The sweepback angle of the horizontal stabilizer is greater than that of the wing. I assume this is to reduce compressibility effects, but it would only make sense if the tailplane sees a higher airspeed than the wing. ¿Is this the case?
$\begingroup$
$\endgroup$
2
-
1$\begingroup$ This doesn't make sense, the tailplane and wing are attached to the same structure moving through the air at the same speed, so the airspeed of both has to be the same. $\endgroup$– GdDAug 28, 2021 at 18:29
-
1$\begingroup$ @GdD The flow is obviously perturbed by the wing and the effective velocity that the tail sees is not the same as the wing. This is implicit in the question. $\endgroup$– stevederekson555Aug 28, 2021 at 18:32
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
3
"The sweepback angle of the horizontal stabilizer is greater than that of the wing." Usually it has to be about 5° higher as the the Wing so the critical Machnumber is higher for the Empennage. You dont wanna lose efficiency due to shock waves on the tails (tails have always to work). There are also rules for the thickness and other parameters of the Empennage. You can find some more of them here.
In basic preliminary design the seen speed of the empennage depends on the position of the empennage, but is usually smaller than for the wing, because of the boundary layer(wake). For more detailed design speeds of the jet flow behind a propeller/engine or other cases can be considered too. So not in all cases the airspeed is lower on the empennage and depends on what you are trying to analyze. But the tails should always be able to stabilize the aircraft.
-
1$\begingroup$ Thanks. So essentially you're saying that the increased sweepback angle on the horizontal tailplane is due to a lower tolerance for compressibility effects on the empennage due to it's function and not due to a difference in effective velocity. I understood that correctly? $\endgroup$ Aug 28, 2021 at 21:06
-
$\begingroup$ Yes, that is correct. But as always there are also "exceptions" e.g. the horizontal tail of the F18. For the YF17(prototype of the F18) the horizontal tail sweep was also initially higher than the sweep angle of the wing. The Horizontal Tail sweep of the protoype was increased further after realising the control authority was not high enough for starts on aircraft carriers. By increasing the sweep the center of pressure is moving backwards increasing the leverarm without other major changes of the configuration. $\endgroup$– ArturAug 28, 2021 at 23:29
-
1$\begingroup$ @steve, the more important factor is that a higher sweep tail is more tolerant to the angle of attack changes: its critical AoA will be higher than for the wing. It is not uncommon to see a swept tail with a straight wing for very subsonic aircraft (e.g. turboprops). $\endgroup$– ZeusAug 30, 2021 at 1:07