Helional is described by the manufacturer IFF as:
It also goes by a number of other names including ocean propanal, tropional, florial and floramelon.Green, floral (cyclemen) with top notes of ozone and new mown hay
The TGSC description is:
[note that these descriptors are presented in order of prominence in the odour]watery, fresh, green, ozone, cyclamen, hay
I find it a deceptive material that is rather underwhelming on first sniff but has a distinctive smooth, fresh, floralising effect in a fragrance blend. It is restricted by IFRA to 5.3% of the final product when used in alcoholic fragrances, but that still allows for quite high usage and is one of the few green notes that can be used relatively freely without overwhelming and dominating everything else (compare with Cucumber Aldehyde, Galbanum, Violet Leaf Absolute or Stemone for example).
In pure form it needs careful storage to avoid deterioration - on an industrial scale it is stored under nitrogen.