3 - Jet and Propeller Aircraft Differences

Question 1

Describe the approach differences between a jet and a piston engine
propeller aircraft.

Answer 1

There are six main handling differences between a jet aircraft and a propeller
aircraft on the landing approach.

1. Momentum.
2. Speed stability.
3. Wing lift values.
4. Engine response rate I acceleration and deceleration
5. Slipstream effects.
6. Power-on stall speed.

In all the differences, the jet aircraft
is worse off than the propeller aircraft in maintaining the approach
profile and in correcting errors on the approach.

The six main differences involve aerodynamics and the engines. The
aerodynamic differences include…

1. Momentum - The jet aircraft has a greater momentum than the lighter
   and slower propeller-driven aircraft.

Therefore, the responses of a jet
aircraft to changes in flight path are much slower, and sudden
changes are virtually impossible.

2. Speed stability - The jet aircraft suffers from poor speed stability
   being an inducement to a low-speed condition.
3. Wing lift values. The jet aircraft’s swept wing produces less lift than
   the propeller aircraft’s straight wing.

The swept wing experiences a
faster increasing drag penalty than lift, resulting in a high sink rate at low speeds.

The engine differences include

4. Engine response rate / acceleration and deceleration.
   The jet engine has a poorer acceleration response at low-rpm speeds known as lag.

The propeller drag produces a decelerating braking action (which can be very useful on the approach) but is absent
on the jet aircraft.

5. Slipstream effects. The propeller slipstream produces an immediate
   extra lift value over the wings at a constant airspeed but is absent
   on jet aircraft.
6. Power-on stall speed. The stall speed is significantly lower when the engine power is increased or on for a propeller aircraft because it generates a slipstream with an increased airflow speed over the
   wing that increases the lift produced and thus reduces the aircraft’s
   effective weight, also reducing the stall speed.

For a jet aircraft,
which does not produce a slipstream over the wing, the stall speed
is virtually unchanged with its power on.

Question 2

Explain the (low) speed control difference between a jet aircraft and a
propeller-driven aircraft.

Answer 2

The speed control for both jet and propeller-driven aircraft is a function
of drag against speed.

The drag experienced by both aircraft has the same dynamic qualities in that to balance drag against a speed, the
thrust/power must be set to a corresponding required value.

However, the value of drag against a speed is different between jet and propeller driven
aircraft, and it is this difference that makes their speed control properties re-markedly different.

(See Q: Describe the drag curve on a
piston-enginedljet aircraft, pages 7 and 8.)

The jet aircraft’s relatively flat drag curve over the low speed range
makes it difficult to select the correct thrust for the required speed
because of the minimal difference in thrust required values across a
large speed range. Therefore, it is easy to select an incorrect speed
control thrust setting, and any slight difference, whether higher or
lower, will set up a speed divergence that is also difficult to detect.

A speed divergence due to poor speed control on jet aircraft can result
in the aircraft’s speed slipping into an unstable region where the aircraft
suffers from poor speed stability.

(See Q: What speed stability
differences are there between ajet aircraft and a propeller-driven aircraft?
page 87.)

In contrast, the propeller-driven aircraft’s drag curve is markedly
different across its low-speed range, and therefore, it is much easier
to determine and set the correct power to balance the drag and to
deliver the desired speed.

Also, incorrect settings give rise to noticeable handling characteristics and more marked speed divergences
that are very noticeable to the pilot, in stark contrast to the jet aircraft’s
qualities.

Therefore, the speed control of the propeller-driven aircraft is easier to manage.

Question 3

What speed stability differences are there between a jet aircraft and a
propeller-driven aircraft?

Answer 3

The jet aircraft’s speed stability is much poorer than that of a propeller driven
aircraft because of the following two main differences:

1. The jet’s recommended threshold speed (1.4xVs) tends to be in
   the neutral or unstable speed range.
2. Thrust changes with speed, which helps to improve the speed stability on propeller-driven aircraft and does not improve it on jet aircraft. (See Q: Explain speed stability, page 22.)

Question 4

Describe the difference between a jet and a piston-engined propeller
aircraft’s stall speed.

Answer 4

The stall speed of a piston-engined propeller aircraft generally is a
slower speed than that of a jet aircraft.

Also, the range of the stall
speeds is much larger for a jet aircraft than for a piston-engined
propeller aircraft.

Question 5

Describe the differences between propeller and jet aircraft wing
performance.

Answer 5

The performance margin of a propeller-driven aircraft’s straight wing is
greater than that of a jet aircraft’s swept wing, especially when contaminated.

The reason for the improved relative performance on a propeller driven
aircraft is that the wing is straigh and there is a high-energy airflow (prop wash) over the upper surface of the wing that partly offsets some of the contaminant effects.

On a jet, the available performance margin with a contaminated wing is practically zero for two main reasons.

First, there is no upper-surface
high-energy flow available.

Second, the wing sweep decreases
the amount of lift generated for a given angle of attack relative to the straight wing so that higher angles of attack are required from the swept wings, which in turn results in greater performance sensitivity to wing contamination.