Plane descending too fast

Bitconnect
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Plane descending too fast

Post by Bitconnect » January 20th, 2019, 12:19 pm

My plane climbs pretty fast and stays up in the air for a decent amount of time but when the plane starts to descend, it drops really fast. It probably takes ~20 seconds or so for my plane to drop from 22ft to the floor. Everything else seems normal.

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Re: Plane descending too fast

Post by CrayolaCrayon » January 20th, 2019, 1:13 pm

It may be running out of winds too soon. Maybe try thinner rubber or a longer strip? BJT would have a better answer than me, so I'd wait for him on this one.
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Re: Plane descending too fast

Post by klastyioer » January 20th, 2019, 5:12 pm

Bitconnect wrote:My plane climbs pretty fast and stays up in the air for a decent amount of time but when the plane starts to descend, it drops really fast. It probably takes ~20 seconds or so for my plane to drop from 22ft to the floor. Everything else seems normal.
how heavy is ur plane?
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Re: Plane descending too fast

Post by Bitconnect » January 20th, 2019, 6:16 pm

I've used a longer motor each time but nothing stops the plane from falling really quickly. Initially, I had a 14" long 3/32 motor and each time after I used a 2" longer motor and the same exact thing happens. My plane is probably 8.07g or something close to that.

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Re: Plane descending too fast

Post by bjt4888 » January 20th, 2019, 7:18 pm

Bitconnect,

If you could please supply more details I could give very specific help (ex. Weight of the airplane as Katy requested, all dimensions of the airplane, propeller diameter and pitch and design, rubber motor length and weight, number of turns in the motor, max torque, number of backoff turns, launch torque, turns remaining, all trim settings of the airplane).

Rapid descent, if the motor has full max turns on it, is usually related to more work needed to match rubber density to propeller design and pitch. However, having all the details above will rule out other possible issues.

Good job getting your airplane built and in the air.

Brian T

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Re: Plane descending too fast

Post by jander14indoor » January 20th, 2019, 7:56 pm

Also be useful to know how long to climb, how high, how long at altitude.
Fast descent means you are out of power, need more torque (fatter motor, less total weight) at that portion of the flight.

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Re: Plane descending too fast

Post by coachchuckaahs » January 20th, 2019, 8:56 pm

Jeff:

Or possibly need thinner motor, if it is completely out of winds. Need to know remaining turns in the rubber upon landing. The description is vague, could be dead stick on letdown.

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Re: Plane descending too fast

Post by Bitconnect » January 21st, 2019, 10:26 am

bjt4888 wrote:Bitconnect,

If you could please supply more details I could give very specific help (ex. Weight of the airplane as Katy requested, all dimensions of the airplane, propeller diameter and pitch and design, rubber motor length and weight, number of turns in the motor, max torque, number of backoff turns, launch torque, turns remaining, all trim settings of the airplane).

Rapid descent, if the motor has full max turns on it, is usually related to more work needed to match rubber density to propeller design and pitch. However, having all the details above will rule out other possible issues.

Good job getting your airplane built and in the air.

Brian T
:
I'll follow up on this later, but heres some general info:
Plane: freedom flight 2019 plane that weighs 8.07g with my propeller. CG of the plane with rubber attached is 6 5/16" from the nose. Because the propeller was so heavy and I had no room for ballast, I moved the wing as close to the nose of the plane as possible. I have a feeling that my nose is way too heavy and that's why the plane won't fly well. Stab is 15 3/4" from the nose of the plane simply because that's about the farthest you can put it due to the design of the plane.
Propeller: flaring ikara prop (2.36g) that has had the blades trimmed so that the portion in front of the spar is gone. Pitch has not been altered at all (23-25 degrees, measuring pitch is never consistent for me). Rubber motor is 3/32" TSS that is18" long (2.07g). I've been winding to 1600 and backing off to 1400. Launch torque is around .35 in-oz. My plane climbs really high for some reason so I usually have to dewind a lot in order to stop it from repeatedly smacking the metal lattice supports top of the gym. The plane lands with maybe 100-200 winds left. LE of wing is 1/8" higher than TE, LE of the stab is 1/16" lower than TE.
Flight characteristics: Circle of flight is around 25ft diameter. Climbs really fast and reaches 20 ft in maybe 2 circles (takes 25 seconds for 2 circles), stays for a while on the top (40 seconds for 4 circles) and eventually descends at like 7 feet per circle (25 seconds for 3 circles).
Extra info that might be relevant: Past planes that I've made glide pretty well while deadsticking, this plane on the other hand just nosedives at a 40-degree angle and barely glides at all when deadsticking.
I'm pretty sure majority of the flight time is supposed to come from the descent of the plane, so I am puzzled as to why my descent is just as fast as my climbing.

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Re: Plane descending too fast

Post by coachchuckaahs » January 21st, 2019, 12:01 pm

BitConnect:

It is crucial to get the CG per the plans, even if it means a bit more weight. The tail is long, so it does not take a ton of clay at the tail to get the CG moved back. The wrong CG will cost you more than weight. It is quite possible that letdown performance is impacted by your forward CG.

Your turns remaining appear reasonable. However, having to unwind "a lot" may be indicative of too strong rubber, or more likely in your case, too little pitch. What is "a lot"? If similar to your remaining winds at the end of flight, might be about right. I assume you are measuring pitch with the FF pitch gauge (if I recall, it measures at 2.5")? I also assume that you have limited choices of rubber (stock dimensions). I would try to increase the pitch of the prop. You may even see more aggressive climb initially, and further increase tame it down a bit.

If you are deadsticking, you have too thick rubber or too little pitch as well. You want to be under power all the way to the ground.

So, move your CG back, then play with pitch. Clean up your pitch gauge until you get accurate repeatable results. See other posts about changing pitch, consider slightly heating for longevity.

At a recent session with my club, World Champion Brett Sanborn came over and suggested more pitch to my kids. I was thinking less, he said "a lot more". We tried it his way and our performance instantly improved! Sometimes things are not intuitive. Matching prop to rubber is crucial, and if you have limited rubber choices, pitch is what you need to master.

Coach Chuck
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Nationals Results:
2016 C WS 8th place
2018 B WS 2nd place
2018 C Heli Champion
2019 B ELG 3rd place
2019 C WS Champion
AMA Results: 3 AAHS members qualify for US Jr Team in F1D, 4 new youth senior records

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Re: Plane descending too fast

Post by bjt4888 » January 21st, 2019, 12:49 pm

Bitconnect,

Great job supplying the details. I agree with Coach Chuck that the pitch of the propeller is the first culprit.

I have a FF pitch gauge and I checked the radius that measures just now to be just short of 2.25”. This means that at 25 degrees, your propeller pitch is 6.6 inches (tangent of 25 degrees times circumference of the circle with a 2.25” radius). As the propeller pitch that my teams are using is private team information, I cannot mention, but you will want to test significantly increased pitch.

Low pitch on the propeller and rubber you are using results in increased horsepower. This is what is causing the rapid climb. Low pitch is also inefficient enough to explain the short “let down” phase of flight.

As we are using the FF kit with modifications, I could double check the instructions and I can see that you CG is close enough to plan specs to not be a problem. The plan r ecommends 2.5” behind the wing TE. So, if your nose is 1”, you CG is actually 1/16” further back than the plan recommends; not a problem.

Also, I believe that you are underwinding your motor. 18” of 3/32” of every vintage of tan super sport that I have tested except the current year (5/2018) will take about 2,300 turns before breaking (after break in; not on the first wind). Test this by winding a motor till it breaks (use adequate lubrication and standard stretch winding techniques). A safe winding would be about 80% of this, or about 1,850 turns. For a 20 ft ceiling, you will still need to back off to .35 in oz, or so, for the first flight with this winding and with more prop pitch. This might mean a backoff of 250 turns or so. Gradually reduce backoff on subsequent flights till you are just below the ceiling during cruise.

Also, your decalage angle is greater than recommended in the FF instructions. Every 1/32” difference in this angle will significantly change how the airplane flies. The stab should have a slight left panel washin and the stab in total should have 1/32” positive incidence (LE higher than TE).

Good luck and good flying.

Brian T

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