Duct Design

Here we see some interesting things happening. With Duct3 as designed, with an exit area of 7.16 square feet, all the thrust is from massflow, none is from pressure. 113 HP is required to produce a thrust of 173 pounds to go 160 knots.

Now, there is quite a jump from 7.16 to 8.10, some intermediate values would show Pressure Thrust increasing while Massflow Thrust decreases. But here, with the change from 7.16 to 8.10, all the thrust reverts from Massflow Thrust to Pressure Thrust. Once Massflow Thrust goes to zero, exit velocity v4 is equal to ambient velocity v0, and all thrust is due to pressure created by the fan HP.

It is probably not desirable to have exit velocity less than ambient velocity, but it is interesting that as you continue to increase exit area, the ability of the HP to produce thrust becomes more efficient. You can see that less HP is required to produce the same 173 pounds of thrust.

Because of this possibility of increased thrust with less HP, I've decided to modify Duct3, to increase it's exit area. This actually makes the cross-section look better. Duct3 was supposed to be a high-speed duct, but it didn't turn out that way, because it lugged the engine down. I'm hoping that increasing the exit area will allow the RPM to come up again, and increase my speed to about 140 knots.

Here are some interesting results from an analysis of Duct2, the duct I've had the most success with to date, and for which I've done most of my 150 hours of flying. With A0=9.76, an A0/A2 ratio of 1.33, A2=7.32, and A4=9.02, an A4/A2 ratio of 1.23, we get the following results:

This shows that Duct2 is using a combination of Pressure Thrust and Massflow Thrust. But the results for Duct3, with A0=8.05, an A0/A2 ratio of 1.10, A2=7.32, and A4=7.16, an A4/A2 ratio of 0.98, shows it's thrust is all Massflow:

So my next experiment, currently under construction, is to modify Duct3 to increase it's exit area. Increasing the exit area to 8.5 square feet, an A4/A2 ratio of 1.16, slightly greater than the A0/A2 ratio of 1.10, gives the following results:

If these numbers are real, then achieving a 140 knot cruise speed with a 6 gallon per hour burn rate and 27 mpg efficiency, would be a little closer to normal Long-EZ performance. Actually I would be happy to achieve 140 knot cruise with only 20 mpg efficiency. Currently, with Duct2, I'm only doing about 110 knots at 6.5 gph and 20 mpg efficiency.

We can see from this picture, that the modified Duct3 has better lines than the original, less external tail cone, hopefully less chance for the airflow to separate and add drag. When this is built and tested, the results will be posted here.

Update, 3-4-02, I'm just flying these days, not doing much experimenting, but here is my latest thinking for the future: A smaller diameter, higher RPM ducted fan, will probably have 5 blades, especially if turbocharged.