by Tommy H. Thomason

Monday, August 21, 2017

F-4 Phantom ACLS Radar Reflector

ACLS is the initialism for Automatic Carrier Landing System. For more, see http://thanlont.blogspot.com/2011/07/look-no-hands.html

Up until now, I'd never seen a good picture of the radar reflector as incorporated on the F-4 Phantom although I'm sure there must be one or more in the F-4 books that I don't have. This was the best I could come up with. There was a door under the nose and a corner reflector extended from the compartment it was housed in.

Thanks to Angelo Romano, we now have one:
It took me a few minutes to figure out how it folded up even with this illustration in hand, which depicts the reflector from behind:
If you look closely at Angelo's picture above, there is a line on the panel on the left (on the right side of the F-4) that shows where it folds in half. It is then sandwiched between the upper panel and the one on the right to form a very compact package that can be stored in the space available.

Tuesday, August 15, 2017

Relying on Museum Pieces for Accuracy Part 3

Restored airplanes, either static or warbirds, can lead a kit manufacturer and/or modeler astray from an accuracy standpoint. Missing parts, ersatz replacement parts, flat oleo struts, one-off test program modifications, etc. have all resulted in kits and built models with errors. Sometimes, however, what's there is ignored or disbelieved. A case in point is the Douglas AD (A-1) Skyraider vertical stabilizer.

In addition to thrust, the propeller on a single-engine airplane creates other forces that must be taken into account. Consider the following for a propeller turning clockwise from the pilot's point of view. When the propeller is inclined nose up to the relative airflow, the down-going blade produces more thrust on that side than the other, resulting in a turning moment to the left (this effect is known as P-factor). The turning propeller also creates torque, causing the airplane to roll to the left; opposing this requires right stick, which increases lift on the left wing and therefore potentially drag and a turn to the left (some aileron-control designs compensate for this). When the airplane is on its takeoff roll, the torque also puts more pressure and therefore more drag on the tire on the left side of the airplane, causing a turn to the left. The swirl from the propeller, equivalent to downwash from a wing, impinges on the vertical fin, pushing it to the right and therefore the nose to the left.

In other words, a lot of right rudder (which results in a turn to the right) can be required to oppose these forces that cause a left turn. They change with the throttle setting and, in the case of P-factor, angle of attack. More powerful engines and bigger, heavier propellers result in higher forces. The flight-control forces to counteract them decreases with airspeed. As a result, the designer of a powerful single-engine, propeller-pulled airplane sometimes provides a built-in assist like a vertical fin with the leading edge angled left, providing a right rudder effect.

The Douglas AD (A-1) Skyraider incorporates such a feature, with the fin angled left at three degrees.

You'll note that the fin also appears to have a cambered airfoil, creating lift to the right as in the application of left rudder. My guess is that this isn't as effective at low speeds during a high-power wave-off as the angling of the fin to the left (right rudder) but is important in a dive (the AD was designed as a dive bomber) when the fin angle created too much "right rudder" at that low angle of attack, reduced throttle setting condition at fairly high speed.

This is my picture of the fin of the AD Skyraider at the National Naval Aviation Museum that shows the airfoil and the angle to the left relative to the dorsal fin that can be seen forward of the red anti-collision beacon.

Byron (SpadGuy) Hukee (see http://skyraider.org/) provided this picture of a Skyraider's rudder.
If you look closely, you'll see a kink in the trailing edge of the rudder just above the location of the horizontal stabilizer. You'll also note a difference in the fairing of the fuselage into the fin between the left and right sides of the airplane.

An even more striking example was provided by Ed Barthelmes (see https://www.amazon.com/1-Skyraider-Walk-Around-No/dp/0897474295) of the AD-5's vertical fin leading edge. Its air inlet and dorsal fin provide an excellent perspective of the fin's offset to the left side of the fuselage.

The old Airfix 1/72 kit of the AD Skyraider incorporated this feature. Some modelers have erroneously gone to the trouble of removing it...

Saturday, August 12, 2017

Relying on Museum Examples for Detail Accuracy: Part 2

Today's example is the gorgeous "F7U-3M" at the National Naval Aviation Museum at Pensacola.
This picture is from the walkaround section of Britmodeller curated by Julien and in this case credited to Bootneck Mike. For more, see http://www.britmodeller.com/forums/index.php?/topic/234949143-vought-f7u-cutlass/

I hadn't noticed it until F7U expert Al Casby of Project Cutlass pointed it out to me, but the external tanks are almost certainly bogus. External tanks are not often seen on the F7U-3 (even though it was short on endurance) but if present, they would have been either the standard Douglas-design AERO 150-gallon tanks, the very similar Fletcher 150-gallon tank, or the bespoke belly-mounted tank. These tanks have no fins and their afterbody has a distinctive upward sweep.
 Don Hinton Photo Cropped

 I suspect that they are either the 200-gallon tank that was carried by F-86s.
Or even more likely, given the flange on the left side of the NNAM tank, the one for the F-100s (see http://www.supersabre.com/200GallonTank.htm).

Note that this F7U was delivered to the Pensacola museum with these tanks installed so they weren't a goof by the workshop at the National Naval Aviation Museum.

These are what the F7U-3 tanks should look like:
Note that the pod under the belly was also a fuel tank that could be jettisoned. (A very similar pod could be carried in its place that contained 2.75-inch folding fin rockets.)

For other detail issues with this airplane from an accuracy standpoint, see http://tailspintopics.blogspot.com/2015/08/its-not-that-easy-to-get-it-right.html