Deriving Best range speed from Vl/d max

[gus99]

Hi Guys,

I'm doing some historical research on aeronautical engineering and hope someone can answer a few questions.
Best range speed is always a little higher than Best glide speed for a prop aircraft due to the fact one has to take engine efficiency into account OK, as I understand it Vbr was traditionally arrived at from Vbg by adding a nominal 10kts. Now I think its true to say that Vbg is generally understood as an indicated speed (although of course there's an associated TAS). Does this mean that Vbr was always calculated on the basis of adding 10kts to the indicated speed before deriving the corresponding TAS? I have some software that calculates Vbr but as far as I can tell, the answer is consistent with adding the ten knots to the TAS corresponding to the appropriate Vbg. Which method is correct in the traditional calculation? I understand that since the 10kts is somewhat nominal, the difference may be regarded as academic but I would still like to know the way the calculation was supposed to be done.
Your help appreciated.
Gus

 

[Sparweb]

The "traditional calculation" isn't "academic". It's part of the determination of the safe operation of the aircraft, and you haven't done your basic work in design if you haven't set a target range and rate of climb. You won't receive a type certificate if you don't bother to meet the regulatory minimums, either.

The question is very broad, so I think it would be most appropriate for you to look up the topic in an aerodynamics textbook. They thoroughly cover the topic because it's very important to designers. Otherwise you are asking us for many pages of typing for "academic interest".

If you are a pilot, then you probably know a few handy rules of thumb to help remember the numbers. This is especially useful when hopping from one aircraft type to another - the figures do swim in one's head. But don't mistake these guidelines for the actual published figures, and a lot of work goes into determining them in the design and certification process.

Steven Fahey, CET

 

[gus99]

My guess is that you and i are talking about different things. The 10 kts you probably have in mind is a safety margin over minimum drag speed designed to prevent one inadvertently slowing to end up on the back side of the power curve in turbulence or when turning. What i am talking about purely relates to estimating the true best range speed from Vbg and the torque curve of the engine. Except at very low rpm/ high MP settings, the general trend is to better fuel efficiency at higher power because the cam profiles and valve timing are biased to give good take-off performance rather than optimum cruise efficiency. Peak torque means peak efficiency. Speeds faster than the theoretical best glide for engine off operation are thus appropriate for power on operation.


Regards Gus

 

[GregLocock]

How far back do you want to go?

http://naca.larc.nasa.gov/search.jsp?N=4294756445&Ns=PublicationYear|0

Cheers

Greg Locock


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