View Full Version : Why is there such a big difference in design philosophy of a prop vs. jet aircraft re
For all you Big Jet Drivers out there, at the moment a very topical subject is tail plane de-icing. Here is a question :
Why is there such a big difference in design philosophy of a prop vs. jet aircraft regarding tail plane de-icing/anti-icing ?
Most jets have no protection at all, on the tail plane.
pineridge
15th Feb 2009, 09:55
c46r said..........
"Most jets have no protection at all."
WTF?
AltFlaps
15th Feb 2009, 15:34
The 737 Classics (300/400/500) do not have any tail plane de-icing capability !
When I converted to the 737, I was very surprised by this!
I have flown several smaller jet types and turbo props (including the Q400) - and all had either hot leading edges or boots.
But I think the fact that most medium (and above) jets have powered flying controls is a big factor ...
The fact that the jets spend most of their time above the weather while the props job are down in it is probably also factored in.
It is simply that the larger swept-wing jet transports have proven, in icing certification trials, that anti-icing for the tail surfaces was not needed.
The 707 had tail anti-icing, and believe it or not, some operators disconnected this feature, with the complete approval of regulatory authorities.
Lets see....I can offhand remember only about half a dozen times I have switched on airframe anti-ice in a large jet.
Engine anti-ice?
Yes, absolutely.
Airframe...not much.
Bullethead
15th Feb 2009, 16:13
The 707 had tail anti-icing,
The B707-338s I flew didn't have it yet the C130Hs I flew had wing, tailplane, rudder and engine thermal anti-ice. Hot bleed air, and turning it all on was like losing an engine it used so much bleed air. :eek:
The significant design difference between most turbo prop aircraft and most, if not all, swept wing jet aircraft is that the the turbo props have a fixed tailplane and an elevator whereas the jets have a movable tailplane and an elevator.
Because the jets have this feature where the whole tailplane is moved when trimming in pitch the problem of high incidence on the tailplane is avoided and so therefore the problem of tailplane stalling. Ice will still build up on the tailplane but because the whole tailplane moves when trimming the aircraft you still have adequate pitch control and so no requirement for tailplane anti-icing.
On the B707, full elevator deflection was equivalent to about four units of pitch trim and there were more than twelve units of pitch trim available. A very powerful control.
Regards,
BH.
BH,
A very good and valid point. thanks.
Mad (Flt) Scientist
15th Feb 2009, 18:18
Bit of misconceptions creeping in.
All-moving tailplanes actually create a larger angle of attack at the tailplane than a fixed tailplane. So they can be actually MORE at risk of stalling than a fixed tail of similar size and location.
[Say the aircraft angle of attack is 5 degrees and the downwash is 10 degrees (a lot of flaps deployed!). Then the flow at the tail is 5 degrees downwards, and with a tail fixed at zero degrees on the aircraft the angle of attack at the tail would be -5 degrees. That generates a certain amount of down force and hence nose-up moment for the aircraft. Say I need more to trim. On a fixed stab aircraft I move the elevator, but the tail is fixed so the AoA at the tail doesn't change. On a moving tail I move the tail nose-down - which directly increases the AoA at the tail.]
The tail on an all-moving tail aircraft usually isnt deiced because:
1. Usually you have powered elevators which makes the problem of tail stall less severe and easier to design against (no control 'snatch')
2. Its hard to get anti-ice bleed to the tail because it's moving around
3. The aircraft is certified assuming ice on the tail - basically, the tail is made big enough to avoid the tail stall problem.
Exposure isn't a factor - if you are approved for flight in icing that includes holding, so the ability to "climb through" the ice isn't a factor in the design.
3. The aircraft is certified assuming ice on the tail - basically, the tail is made big enough to avoid the tail stall problem.
Spot on.
The powered stabilizer is a very powerful tool, make no mistake.
The L1011 is a perfect example.
The entire stab moves, for pitch demands.
A classic design that works very well, indeed.:ok:
bcgallacher
16th Feb 2009, 12:35
The 747 has only wing l/edge deicing none on vertical or horizontal stab - same with Tristar.The old Viscount was a turboprop that was susceptible to stab icing - more than one hull loss if I remember even with stab deicing[hot air].Never had an explanation for the different philosophies but I think the replies given are about right