Mods and new cruise speed

[hlangebro]

Well, I have a pressure plenum, so all the high pressure is in there. When it exits into the cowl, you really want a pressure drop so the air gets sucked out. In my case, the high pressure from the plenum goes into the cowl volume, and since the cowl flap is reducing the exit, the pressure builds up, and yes, it increases exit velocity, however obviously, the inlets will supply as much pressure as they can, and what I need to do, is to seal the oil door and make sure the latch works properly.

[DanH]

Originally Posted by hlangebro

Well, I have a pressure plenum, so all the high pressure is in there. When it exits into the cowl, you really want a pressure drop so the air gets sucked out.

Nope, no sucking.

There is a pressure drop across the fins, that drop being related to mass flow. You can read the mass in lbs per second directly from a Lycoming cooling chart.

The initial goal is maximum conversion of dynamic pressure to increased static pressure in the chamber above the fins. Pressure coefficients (a measure of that conversion) typically range from 0.6 to 0.85 (see NASA CR3405 or similar). Let's assume 0.75 for your James cowl (which may be a generous assumption). Standard day max dynamic pressure at 8500 altitude and 160 TAS will ballpark around 12.9 inches H2O. From the O-320 cooling chart, at 8500 an approximate 7" drop across the fins would result in 2 lbs per second mass flow, more than enough to keep CHT happy given winter OAT at altitude (say, 40~50F in SoCal?).

12.9 inches x .75 = 9.675"

9.675" - 7" = 2.675"

That 2.675" is positive pressure, the sort that would blow open a wimpy oil door. The atmosphere immediately outside your cowl exit is (usually) at freestream static pressure, so the 2.675" difference provides the force necessary to generate velocity through the exit opening.

Here's a key point. A cowl with a better pressure coefficient would have a higher static pressure in the upper cowl volume. For example, 0.85 would mean 10.965" in the upper cowl under the same flight conditions. The exit could be throttled so as to have the same 7" drop across the fins, thus the same 2 lbs per second mass flow and the same CHT. However, the lower cowl pressure would 3.965", resulting in higher exit velocity. Cooling drag = mass x loss of velocity, so the same mass and less velocity loss equals the same cooling with less drag.

... and what I need to do, is to seal the oil door and make sure the latch works properly.

Yes. From the beginning, my cowl was set up with an eye toward increasing pressure, so the oil door has 10 additional plies of 9 oz glass on the inner surface. Too bad I didn't have any honeycomb handy at the time, as it still bulges a little when cruising at 185 knots with the cowl exit pulled down to about 30 sq inches. That air leak is another drag source.

[hlangebro]

Yes, I agree, you are correct.
My door was made quite stout with Carbon stiffeners. I checked it today and it is pretty stiff....
I might have to add some more stiffeners though.
Still a little disappointed that it didn't do anything for the performance at the lower altitudes, but higher up it will

[Jrskygod]

Hakan would you please share some pictures and details of your cowl flap. Is it electrically or cable actuated?

[revenson]

What is the rationale for 'negative reflexed flaps and ailerons'. Wouldn't that tend to raise the nose? Why would that lower drag or increase speed?

[hlangebro]

Negative reflex is a strange term....as that implies that the flaps and ailerons would move trailing edge down....
Anyway, the reflex moves the nose up a tad and actually reduces wing shape drag. The big issue however is that there is always a point when shape drag of the fuselage will be higher then the shape drag of the wing. It is seems though, though my flight tests that even with a slight nose up of the fuselage ( and aircraft) the reduced drag of the wing wins. It could be that our RV's are flying a tad nose down to begin with...

Regarding the cowl flap, I will check to see if I have any pics. It is controlled by a vernier cable and closes beyond nominal and have cutouts for the exhaust pipes.

[revenson]

Originally Posted by hlangebro

For all of you with 180-200hp in your RV-7A's, what is you true cruise speed at 8000-8500ft density altitude, at 65 and 75% power?

The best I could do at 8000 DA, 2600 rpm: 164 TAS.
This at: WOT (76% PWR), 9.8 GPH, 1605 lbs, 84.1" CG (some aft of center CG)

Airplane: 7A, ECI parallel valve 360, Vertical Induction, AFP injection, WW200RV prop, dual PMAGS, louevers on bottom of cowl, no antenna fairings.

With the lightweight prop, I tend towards aft CG, so not interested in reflexed ailerons/flaps, but would like to consider and learn more about: tire seals, antenna fairings, internal air ramp, and closing my louevers.

[Bevan]

Originally Posted by hlangebro

Anyway, the reflex moves the nose up a tad and actually reduces wing shape drag. The big issue however is that there is always a point when shape drag of the fuselage will be higher then the shape drag of the wing. It is seems though, though my flight tests that even with a slight nose up of the fuselage ( and aircraft) the reduced drag of the wing wins. It could be that our RV's are flying a tad nose down to begin with...

Could it be that at the higher speeds, the wing is making much more lift than we need (particularly aft of CG) requiring more nose down trim causing drag. Reflexing the flaps/ailerons effectively shortens the effective chord (on the aft part of the wing) thereby reducing lift and moving the center of lift forward.

Bevan

[hlangebro]

Reflexing doesn't shorten the chord, it changes your zero lift angle of attack, drag and pitching moment. You will also produce the same amount of lift, regardless of CG, however your pitching moment changes.
It is true that the faster you go, the less angle of attack the wing needs to maintain level flight. For aft CG, you elevator now moves trailing edge down a tad to compensate for pitching moment change, and this adds to the trim drag. I used to have the ground adjustable Whirlwind and it worked great, and with my 160hp I had a top cruise at 8500ft DA, 75%, 2700rpm of 164ktas. Since then I changed to the CS Hartzell and my CG became more manageable, plus I did many other drag reducing changes as well, to get the cruise to be 170ktas.

[glenn654]

Did you make or buy your comm antenna fairings?

I doubt I have the artistic talent to make good ones.

Glenn