wright Stuff" thread. The "soft flaring" prop took about 12 grams of force to flatten the blade on the pan of our scale and the "stiffer flaring" prop took about 16 grams to nearly flatten (a little hard to compare as the props were so different, but the soft flaring was a copy of Bill Gowen's carbon rod hub flapper and the stiff flaring was the sanded Ikara) Sorry if I am providing info that you already know.

I suspect that what you observed in 1/2 motor test flights with your flaring prop is what is called "blade stall" due to blades that flared a little too easily resulting in excessive pitch during that portion of the flight. However, all that being said, your work with the flaring prop and your other configurations is top-notch and I hope your Nationals results are good.

Brian T.