Radial Rocket TD Pilot Report #1:  Let’s Go for a Flight!

By Jeff Ackland
If you have checked out the Radial Rocket specs page on this website, you now know that the Rocket is a low wing tail-dragger with fixed gear, is powered by an M-14P 360 HP or M-14P 400 HP radial engine, and seats two, in tandem. Although the 25 foot wingspan and 22 foot length dimensions would indicate a fairly compact airframe, as you walk up to it the Rocket seems much larger and more substantial, partly due to the round engine cowl (40.5 inches in diameter), and partly due to how high the plane sits on the gear – those main gear legs are long, and they have to be to keep the 98 inch diameter 3-blade MT prop out of the dirt!

Preflight:

In addition to a typical preflight inspection, radial engine airplanes require a bit of extra attention before any engine start, and the Radial Rocket is no exception. Care must be taken to prevent a hydraulic lock (due to excess oil accumulating in the lower cylinders), and subsequent damage to the engine, during engine start. First item in the preflight (or pre engine start) operation is a check of the cockpit switches and engine controls – throttle closed, master and mags “off”, oil valve control to “closed” (more on this later), and firewall fuel shutoff control to “closed”. Because the M-14P carb is equipped with automatic mixture control, it does not have a mixture control lever. Therefore the engine is shut down after flight by switching off the mags instead of pulling the mixture control to idle cutoff – thus the engine fuel system is charged, which makes it more likely that, if the prop is moved (and it will be shortly), the engine will accidentally start and run with a hot mag. So, make doubly sure that the throttle is closed, master off, and firewall fuel shutoff is off

Now we can proceed to complete a very typical walk-around inspection of the aircraft, starting at the left wing root and circling the plane in a counter-clockwise direction. The next step is to ensure that the intake drain valve is open, and that the air start air bottle main valve is closed. Although new production M-14P’s can be ordered with electric starters, most M-14P engines use an air start system in which high pressure air is distributed to the engine cylinders to turn the engine over during startup. An engine- mounted compressor refills the air bottle after startup.

With switches off and the walk around accomplished, pull the prop through 18 blades – 6 revolutions of the prop shaft – 9 revolutions of the crankshaft. As the prop is rotated, it will become immediately apparent if one or more of the lower cylinders is filled with oil. An oil filled cylinder must be cleared before engine start, by removal of a spark plug. With the airframe pre-flight’ed and the engine ready for start, it’s time to climb aboard, start the engine, and go flying, but not before ensuring the intake manifold drain is closed, and the air bottle valve is open.

Mount Up and Start Your Engine!:

To reach the cockpit, step up onto the left wing-root from behind the wing, taking care not to step on the flap – just keep your feet on the non-skid wing-walk areas. For entry into either seat, the sliding canopy travels quite far aft – So far aft in fact, that when I fly solo I insert a pin into a hole in the left canopy rail to limit the aft movement of the canopy, making it easier to reach for closing prior to take-off. One of the reasons a sliding canopy was designed into the Radial Rocket is that it can be opened as required for ventilation on the ground without worrying about it getting caught and damaged by the wind or prop wash. We have accomplished high power engine runs on the ground, during which the open canopy is very stable, so having it open during engine run-up is no problem.

Once settled into the cockpit and strapped in, switches and engine controls are set up for engine start – Master switch “on”, boost pump “on”, Fuel on-off valve “on” and oil valve “open”. The M-14 likes a lot of prime fuel for a cold start, so the spring loaded electric prime switch is held in the “on” position for about 12 seconds. (Some start checklists I have seen recommend priming the engine, then pulling the prop through several blades in order to distribute then prime charge in the cylinders. I have to tell you that I am very uncomfortable with this procedure, so I don’t use it, as it is possible for a broken mag lead or other electrical fault to allow the engine to fire, with potentially disastrous results.) Throttle open about 1 inch, and engage the spring loaded start switch, which energizes the starting booster coil and also enables the air start system to turn the engine over – one or two blades at most and the engine fires. Once the engine has fired and is turning over, flip the mag switches “on”, let off the start switch, and be ready to tickle the prime switch for a few seconds, until the engine is running smoothly. Check the oil pressure, and we are ready to taxi.

