Vmo reduction with alt? help!

[Willy Miller]

On the Q400 Vmo is max at FL180 then decreases back with altitude (248kts at FL250)


Explain please!

:confused: :confused: :confused:

[alexban]

' EAS is a measure of the dynamic pressure exerted on the aircraft. This dynamic pressure plays a key role in the lift and drag created by the aircraft. For a given EAS the aircraft feels the same dynamic pressure, and therefore lift and drag, regardless of altitude. The higher the density altitude, the thinner the air, and the faster an aircraft must travel through the air mass to obtain the same EAS. The actual speed of the aircraft through the air mass is called the true air speed (TAS).'
So,if at FL180 your Vmo will be ,for ex,300kts, if you climb,and want to maintain this IAS the result will be in an increase in TAS,resulting in disruption of airflow over the wing.This is why you'll have to decrease the IAS,in order to mainatain a maximum TAS..your ex of 248kts at FL 250 will corespond to the same TAS as Vmo at FL180....I hope I've not mixed it up,I'm a bit tired now..:ugh:
Brgds..
Alex

[Mad (Flt) Scientist]

The reduction in EAS with increasing altitude may well be to reflect Mach number concerns (which is not dissimilar to the TAS variation, but with some nuances due to variation of the speed of sound with altitude too) but the concern may well be propellor tip speed approaching sonic, not the wing itself.

[Willy Miller]

What about the reducing TAS!

red line FL180 284 IAS = 360 TAS
red line FL250 245 IAS = 340 TAS

why the lower TAS

(props not reaching Mcrit, tips travelling at 350 kts ish at 850RPM)

[Hudson Bay]

Firstly. Nothing to do with the prop tips!

Put very simply.

All aircraft have an optimum cruising flight level for best speed. It does not follow that the higher you go the faster you go.

Alex is right with regards to EAS, so I won't repeat that.

Your Aircraft's optimum cruising level for greatest speed is FL180 depending on variations in temperature. It is the point when the mmo begins to decrease. If you continued to climb your barbers pole would begin to decrease resulting in a reduction in IAS and therefore a reduction in TAS.

If the mmo stayed the same your aircraft would exceed crit mach and I'm sure you have seen the TV footage of aircraft that exceeded crit mach!!!

I am not familiar with the Q400 but my guess is it's wing has very little sweep and that is why your IAS penalty for an increase in FL is considerable. A jet with alot of sweep would not suffer such a penalty because the pressure exerted on the Aircraft's wings would be less.

So, if your late FL180. If you've got limited fuel climb!!!

[Mad (Flt) Scientist]

What about the reducing TAS!
red line FL180 284 IAS = 360 TAS
red line FL250 245 IAS = 340 TAS
why the lower TAS
(props not reaching Mcrit, tips travelling at 350 kts ish at 850RPM)

360 KTAs at 18,000ft is ~ M0.58
340 KTAs at 25,000ft is ~ M0.57

(Speed of sound drops by ~20 kts between those altitudes, so the Mach number is near constant).

That seems to indicate it's a Mach Number line, not a speed dependent line. But M0.60 is pretty slow for a critical Mach even for a straightish wing like the dash8.

Also, if the aircraft is doing 360 KTAS the prop tips are certainly going faster than that. If that 350kts is based simply on prop diameter and revs, which I think it is (gives a 13ft dia prop, which sounds about right) then you really need to combine the tip rotation speed with the aircraft forward speed.

If you do that, then at 350kts rotation plus 360kts forward you get about 500 KTAS - which is about M0.83. Starting to become important.

[Old Smokey]

Firstly, it has EVERYTHING to do with the prop tips!

This is such a common one that it should almost become a Sticky!

Every aircraft has two basic Speed limits, Vmo and Mmo.

(1) Vmo is a structural limit, defined as an Equivalent Airspeed (EAS) which is a direct measurement of the dynamic pressure of the air stream. Aircraft do not have EAS indicators (they should have), and, instead, we refer to CAS indication, which aircraft do have. For a given EAS, CAS increases with increasing Pressure Height, thus, Willy Miller, from the information you've given us, the EAS limit for your aircraft is 284 KEAS.

(2) Mmo is an aerodynamic limit. It is a Mach Number beyond which control problems, and severe loss of thrust from propellers occurs. It has NOTHING to do with the aerodynamic load on your aircraft, but may contribute to undesirable propeller vibration and flexing. High speed propeller aircraft usually do not have Mach Meters, they should have, and the Mmo limit is usually reflected as a decreasing Vmo with increasing Pressure Height.

Mmo is determined at a Mach Number somewhat beyond Mcrit (NOT at Mcrit), where the control problems and/or loss of propeller thrust due to Mcrit exceedance is acceptable. Thus, it is related to Mcrit.

Every component of the aircraft has it's own Mcrit, wings, propellers etc., and Mcrit is usually found at the fastest moving part of the aircraft. For the propeller aircraft, the fastest moving part is the propeller tips, their speed being the VECTOR SUM of their own speed due to rotation and the aircraft forward speed (TAS).

From the information you've given -


red line FL180 284 IAS = 360 TAS
red line FL250 245 IAS = 340 TAS
Prop tips travelling at 350 kts ish at 850RPM)

We can establish several numbers for the aircraft (for ISA) -

284 CAS at 18000 Ft P.Ht. : EAS = 278.1 : TAS = 368.4 : Mach No. = 0.595
245 CAS at 25000 Ft P.Ht. : EAS = 238.6 : TAS = 356.5 : Mach No. = 0.592

Of interest is that whilst all of the other data varies, Mach Number is almost the same!

If we now calculate the Vector Sum of the Prop tip speed and the TAS, we can establish several numbers for the propeller, accepting your prop tip speed of 350 Kt at 850 R.P.M. (for ISA) -

At 18000 Ft P.Ht. : TAS = 508.1 : Mach No. = 0.821
At 25000 Ft P.Ht. : TAS = 499.6 : Mach No. = 0.830

Mach Number again is very similar.

For both examples the Mach Numbers are very similar. Don't forget that in aircraft certification, the highest temperature approved in the Environmental Envelope (Typically, ISA + 35°C) is used to find the TAS, and the highest permissible propeller speed is considered (Maximum R.P.M. is probably above the 850 R.P.M. that you quoted) to find the Vector Sum speed of the propeller tips. M0.82 to M0.83 does sound about right for a thin propeller blade, as compared to a much thicker wing. (If the certification criteria had been used in lieu of the example worked at ISA, I'm sure that the closely matching Mach Numbers would become equal).

Hudson Bay, you seem to have resurrected the old "aircraft don't fly above Mcrit" bogie again -"your aircraft would exceed crit mach and I'm sure you have seen the TV footage of aircraft that exceeded crit mach!!!". As recently as yesterday I exceeded Mcrit during Climb, Cruise, and Descent in a normal sub-sonic airliner. Tomorrow, along with Tens of thousands of other pilots around the world, I intend to do the same again.It's normal operations.:ok:

Regards,

Old Smokey

[Willy Miller]

Thanks very much for the replies ( sure you've heard it all before!) I didn't account for the VECTOR SUM of the forwrd moving props, just the rotational speed. Also, didn't know that in certification they use max prop RPM (1020).

Proves two things:

There is no such thing as a stupid question

You NEVER stop learning in aviation

Thanks again,

Mat.

:ok: