Vmo increase with altitude

[Boomerang]

Despite a search through tech log and my texts I can't find an explanation why there is a slight increase in Vmo as altitude increases. (Up to Mmo boundary).

If I was to guess it's because of compressibility effects on CAS/IAS, and hence EAS Vmo is constant.

Anyone who actually knows?

[donpizmeov]

Not sure if this is true for all types, but one I fly had a constant Vne upto 8000', then increased up to Mne and reduced at that mach No.
Reason from manufacturer was that the lower speed upto 8000' was to do with bird strike protection on front windows!

[john_tullamarine]

The ASI is only "correct", at standard SL conditions.

Same thing happens with the indicated stall speed.

Try a search on ASI design and/or errors and/or corrections ... might produce more success than before ...

[jetblues]

Try www.smartcockpit.com

Check the 767 or Operations section (can't remember which) and there is a Vmo/Mmo Revision summary from Boeing. Included are some quite good diagrams etc.

Hope this helps.

[Boomerang]

A very interesting site!

Unfortunately the Vmo for the 767 example is a constant IAS. The example I have in front of me is the buffet boundary chart for the B727, as shown in Aircraft General Knowledge, T.Thom.

Any 727 drivers who can explain the increase in Vmo (357 KIAS@10,000 up to 370KIAS@FL255)?

[john_tullamarine]

May I try, once again ... perhaps with a little more detail ... doesn't matter what heavy we are considering .. the basic rules are along the following lines ....

(a) the limiting design speeds are EAS as we are interested in forces acting on the airframe .. in particular ... ripping off D-cell leading edges, aeroelastic problems and such like.... which are the sorts of nasty things which might happen if one is unduly careless or wilfully naughty.

(b) Vmo/Mmo, amongst other things, are related to the design speeds .. hence a basic interest in EAS values at lower levels and mach number at the higher levels ...

(c) the ASI is a mickey mouse, albeit fancy, differential pressure gauge, calibrated in IAS on the basis of certain assumptions as to what the air properties are .. and we all know that the air properties vary in the real world .. therefore the ASI has lots of real world errors which we allow for with the usual corrections

(d) the ASI reads EAS at sea level standard only

(e) as the aircraft goes UP, the ASI reading goes UP for a constant EAS .. ie the ASI tells fibs ... the main reason that the AFM and OM speeds are in IAS or CAS rather than EAS

(f) in general, you will see the Vmo and Vs lines increase once these errors become significant .. typically above around 10,000 to 15,000 ft .. the ASI errors cause a noticeable divergence of the IAS reading from the desired EAS

(g) the specific constant IAS limit referred to in the earlier post will be due to some other reason causing Boeing to restrict the EAS, or even a design or operational compromise within the design camp not related to the basic design standard. We would need some comment from a Boeing chap or chapess to resolve the specific detail .... and, it doesn't really matter, the basics are not altered

If you are interested in looking at the current basic design rules, try FARs 25.253, 25.335, and 25.1505.

[Boomerang]

Ok thanks very much.

[max motor]

Boomerang you were basically right first time. The ASI errors john_tullamarine refers to are basically due to compressibility effects in the pitot tube.

EAS equates to the dynamic pressure that acts upon the airframe.

At sea level this pressure will be the same as the pressure felt in the pitot tube.

At high altitudes and speeds the pressure felt in the pitot tube will be slightly higher than the pressure acting on the airframe. The increase in pressure is due to compressibility effects in the pitot tube. This is the pressure felt by the ASI and of course is known as IAS/CAS. Therefore relative to EAS, CAS will over read at altitude.

EAS is therefore CAS corrected for compressibility. Your ASI reads CAS but the Vmo pointer represents the corresponding CAS for the maximum allowable EAS, hence the difference at altitude.