brons2
Posts: 2462
Joined: Tue Sep 25, 2001 1:02 pm

Airspeed Indicator During Aerobatics

Tue Jul 05, 2005 6:28 am

Today, I took a flight in a Commemorative Air Force AT-6C warbird for the 4th of July. The plane in question is:

http://warbirdregistry.org/texanregistry/at6c-8812054.html

I sat in the back seat, and had what looked to me like a pretty complete set of instrumentation. It had airspeed, vertical speed, horizon indicator, engine RPM, manifold vaccuum, fuel pressure, engine temp, fuel level, etc.

The pilot up front let me do some shallow turns once we had reached 3000 feet, which was fun and interesting for this non-pilot.

Afterwards though we launched into some aerobatics. I wasn't scared by the g-forces involved, but it seemed like during several of the manuvers he let the airspeed bleed off a little more than I was comfortable with. We were in a figure-8 at a near 90 degree bank and I looked at the airspeed indicator and it was barely over 40 kts. I was scared shitless. I felt like blurting out "WE'RE GONNA STALL!!!" into the headset but I kept quiet.

So my question is thus. We did not in fact stall the plane, so I was wondering if the airspeed indicator is actually accurate when you are doing aerobatics and/or steep turns.

When we were doing loops, I couldn't even bring myself to look at the airspeed. I know we entered the loop at around 180 kts, but then were quickly losing speed. I closed my eyes for a second, and then just numbly looked at the ground.

Anyways, is the gauge accurate when doing this stuff? Obviously I am still here typing, so what we're doing couldn't have been too unsafe. I guess.
Firings, if well done, are good for employee morale.
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Airspeed Indicator During Aerobatics

Tue Jul 05, 2005 7:42 am

Not real familiar with the T-6, having only flown it one time but I'll give it a shot here.

Extreme angles of attack can make airspeed indicators read low, as the airflow will be across the opening as much as into it. Also airspeed indicators and the pitot system that drives them were designed to read most accurately in the range of the intended speed envelope for the plane in question. I would not expect a reading of 40 knots to be accurate in anything except a helicopter with a helicopter airspeed indicator and pitot.

So if the T-6 indicated 40 knots, it was probably slow. In fact it was probably below stall speed.

So why didn't it stall?

You said it yourself, you were in a 90o bank. The wings weren't supporting the plane - it was in trajectory flight. Inertia was carrying it over the arc the pilot had committed it to.

That is a pretty common thing for aerobatics in underpowered planes or those with high wing loadings. Delmar Benjamin's GeeBee replica (below) is famous for its knife-edge flight demonstrations. Hey, why not? The wings hardly do anything anyway. I've been in the same corner you describe in a T-6, a DC-3 and other planes. You get there with the kinetic energy you built up in a shallow dive before the pullup and you just arc over the top light-in-your-seat and begin to accelerate down the other side. It is fun, it looks pretty cool and it is really pretty easy on the airframe.

As for the loops, you are correct in your judgement about running out of energy over the top. It does take some speed and a good entry to make the loop look good in the T-6. Coming over the top you may be on the verge of a stall but you are pulling the nose down anyway and it'll gather speed again on the downline. I wouldn't try to hold it inverted at that point!

T-6 is a lot more fun inside than outside. Watching one it is all noise and no performance. Being aboard though, is really fun. I could never see the attraction until I flew one. Good for you on taking the ride!


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Happiness is not seeing another trite Ste. Maarten photo all week long.
 
MrChips
Posts: 933
Joined: Wed Mar 30, 2005 2:56 pm

RE: Airspeed Indicator During Aerobatics

Tue Jul 05, 2005 1:30 pm

If the airplane is in any sort of sideslip, the airspeed indicator may read erroneously as well.

This is because many aircraft have their static ports on the side(s) of the fuselage. If the aircraft is in a sideslip into a static port, dynamic pressure from the air hitting the static port straight on will increase the static pressure at the static port. Since an ASI shows the relationship between static and pitot (static+dynamic) pressure, the airspeed will appear to drop considerably and remain much lower than it actually is.
Time...to un-pimp...ze auto!
 
AR1300
Posts: 1686
Joined: Mon Feb 07, 2005 1:22 pm

RE: Airspeed Indicator During Aerobatics

Tue Jul 05, 2005 3:06 pm

Nothing more left, SamClick already took the cake....  Wink Big grin

Mike
You are now free to move about the cabin
 
abbs380
Posts: 118
Joined: Mon Mar 21, 2005 11:25 am

RE: Airspeed Indicator During Aerobatics

Wed Jul 06, 2005 5:06 pm

SlamClick, Are you saying you have done aerobatics in a DC-3?
 
Whiskeyflyer
Posts: 223
Joined: Mon May 13, 2002 3:07 pm

RE: Airspeed Indicator During Aerobatics

Thu Jul 07, 2005 12:47 am

Abbs380 I once got pulled on a CAA inspection on a DC3 because I was missing the decal "NOT TO BE USED FOR AEROBATIC FLIGHT", in the cockpit and sure enough the decal was in the IPC. (Bureaucrats...... always looking for something, to catch you on)
Must be honest having seen and taken the wings taken off a DC3 for a Check 5 inspection, I would flip it (remember the fuel tanks near the fuselage, so not much weight in the wings)
Strong bird the DC 3.
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Airspeed Indicator During Aerobatics

Fri Jul 08, 2005 12:09 am

Quoting Abbs380 (Reply 4):
SlamClick, Are you saying you have done aerobatics in a DC-3?

Oh heavens no!

That would be illegal.



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