Speed stability differences between a jet aircraft and a propeller-driven aircraft

[akafrank07]

Hi folks in the book i'm reading it states;

The jet aircrafts speed stability is much poorer than that of a propeller driven aircraft because of the following reason;

Thrust changes with speed, which helps to improve the speed stability on propeller-driven aircraft and does not improve it on jet aircraft

Basically i don't understand it, and would appreciate it if somebody could elaborate :)

[paco]

Very briefly, 'cos I'm in a rush:

The speed range below L/DMAX in a jet is the speed unstable regime. That above it is the speed stable regime. In the speed stable regime, power is added to increase speed, as normal. In the speed unstable regime, however, power has to be added to maintain a slower speed, and there is less of a tendency to return to trim speed anyway, so control movements must be more precise.

It's not all you need but it's a start.....

[akafrank07]

Thanks for your help paco i just noticed the reply

'In the speed unstable regime, however, power has to be added to maintain a slower speed and there is less of a tendency to return to trim speed anyway, so control movements must be more precise.'

In that quote does that mean that the power was added to maintain a slower speed from when it was initially disturbed? if that's so, it makes scene now
cheers

[paco]

Yes it does!

Phil

[akafrank07]

Thanks for the help Phil, and note i am trying read all background before asking questions

[paco]

We don't mind! That's what schools are for!

Phil

[BobM2]

What has been stated above is true for both jet & prop airplanes at low speed where decreasing airspeed creates a greater drag rise than lift increase. It's known as the "back side of the power curve".

The difference in speed stability between jets & props is the effect of prop wash which varies the lift of the wing portion exposed to it, but most importantly varies the downward force created by the horizontal stabilizer.
This means that adding power tends to pitch the nose up, while decreasing power pitches nose down. In a well designed prop airplane the airspeed will remain very close to the trim speed with varying power settings mainly affecting vertical speed. On jets with engines mounted on/in the fuselage, power changes mainly affect airspeed with little pitch change.

The exceptions to this are low-wing jet aircraft with under-wing mounted engines like most large transports today. The low thrust line (below the cg) creates a pitching moment with thrust changes very similar to prop aircraft.

Pusher props and/or canards are different.

[keith williams]

The jet aircrafts speed stability is much poorer than that of a propeller driven aircraft because of the following reason;

Thrust changes with speed, which helps to improve the speed stability on propeller-driven aircraft and does not improve it on jet aircraft


To understand the above comments in the original post we need to look at the thrust curves for jets and props.

For conventional turbojets (with no bypass or low bypass) the thrust is a maximum when airspeed is zero, then reduces as speed increases from zero up to about 250 knots. Thrust then increases again due to ram effect as speed continues to increase. If you think about it this thrust curve looks pretty much like a standard drag curve (allbeit with less pronounced slopes).

This means that throughout much of the speed range, as aircraft speed changes the thrust changes in the same sense as the drag. So if Vmd is in the region of 250 knots then below Vmd if speed increases, the decreasing drag is matched to some extent by the reducing thrust. And if speed increases above Vmd the increasing drag is matched to some extent by the increasing thrust. The overall effect is a reduction in speed stability.


For a propeller aircraft the thrust is maximum when airspeed is zero, then thrust gradually decreases with increasing airspeed. The airspeed never gets high enough for ram effect to be a major factor.

This means that as airspeed increases above Vmd, the drag increases while the thrust decreases. This makes the aircraft more speed stable above Vmd.

[akafrank07]

Thanks very much for your help Keith, very well explained indeed :ok: