Usually after “short” vowels in words of more than one syllable.
appal
appeal
appear
appease
append
applause
apply
appoint
appraise
apprise
approach
approve
bopping
capping
chipper
chipping
chopper
chopping
clapper
clapping
clipper
clipping
clopped
clopping
copper
copping
cropper
cuppa
cupping
dapper
dappling
dipper
dipping
dripping
dropper
dropping
equipped
flapper
flapping
flippant
flipper
flipping
flopping
foppish
grappa
gripper
gripping
happen
hipper
hippest
hippo
hopper
hopping
kipper
lopping
mapping
moppet
mopping
napper
napping
nipper
nipping
oppose
oppress
pepper
pipped
pippin
pipping
poppa
popper
poppet
popping
propping
puppet
quipping
rappel
rapper
rapping
ripper
ripping
rippling
sapper
sapping
scupper
shipping
shopper
shopping
sipper
sipping
skipper
skipping
slapper
slapping
slipper
slipping
slopping
snapper
snapping
snippet
snipping
sopping
stepper
stepping
stopper
stopping
strapper
stripper
supper
supplant
supply
support
suppose
suppress
swapped
swapping
tapping
tippet
tipping
topper
topping
toppling
trapper
trapping
tripping
uncapped
untapped
unzipped
upper
upping
whippet
whipping
whopper
whopping
wrapper
wrapping
yapping
yipped
yuppies
zapper
zapping
zipper
zipping