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1. 03:29 PM - Re: Landing the KIS TR-1 (BlueSkyFlier)
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| Subject: | Re: Landing the KIS TR-1 |
Since before buying the KIS TR-1 late last year I have been researching the possible
reasons for the nose dropping tendency which the TR-1 is sometimes alleged
to display on occasion. During this exercise I learned quite a bit about laminar
flow wings and ground effect, the sum of which led to me to conclude in
brief that:
1) NACA six digit laminar flow wing (even with straightened rear profile) definitely
prefers to be operated within its design range which for the KIS is nominally
where 0 CL 0.4
2) Angles of attack which pushes CL > 0.8 for the 63(2)A215 profile profile rapidly
increase the negative moment (i.e. the forward pitching moment) [where A
in the profile code accounts for the flat surfaced flap and ailerons at the rear
of the wing].
3) The flat surfaces on the flaps and ailerons could apparently be the reason (inter
alia) why the TR-1 tends to mush gently in the stall instead of popping
down the nose.
4) When entering ground effect whilst maintaining a fairly conservative angle
of attack, the vlaue of CL will increase automatically by between 5% to 8% [refer
attached diagram]. Viewed on its own this would seem to be a beneficial effect.
However, at the same time, the critical angle of attack (i.e. the stall
angle where boundary layer separation occurs) and associated CLmax decrease due
to changes in pressure distribution. It follows that, if ground effect is entered
with an angle of attack which is too close to CLmax , the combination of
resultant increase in CL , decrease in CLmax and increasing negative moment
may tip the balance over the edge and conspire to upset the stable state of affairs.
5) Furthermore, the changed angle of airflow across the low tail section could
also reduce the elevator authority required to offset the increased negative moment
and/or counteract the imbalance which may arise.
Over the weekend I finally came across two references below which seem to pull
all these fragments together into a whole that makes sense.
(i) Synthesis of subsonic airplane design by Egbert Torenbeek, page 552 in particular.
(ii) Analysis of the aerodynamic chracteristics of devices for increasing wing
lift Part III Influence of ground proximity on the aerodynamic characteristics
of the flaps; by Rafael Garncarek translated from the original for NACAs library.
(Small excerpts from the relevant pages of these two reference are attached with
due acknowledgement to the respective authors.)
So what does this all mean? To me at least, it indicates that the TR-1 (which is
but one example of such laminar flow low wing aircraft) would provide a good
margin of security during landing if the main gear is touched down at circa 55
to 65 kts with a fairly flat aircraft attitude and with only one stage of flaps
- providing runway and other factors allow. Attempts to hold off at high angle
of attack with full flaps close to the ground may well invite the unwelcome
effects mentioned earlier, all of which can be avoided by getting the gear
down before bleeding off the residual speed by carefully keeping the nose up while
she runs out. Setting RPM in descent/glide to the zero prop drag value will
also help. Such RPM setting can simply be calculated from the propeller pitch
and intended touchdown speed and does not need to be very precise at all. Somewhere
between 1300 and 1450 would work well for most propellers and of course
this has to be reduced gently to idle as main gear settles down.
NOTE: I know that the matter of appropriate landing technique may be an emotive
subject to some readers. So let me make it clear that I am merely sharing this
information and what little understanding was (maybe erroneously so) gleaned
from it for general interest. This does not constitute a recommendation in any
way shape or form and readers are urged to draw their own conclusions from the
original material and relevant further references therein if so inclined.
Happy landings,
Alfred
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http://forums.matronics.com/viewtopic.php?p=336224#336224
Attachments:
http://forums.matronics.com//files/aerodynamic_effects_in_ground_effect_145.jpg
http://forums.matronics.com//files/ground_effect_on_lift_coefficient_155.jpg
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