Page 65
rv620mr said:
George & Walter,
Thanks for replying. I'm looking at pg. 65, figure 1-4 of Kent Paser's book (first printing 1994). Monte Neilan, RV6, Flying, Kimball, NE
Monte, yes the idea's shown on pg 65 are very sound ones, which work absolutly well. Basically it's a cowl flap found on many factory planes. The idea is to play with exit area in flight. A good idea but adds complexity, weight, pilot work load and maintenance. Page 65 also incorporates a concept he called a "jet-pump" or "venturi-pump". It's most commonly called an exhaust augmentor.
The Fig 4-1, pg 65 shows a flush ramp (reverse flush scoop) w/ parallel sides and proper exit angle. Positioning the exhaust as shown provides an exhaust augmentor function. Exhaust augmentation is widely exploited in rocket and jet engines. Square (parallel sided, non diverging or converging) flush ramp is optimal for exits. (A NACA scoop is not ideal or used for exits, (The RV-4 has a slight ramp, but not of proper depth to exit the exhaust.)
An augmenter is a divergent (ie: bigger at the back than the front) cone into which the exhaust gases are expelled. In this case we have a square X-section. The efficency of this "augmenter" is less than ideal. I did a quick check, NASA has a 1957 reseach paper on "Internal performance of two-dimensional wedge exhaust nozzles". This is theory. The trick is to make the theory practical.
Mr Bernoulli figured out pressure exerted by gas is dependent on speed which it flows. The faster the flow, the lower the pressure. The "Bernoulli effect" creates a low-pressure area around the hot gas coming out the tailpipe; this low pressure draws cooler, denser air into the gas-flow. With out an augmenter tremendous amounts of turbulence and large swirling vortices's occure where hot and cold air mix. As they mix, the cold air is heated by the exhaust and expands but, since it's not confined in any way, this expansion doesn't add to the thrust -- it expands in all directions and reactive forces cancel out. This is what we have when we dump out pipes into the free air.
This idea is found on a megaphone or "Coanda" (solid cone inside a megaphone) at the end of an exhaust pipe. ("Coanda effect"
http://www.thermofluids.co.uk/effect.php;
http://www.rexresearch.com/coanda/1coanda.htm)
When we add an augmenter we have a degree of control over the expansion process. The heated air no longer expands evenly in all directions; instead the only direction it expands is towards the rear of the cone. Mr Newton then states "for every action there's an opposite and equal reaction". This means the heated air now is travelling only towards the rear of the cone; There is a reactive force exerted on the cone pushing it forward. This reactive force is "thrust."
The geometry does not exist on a RV to lend itself to a cowl flap or augmenter tubes. We just don't have the room unless an exhaust ramp was incorporated into the lower fuselage. Some homebuilts have "exhaust tunnel's". My 1958 Apache and friends C-310 (twins) have augmentor tubes. Nothing new.
http://www.airspacemag.com/ASM/Mag/Supp/AS05/M202.html
Van has developed a very good basic non-cowl flap design of very low drag compared to most aircraft, so incorporating a cow flap or augmenter is not easy. Van has recognized the advantage of these ideas and published pictures and ideas in the news letter, "The RVator" over the years.
How do you make all this work in say a RV-7? Well, not easy (click)
To incorporate a flush exit ramp takes a major structural change to a RV. Van crunched the numbers of cost, weight, building complexity and simple efficient configuration, coming up with the current design. There are mods and gains but how much are you willing or able to do.
though.
