Why a reduction in Vmo?

[Mike Huntaches]

Have read various topics related to this matter but none answer my ponderings. Here goes.
Why is it on (some?) turboprop a/craft once you hit a certain level (ie flight level) that the Vmo starts to decrease as you approach the maximum operating level?
Have seen for example 230ias to 22000 then reduces to 215 ias by 28000 etc etc.?

thanks for any replies.:confused:

[Dan Winterland]

Not certain, but I suspect that it may be to prevent aileron flutter. Prevalent in aircraft with manual rather than powered controls at high TASs, reducing VMO should keep you out of the flutter regieme.

Having experienced aileron flutter once, I think it's a jolly good idea! :ok:

[john_tullamarine]

Alternatively, if you have a closer look at the AFM, or you might have to resort to the TCDS, you might find that it is Mmo related ?

What aircraft, specifically, and we might be able to throw some light on the matter ......

[411A]

Not just related to turbopropeller aircraft.

Was the same on the DC-6, DC-7, and Lockheed 1649A.
The latter two had Mach number limitations as well as CAS.

In the case of the DC6, was indeed related to aileron/elevator flutter at very high speeds. In the case of the elevator, an additionl hinge was added at the outboard end on the -6B models, and eventually added to the straight six as well.

When the DC6 was designed, it used the same wing as the DC4 (with added washout at the tip), but with much larger engines (ie: 60knots faster), so in descent with cruise power selected, flutter bacame a problem. Likewise with the DC7, the DC6 wing was used (slightly modified again), but with much larger engines (and 60knots faster still), compressability effects were noted.

A lot was learned in the late 40's/early 50's about trans-sonic airflow.

Of course, the wing on turbopropeller aircraft can be designed for very high speed flight (with big enough engines) but then there becomes a problem with landing speeds, and the rather complicated flaps to keep these within reason.

Turbojet aircraft as well have decreasing Vmo as altitude is increased...and as indicated mach number increases.

As altitude increases, TAS increases for a constant IAS/CAS.
At higher altitudes, trans-sonic flow can become a problem, so IAS is limited, to allow for this condition.

[onno]

I being new here, it is a very good forum. This is in the aircraft i will be flying fokker 50. In this one it is 224 upto fl200 and then by fl250 it is becomes 206 knots. Is this the same problem to which you refers?
I am sorry for my untidy commands of english at this time. thankyou.

[Mike Huntaches]

Thanks for the responses people. I am only able to provide a 'real' example of the 1900D courtesy of a friend who flies it. The manual states "Vmo is 245 from Sea level to 13500 feet
and decreases to 195 from 13500 feet to 25000". I asume as a similar turboprop (only a bit bigger) that the F50 in the post by omno has the restriction for possibly similar reasons? Have heard of the flutter aspect before but would it be the case for these examples?
Someone must have an idea?
:confused:

[Crash & Burn]

G'day,

In turbo props the decreasing limit with altitude is usually caused by the speed of the prop tips nearing the speed of sound. As the prop tip speed gets close to the speed of sound the prop becomes less efficient.

One thing to remember:
The speed of sound decreses as temperature decreases.

Let's say we are doing 224 knots IAS at Sea Level on an ISA day - this equates to approx. Mach 0.339

Let's then say we are doing 224 knots IAS at 20,000' on an ISA day (-25 deg C) - this equates to approx. Mach 0.492

The Fokker 50 manual doesn't give a specific maximum Mmo limit (Well mine doesn't!) but does say that you can maintain 224 knots up to approx 20,000 feet. This would tend to indicate that the Mmo limit is around mach 0.492 on the Fokker 50.

If we were to then do 224 knots IAS at 25,000' on an ISA day (-35 deg C) this would equate to Mach 0.544

As you can see this now exceeds the MMo limit. Consequently, to stay under the Mmo limit we need to fly at a slower speed.

The 'barbers pole' indicates the most limiting figure of Vmo or MMo - in the Fokker 50 this would be 224 knots up until approx. 20,000 feet and then the barbers pole starts moving back on the airspeed indicator until it gets back to approx 206 knots at 25,000' (Max ALT for the F50)

One thing to note on the Fokker 50 is that if you turn the Prop electronic control system off then the prop RPM will increase from 85% (in cruise) to 104%. This increases the tip speed of the props significantly and puts them closer to the speed of sound - to compensate for this there is a table of reduced speeds in the QRH to fly if this happens.

As a matter of interest the Max IAS at 20,000 feet goes from 224 knots with the prop control working down to 161 knots if it fails - quite a difference!

[411A]

Not related to prop tip speed, nor prop rpm, so far as I know.

Recall the F.27Mark600 (same basic airframe as the F.50) had a Vmo of 224 knots.
Yet the Fairchild FH227B, with the larger diameter props (same engines, RDa7's) had a Vmo of 250 knots.
However, the FH227B had an elevator trailing edge 'wedge' fitted (Fokker F.27 did not), so suspect control surface flutter was the concern with the slower designs. Visited the Fairchild factory years ago to have the larger diameter props fitted (for high altitude performance considerations), and when the change was made, off came the elevators to have the wedge fitted.
Acording to the old timers there, was for elevator flutter concerns at the higher Vmo.
And, they had the flight test data to back this up.
Interesting folks.

[Mike Huntaches]

Thankyou for your responses people, but the jury seems to be out! Is it related to prop speed(ie why a faster TAS means higher tip speed? -can you explain please?) or is it a control flutter issue. Crash &Burn is your info from the manufacturer or elsewhere? Interesting none the less. Does anyone else have anything to offer on this issue?:confused:
In the Fokker example does anyone have any enlightening ideas?
What about the B1900 also?

[Tinstaafl]

Mike, the prop. tip speed is the combination of the rotational speed + the aircraft's forward speed. Whether or not the reducing IAS limit is a prop limit due tip mach effects or not I'm not sure. I tend to think lack of flutter damping at the high altitudes is the usual reason.

[Cardinal]

Mike, the Beech 1900D example is really the case of two limitations overlapping each other. Vmo is 248, and the reduction in Vmo above 13200 is an approximation as the Mach .48 Mmo takes over and becomes more limiting.

[Mike Huntaches]

Does anyone know the formula for deriving tip speed given prop diameter, rpm etc etc?

[28L]

Mike,
I would guess the formula at zero forward speed would be:
Pi x D x rpm. If you assume a prop diameter of 2m and an rpm of 2000 this works out to around 720kph.
If you are doing a forward speed of 300kph, then by the velocity triangle (Pythagorus) the tip speed is around 780kph.
But I left school 30 years ago, so waiting to be corected on this one!