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Last year I picked up two Hubsan parachutes (H109S 27) for really cheap and like a fool assumed that the 27 in the model number was the diameter in inches of the parachutes. I put together a spreadsheet of parachute size vs drop velocity vs impact force and I ran into an interesting issue.
Many parachute manufacturers calculate their vertical drop velocity using a Drag Coefficient of 1.75 (unitless), but that is for a truly parabolic chute. This model is actually a parasheet, a single piece of roundish cloth with a much lower drag coefficient of 0.75. So given that assumption, I quickly concluded that 27" was inadequate for the solo (4 lbs = 1.8 kg with gimbal). That would have resulted in a drop speed of 34 fps (= 23 mph = 10 m/sec) and an impact force of about 85 Joules (think throwing your solo at someone across the room at nearly full force...)
The "maximum recommended impact force" of 69 Joules is from the French Aviation Authority or DGAC (cited on the Mars Parachute website) for which the max drop speed would be 29 fps (= 9 m/s = 20 mph). That is still pretty darn fast. I would want a much slower drop velocity.
That calculation was correct but another assumption was wrong...
Investigating a little further -- not wanting to give up so easily -- I actually opened the chute and measured it. That 27" must have been the radius, if anything at all meaningful. It was 54" across! So I quickly cursed my stupidity and recalculated the impact force and drop speed. Initially I used 54" as the diameter but subsequently found that Hubsan claims it only has 49.2 inches of diameter (buried deeply on its accessory web page).
Therefore, given that the parachute tube (including self-contained battery and actuator) is about 0.4 lbs, and a total weight of 4.4 lbs (2 kg), the Hubsan H109s 27 parachute is capable of either:
1. (drag coeff. = 1.75): Drop speed = 12.7 fps = 3.9 m/s = 8.6 mph
for an impact force of about 13 Joules. This matches the manufacturers literature values (adjusted for the solo) but not my skeptical mind.
2. (drag coeff. = 0.75): Drop speed = 19.4 fps = 5.9 m/s = 13.2 mph
for an impact force of about 30 Joules. This is likely more realistic and I'd like to test it someday without using the solo.
I guess I can blame this partially on very poor documentation (no specs given for the parachute cloth itself) but mostly on my laziness to open the thing and measure it. the good news is that one of these parachutes is plenty for the weight of the solo and the gimbal, at least for the recommendations I have seen.
If anyone is interested in my spreadsheet, I'll have to send you a copy.
btw I still have not used these...
Many parachute manufacturers calculate their vertical drop velocity using a Drag Coefficient of 1.75 (unitless), but that is for a truly parabolic chute. This model is actually a parasheet, a single piece of roundish cloth with a much lower drag coefficient of 0.75. So given that assumption, I quickly concluded that 27" was inadequate for the solo (4 lbs = 1.8 kg with gimbal). That would have resulted in a drop speed of 34 fps (= 23 mph = 10 m/sec) and an impact force of about 85 Joules (think throwing your solo at someone across the room at nearly full force...)
The "maximum recommended impact force" of 69 Joules is from the French Aviation Authority or DGAC (cited on the Mars Parachute website) for which the max drop speed would be 29 fps (= 9 m/s = 20 mph). That is still pretty darn fast. I would want a much slower drop velocity.
That calculation was correct but another assumption was wrong...
Investigating a little further -- not wanting to give up so easily -- I actually opened the chute and measured it. That 27" must have been the radius, if anything at all meaningful. It was 54" across! So I quickly cursed my stupidity and recalculated the impact force and drop speed. Initially I used 54" as the diameter but subsequently found that Hubsan claims it only has 49.2 inches of diameter (buried deeply on its accessory web page).
Therefore, given that the parachute tube (including self-contained battery and actuator) is about 0.4 lbs, and a total weight of 4.4 lbs (2 kg), the Hubsan H109s 27 parachute is capable of either:
1. (drag coeff. = 1.75): Drop speed = 12.7 fps = 3.9 m/s = 8.6 mph
for an impact force of about 13 Joules. This matches the manufacturers literature values (adjusted for the solo) but not my skeptical mind.
2. (drag coeff. = 0.75): Drop speed = 19.4 fps = 5.9 m/s = 13.2 mph
for an impact force of about 30 Joules. This is likely more realistic and I'd like to test it someday without using the solo.
I guess I can blame this partially on very poor documentation (no specs given for the parachute cloth itself) but mostly on my laziness to open the thing and measure it. the good news is that one of these parachutes is plenty for the weight of the solo and the gimbal, at least for the recommendations I have seen.
If anyone is interested in my spreadsheet, I'll have to send you a copy.
btw I still have not used these...
