1
$\begingroup$

Here are the background facts:

This excercise has the objective to familiarise the student with the slow flight characteristics.

The steps are:

1)Establish straight and level, 90% RPM, 15 units AoA, hands out trimmed. (No specific airspeed, it just happens to be appr. 110 KIAS)

2) Add 5% RPM (so i set it at 95%) and establish a climb with 15 units AoA for 500 ft climb. No bank, whatsoever. (Again no specific airspeed, it just happens to be appr. 100-105 KIAS)

3) Reduce 5% RPM so you fly straight and level at the new altitude (Same parameters as step 1)

4) Reduce 5 % RPM (so i set it at 85%) and establish a dive with 15 units AoA for return to the initial altitude. No bank whatsoever. (No specific aispeed during the dive, it just happens to be appr. 115-120 KIAS).

My question is simple. Why is the airspeed different? The answer might be simple. I simply can't figure it out. Hope i clarified it as much as possible.

$\endgroup$
  • $\begingroup$ What kind of airplane is it? And are the speeds you observe hands off trim speeds having not touched the trim in each case? On a tractor engine airplane, thrust and slipstream effects have an impact on both AOA required to support a given weight, and trim speed, going down when you add power and vice versa. $\endgroup$ – John K Apr 20 '19 at 13:10
  • $\begingroup$ Military jet trainer plane. Trimmed for 15 units straight and level landing configuration, approximately 110 kias. Then when i change the attitude i just maintain those 15 units. It is a steady climb/dive with 15 units. I dont think i had to do anything with p factor slipstream etc. $\endgroup$ – viper Apr 20 '19 at 15:57
3
$\begingroup$

You should see the same airspeed for the same AOA and non-accelerated flight with a mild climb or descent.

Except with a Buckeye?

The Buckeye has its thrustline low enough so that at different power settings you have different pitch trim settings. Add power to climb and the thrust pitches the nose up. You now have to trim nose down. Nose down trim means less down force on the tail, which means less lift required by the wing, which means a slower speed for the same AOA.

Conversely, pull power off and the nose wants to drop, leading to nose up trim, more down force on the tail, more lift required by the wing, higher IAS for the same AOA.

I frankly never noticed this effect, and never taught it. Wish I could go back and check it out!

$\endgroup$
  • $\begingroup$ i edited the original post trying to clear things up. Thank you for your help. $\endgroup$ – viper Apr 21 '19 at 13:48
  • $\begingroup$ Ok, are you actually in a real airplane, or a simulator? I’ve got a 1000 hours in Buckeyes, and you should see the same IAS for your cases. $\endgroup$ – MikeY Apr 21 '19 at 13:50
  • $\begingroup$ Actual flight. Normally i execute the Slow Floght exercise at FL140, sometimes FL130. The numbers are a fact. I was prompted by the instructor to see the speed when climbing and diving. And explain to him why this happens. $\endgroup$ – viper Apr 21 '19 at 13:52
  • $\begingroup$ Well then my swag would be that the Buckeye has a thrustline below the CG, so for the same speed but different thrust you have a different trim condition. Adding power leads to nosedown trim, and pulling it does the opposite. This would be an effect particular to the aircraft, not a general property. $\endgroup$ – MikeY Apr 21 '19 at 13:57
  • $\begingroup$ You are talking about a Buckeye, correct? Saw your post on another forum. $\endgroup$ – MikeY Apr 21 '19 at 13:58
-1
$\begingroup$

Consider three airplanes: a normal airplane, one that is heavier than normal and one that is lighter than normal. All of them are travelling at 15 units AoA. You will agree that the lighter one will be the slowest (it needs to go very slow as not to have a smaller AoA) and the heaviest one will be the fastest (it needs to be very fast in order not to have a greater AoA).

In the above example we vary the weight of the airplane, but ultimately the change is due to wing loading. In your example, you do not vary the weight, but the tailplane force.

For example, if increased thrust causes a pitch-up moment on your particular airframe, the tail will have increased lift, reducing the wing loading. To maintain a high AoA, the speed will have to be reduced.

(Edit: Initially, I blamed the effect on the thrust vector, which takes part of the weight in a climb. However, @MikeY correctly identified that the drag vector does the same in a descent so that does not explain the higher speed in a dive)

$\endgroup$
  • $\begingroup$ I agree that in a steep climb the thrust would carry more weight and so IAS could be lower for the same AOA, but that property should also hold for a dive, where the drag is now carrying some of the weight. The extreme is an non-accelerating almost vertical climb or dive. For both conditions, you'd have to be at a very low airspeed to see 15 units AOA. $\endgroup$ – MikeY Apr 22 '19 at 13:09
  • $\begingroup$ @MikeY You make a good point. I suppose the main factor must then be the tailplane lift. Thanks for the correction. $\endgroup$ – Sanchises Apr 22 '19 at 18:25
  • $\begingroup$ Little bit of a "eureka" moment realizing the stabilized speeds for climb and dive in this exercise matches the effects of spoilers vs flaps discussed in another question, except the downforce difference is on the tail. Here the trim position of elevator gives more down force in the dive (just like spoilers, makes plane "heavier") resulting in faster speed. This is because of the reduction of the power from 95 to 85% (the thrust providing nose up force from "underslung" engine) requires more tail down force trim to maintain same AOA. Climbing, with more power, tail down trim is less. $\endgroup$ – Robert DiGiovanni Apr 22 '19 at 23:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.