The AC-4B Glider

AC-4B 2-view

Why the AC-4B?

On booming weekend days at the Illini Glider Club, I sometimes had to wait in line for a long time to fly one of the single-seat Club gliders, and then, unless being the last of the list, and nobody radioed me down, the flight is limited to one hour by Club rules.

As a result, I started to mull the idea of having my own private glider (there are now 6 based at Monticello). After researching all the pros and cons, for the type of flying I wanted to do, I narrowed down my choice to the Aviastroitel, commonly named "Russia", AC-4, designed by Vladimir Federov. In England it is named Aircraft Cooperative Mechta Me7.

A big factor in the choice is that a member of the Club had one, and I could see it in action both on the ground and in flight. When this member suddenly had to move to Chicago and put up his glider for sale, I just could not pass that opportunity. So, before I realized it, I became the proud owner of N29HS (serial 22, made in 1995); it took me a while to get accustomed to the idea of being an "aircraft owner"!

AC-4B Data and Performance

Data in this section below applies to AC-4B serial 22 at Max Gross Weight

Parameter SI US
Span 12.6 m 41.3 ft
Length 5.24 m 17.2 ft
Height 1.31 m 4.3 ft
Wing Area 7.7 m2 82.9 ft2
Empty Weight 141 kg 311 lb
Max Pilot Weight 109 kg 240 lb
Max Gross Weight 250 kg 551 lb
Wing Loading 32.4 kg/m2 6.6 lb/ft2
Never Exceed Airspeed Vne 220 km/h 119 knots
Maneuvering Airspeed Va 156 km/h 84 knots
Zero-wind Approach Speed 85 km/h 46 knots
Clean Stall Speed Vs 68 km/h 37 knots
Best L/D 31:1 Airspeed 95 km/h 51 knots
Minimum Sink 0.79 m/s 158 ft/mn

Wing Airfoil: modified Wortmann FX60-157 laminar
Aspect Ratio: 20.6
Roll Rate: 90 degree roll in 3.5 s
Best L/D: 31:1 (without after-market wing fairings)
Load Factor: +5.30 g -2.65 g

AC4A Polars
Polars salvaged from RussiaSailplanes.com in 2000, for different Gross Weights. Data applies to AC-4A and should be very similar to AC-4B (both fixed-gear). L/D gets to 33:1 with fairings.

AC4A_McCready
Comparative McCready Speeds To Fly generated by Carl Herold. The AC-4 without fairings is midway between the L-33 and the Std Cirrus

Russia AC-4B Flight Manual

This 8-page Flight Manual only applies to serial #22 (factory-built in Russia); it may or may not apply to another serial number.
Page 1 Page 2 Page 3 Page 4 Page 5 Page 6 Page 7 Page 8

This Weight and Balance Sheet only applies to serial #22 (factory-built in Russia); it may or may not apply to another serial number.

Russia AC-4B Review

Others have made reviews of the AC-4, with whom I agree,
Jim Hicke AC-4B serial 30
Jim Cooper AC-4C in the UK
so I'll just complement with a few personal points of view.

N29HS_AirToAirOct2006_2.jpg Wings forwards provide a wider range of pilot weights. Note wide-brim hat (40 deg latitude). Photo by Kevin Ford.

Pros

The AC-4 has a very well-thought no-nonsense design. It is strongly built and aerobatic, which gives me a lot of stress-strain reserve since I don't fly aerobatics. Initially I was a little suspicious of those flexing wings, but now I respect fiberglass as much as other aircraft material.

The total time between opening the fully trailer and being ready for take-off is less than 15 minutes, without hurry. An enclosed trailer is a great peace of mind, not having to worry about "hangar rash" and dubious handling by people who are not always glider pilots (we have many students and helpful visitors). All of the unpainted metal parts (bellcranks, etc.) are factory-covered with a oily compound to prevent corrosion.

The cost of the FAA Annual is $50 (powered airplane owners will weep at this). It is also much cheaper than the Polish PW-5, to which it lost the competition to be the "World Class" monotype, for reasons that must have been political.

Originally a Con, but now a Pro, the cockpit seating is practically inexistant. You have to make your own padding. On top of that I removed the adjustable back plate because I use my parachute as a back rest instead, against the bulkhead. However, it is possible to majorly shift position during long flights (unlike the "molded-in" PW-5, for which I am too tall at 6'1"). When high AGL at the top of a wide thermal, I sometimes slide forward like in a bathtub, with the head on the front spar, and then the only things I see are the whisps of clouds above me, and the horizon slowly circling around, an incredibly relaxing feeling.

