NACA Drag

So, today is the first day that I used my b-kool air conditioner in the -10 and I discovered something quite remarkable. I turned the unit off when I reached 6500' and opened my pilot side vent and noticed air speed drop 2 knots. Confused I closed it again and my airspeed came right back. In fact, I played around with a lot of scenarios and found that if I had both front vents open and the rear NACA controller open, I would drop 7kts of air speed.

I repeated the tests on the way home at 9500' with the same results.

Is this common?

Thanks,
Shannon

My first thought is: are you sure you don't have a leak in your static system?

I had the same thought. Ground speed changed the same amount.

civengpe said:

...if I had both front vents open and the rear NACA controller open, I would drop 7kts of air speed...

Click to expand...

Classic symptoms of a static system leak. The vents are pressurizing the cabin by a few milibars. That increased pressure is leaking into the static system and decreasing the pressure differential between the pitot and static references, yielding a lower IAS. I'd suggest you start tracing your static system looking for likely leaks.

Common causes are the plastic line having been pulled off of a static port in the aft fuselage, the line having been slit or crimped when installing interior parts, or fittings not having been tightened properly. Actually, the most common cause I've seen is where builders just forgot to connect the static line to the back of the altimeter, ASI, or VSI, and it's hanging there in space.

I'd start by checking the backs of the instruments, then the static ports, then all the fittings. If all that stuff is good, then start looking for holes in the hoses themselves.

Thanks, Bob K.

If you'd really lost 7 knots, wouldn't you have had to re-trim? And did you?

Have AP?- of course you do. SO - with altitude hold on, does it dive when this happens? If so then static issue (and you really don't want that to happen), if not, it becomes more interesting.

DaleB said:

If you'd really lost 7 knots, wouldn't you have had to re-trim? And did you?

Click to expand...

So, that was the interesting part and how I first noticed something was different. I had the autopilot engaged and was in very smooth air. When I opened the pilot side vent the nose pitched up enough for me to notice. Close, pitch down....

My first reaction was a static leak as well, but my gps ground speed dropped the same amount. I recently checked all of my static fittings and couldn't find any issues.

Edit: if I had a static system leak, the pressurizing of the cabin area would cause my altitude indication to drop and my autopilot would pitch up slightly to correct...causing the drop in airspeed. I will take a look at this on my flight today.

One of the things I really like about this site is the challenging of initial assumptions. I think I have some more investigating to do!

Yes, theory says airflow through all the interior volumes of an airplane results in drag, the product of mass x velocity loss.

The vents can probably move many pounds of air. For most light aircraft, velocity loss would be worse than seen in the case of engine cooling flow, as there is rarely a dedicated (and preferably converging) exit. Instead, the exit air is generally excreted through holes and gaps, many perpendicular to the freestream, resulting in both a complete velocity loss and plumes which act similar to protruding objects.

Quantifying the drag is where things get interesting. For conceptual design, Raymer suggests 5~10% of total parasitic drag is due to leaks and protuberances. I seem to recall an old NACA study assigning 3% of parasitic drag to accessory flows (things like oil coolers and blast tubes), which is probably quite similar to fuselage vent flow.

Can it slow you down by 7 knots? Sounds like a lot to me, but hey, it's an opportunity to earn that "experimental" sticker! Get a formal static system leak check, and if it passes, fly a few dedicated test flights. Observe airspeed changes with different mass flows. Remember that every molecule of air entering via the vents has to exit somewhere, so experiment with exit control.

One of my unfinished test projects is to work on cockpit air drag. With many of our airplanes we have two 2" vents directing air into the cockpit. As Dan mentioned it obviously gets out somewhere and while it is doing so it is adding drag. There are quite a few production planes that have dedicated exit ducts for cabin air. We all make such a fuss over engine cooling drag and but we have given no consideration to TWO 2" hoses squirting air who knows where!

One would think that a lot of the air would get out around the tail, and it might, but how does that explain that cold draft of air that comes forward from the tail cone?
A scoop, facing aft might just do the trick. I have one for my oil cooler exit air on the side of my cowling and it works well. The aft inspection fuselage inspection cover might be good place to instal one.

I picked up a bit of speed with the rocket by sealing the ends of the wings, between the wing and the wing tip. It was easy with my flat tip EVO wing but not too hard to do with the standard wing tips either. Any air squirting out, and not going straight back, is going to cause drag.

This thread has been a good reminder that I need to do some testing with cabin air exiting the cockpit.
Another benefit in having a system that "draws" air would be that we might just be able to use less air for the same amount of cockpit cooling, thus even less drag.

In this case 6 knots seems a bit suspect. I know that a leaking static system will cause this scenario and it should be explored. The fact his ground speed also decreased is interesting but I would like to see a bit more flight testing with zero wind conditions.
But if it is real on this particular plane the man what an easy way to pick up some serious efficiency gains!

The corrugated baggage compartment bulkhead provides quite a bit of flow passage area on all of the RV's (except the RV-8) to move vent and cabin heat air out of the cockpit. Once the air has made its way into the tailcone it can exit through the openings in the emp. fairing. Not the ideal solution, but some of the openings do face aft and the others are in an area of rather disturbed flow and possibly low pressure (elevator torque tube openings) so probably not too bad.
Unfortunately, a lot of people install an upholstered panel over the baggage bulkhead panel, effectively sealing this flow path. The air has to go somewhere, so it starts squirting out through gaps around doors and canopies (probably much worse for inducing drag than exiting at the emp. fairing).
If this RV-10 has such a panel, and the airplane is found to not have any static system leak, it would be very interesting to see what happened if this panel were temporarily removed

BTW, because of fuselage shape, there is lower pressure on the sides and portions of the top of the canopy or doors on an RV. This further complicates the problem because air wants to take that route out of the cockpit (towards the lower pressure). This is one of the biggest causes of air coming fwd through an unsealed baggage bulkhead.
Assuring a good seal of a canopy or doors to prevent out flow will go a long way towards improving heater performance, and reducing drag.

time

A static leak would show a very quick speed increase. Reduced drag would take a good 20-30 seconds to show a 7kts increase.

Mark's right and that's a good clue right there.

When you test for this, do it with the autopilot off, to remove one system from the test matrix.

A few aircraft have dedicated cabin-air exits specifically to manage the cabin air flow removal. Most Beechcraft have one.

Dave

Confirmed static leak today. Altitude changed every time I opened or closed a vent.....now I need to figure out where. I have the safeair1 system that uses rigid plastic brake line, so a slit is not likely. I just verified all connections in the tail a few weeks ago.

My major suspect is efis 2. I just got it back from AFS for an upgrade.

Thanks everyone for challenging my initial assumption.

The static leak that was hardest for me to find was a leak on the airspeed instrument glass to case seal.

Carl

So, to close this loop, I have not found some undecsovered speed mod.

My alt static plug was just loose.

Thanks for the advice guys.