The AC-4 does not fly at all like the PW-5. It is more reactive, essentially it is more of a hot-rod, and you have to fly it that way, quite aggressively. The AC-4 can accelerate in a blink, and when flying fast it outperforms gliders of a class above.

../GlidingAndSoaring/DerekPiggott_Me7.jpg Derek Piggott, aged 81, after completion of a 505 km task in England aboard a Me7 (AC-4) where he beat many younger pilots with superior machines (Wikipedia).

The AC-4B is the "tricycle" fixed gear version, as opposed to the AC-4A taildragger with fixed gear and AC-4C taildragger with retractable gear. Having a nosewheel allows full main wheel braking without risking ending up on the nose, which allows landing rolls of 150 feet. You could land across some runways if you needed it!

AC4_B_C_1.jpg AC4_B_C_2.jpg Nose-to-nose comparison of factory-built AC-4B (left) and homebuilt AC-4C (right).

Seeing me setting up and fly the AC-4 was not lost on someone else in the Club, who bought one too (see above pictures). Too bad our runway has lights, otherwise we could be towed up together! The level tow drag for one AC-4 is approximately 551:31 = 18 lb; that is nothing for the Club's Pawnee.

Cons

The polyurethane paint finish is, well, Russian. The junction between the two halves of the fuselage would have needed one more coat of paint. Now it looks a little like a plastic model kit as built by a 8-year old. This will be fixed when repainted, years down the road.

The tongue fairing in front of the tail is easy to damage when inserting the stabilizer.

There is excessive buffeting at 45 knots and below. Since the pitot tube is on the tail, the airspeed indications are erratic, and the Total Energy probe jumpy. The absence of fairing between fuselage and wings (unlike the PW-5) is probably a factor in this. There is an after-market kit for sliding a fairing onto the wings, and then taping it to fuselage and wing. Also I understand that later models of the AC-4 have a different incidences so that it rides more tail-high.

The spins are apparently not for beginners (me too). From hearsay, it accelerates very fast in the dive that follows the spin recovery. This was scary enough for one pilot to bail out at 2000', during the acceptance flights of the first N29HS, serial 21. The owner refused to take posession of a damaged hull (which apparently landed on its own) and transferred the N-number to serial 22!

Because of the wing upflex in flight, both ailerons are up by a quarter inch. I do not know if it is by design or not, but given the thoughtfulness of the whole ship I presume it is. At 80 knots this is beneficial to reduce lift (that curves the wing upwards) and drag. The Lockheed L1011 trijet had an active system to do right that on cruise, to save a few percent of fuel.

The tail wheel leaks, so I need to pump it up every two weeks. It seems to be a standard joke that affects all AC-4s. No big deal.

The RussiaSailplanes.com importer for the USA is no more. I think they lost because they got engulfed in the more complex AC-5, the motorglider version of the AC-4.

N29HS_AirToAirOct2006_1.jpg Illinois roads look flat and straight but in fact always bordered by utility poles! Land next to them to avoid dinging your ship. Photo by Kevin Ford.

Repairs

[ NOTE TO ALL AC-4B OWNERS: There are no ADs on the (experimental) AC-4, but I would like to post here a list of fixes and repairs (preventive and curative) that you have had to do to your aircraft. Mine follow. = ] Over the years I had to do some repairs to N29HS. In the US there are not (yet) many mechanics familiar with fiberglass airplanes and gliders.

(1) Probably because the canopy got slammed shut by a gust of wind, the aft canopy locking cone-pin caved in and fell between the sides of the fuselage. It had to be fished out and re-clothed and epoxied from a 4" diameter hole made in the inside wall on the fuselage.

(2) During a FAA Annual, a hairline crack was discovered just next to the forward and single pin attachment of the stabilizer, a scary place for a crack. This involved the standard layer by layer inspection and repair, but it turned out that the crack affected only the outermost shell, ifnot just the paint. The only thing now is that the "white" paint there is not the same as the rest of the glider.

This may look like a lot of maintenance, but when I was Crew Chief for the practically brand new PW-5 of the Illini Glider Club, we had to repair a wing-tip split in half and completely redo the cockpit edge over three feet because of an extensive longitudinal crack, major work in comparison.

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