Taxi Ops:

Taxi is easily accomplished by using a combination of differential braking and tailwheel steering. For tight turns, hard application of a brake allows the tailwheel to enter the castoring mode. Rolling straight ahead for a few feet straightens the tailwheel, returning it to the steering mode. As expected, visibility straight ahead over the nose is blocked, but the high seating position and pilot-in-command up-front arrangement, coupled with shallow S-turns and an open canopy, allow for comfortable taxi ops. Brake pedal modulation feels good. And that deep engine rumble during taxi sounds even better with the headset off!

Run-up and Takeoff Checklist:

Minimum oil temp for takeoff is 100 degrees F, and on a cool day it can take 5-10 minutes or so for the 3-5 gallons of oil in the tank to come up to temp. Once the oil temp is above 90 degrees, prop and mags can be checked. Mags are checked at 2000 RPM, prop is cycled at 1800 RPM, (with the stick held back!). Check temps and pressures, carb heat, controls, instruments and avionics, fuel, flaps (15 degrees down), trims, canopy latched, belts and harnesses secure, and we are ready to go. Cylinder heads will be as warm as they are going to get, on the ground, by now. Cylinder head and oil temps can be regulated by adjusting the large cowl flap. On all but a fairly hot day, the cowl flap is usually kept closed, even for climb.

Takeoff and Climb:

Now the fun begins for sure! My usual takeoff procedure is to line up on the runway and smoothly bring in the power on a five-count. Acceleration is awesome, and the thrust from the prop combined with the steerable tailwheel gives immediate and positive rudder control. Depending upon load in the back seat and aft baggage compartment, the tail can be raised to the level attitude early on in the takeoff roll, but doing that is almost wasted motion, as the plane will happily fly off in the tail low attitude quite quickly – at the 6-9 second mark, depending again, upon load.

As soon as the plane lifts off, I retract the takeoff flaps – acceleration as the plane becomes airborne continues to be very positive and there is no tendency for the plane to sink .

Best rate of climb speed, Vy, is 120 mph IAS, which can produce solo-weight climb rates approaching 4000 fpm, with a hefty deck angle to match. Cruise climbing at 140+ mph IAS still gives plenty of climb rate, and better forward visibility.

Cruise / Aerobatics:

Cruise power selection results in true air speeds ranging from 190-230 mph, with associated fuel flows of 11-17 gph. A notable aspect of takeoff and cruise in the Radial Rocket is the absence of the noise level usually associated with these phases of flight. The deep exhaust rumble, combined with prop speeds of only 1900 rpm at takeoff and 1300 rpm or so in cruise, result in a very pleasant sound sensation.

Stick forces for cruise and aerobatic flight are precise, with positive force gradients. You won’t wear yourself out flying the Radial Rocket. Aerobatics are a comfortable one-handed operation. Roll rates are in the 150 degrees per second range – fast enough to be sporty, but not so quick as to make cross country flying a chore. Ailerons have positive centering feel, but with breakout forces that compliment the aerobatic capability of the Radial Rocket.

Going uphill in over-the-top maneuvers is no problem, given the thrust output of the 98 inch diameter prop, which can also serve as an awesome speed brake when coming downhill on the backside of vertical maneuvers – a welcome capability given the cleanliness of the Radial Rocket airframe and its ability to pick up speed, especially when headed downhill.

Descent / Landing:

Although speed can build quickly in a descent, increasing prop rpm and reducing manifold pressure will combine with that big prop disc to produce effective deceleration as we approach the pattern. Flaps can come out at 140 mph IAS, and I usually select full flaps at the midfield downwind point in the pattern, resulting in 100 mph IAS opposite the numbers, with about 15 inches of MP. Speed can be further reduced on final – you can hang the plane on the prop with all that power and thrust available – but I usually stay at 95-100 mph IAS and let speed fall off during the round-out for a wheel landing. I prefer to wheel land the Radial Rocket, which gives great over the nose visibility. Once the mains are on, reducing throttle to idle brings that big prop disc and its drag into play, resulting in a short landing roll. The horizontal tail remains quite effective during roll-out, allowing for a gentle letdown onto the tailwheel. Clear the runway, flaps up, cowl flap set as needed, canopy open if desired, and back to the ramp.

Shutdown:

When parked on the ramp, the engine is run at1200-1500 rpm for 30 seconds to help fully scavenge excess oil from the oil sump. Then throttle to idle and mags switched to off. At this point, the three component clean kit / hydraulic lock prevention system comes into play: First, the oil shutoff valve, located between the oil tank and the engine, is closed using the cable control in the cockpit, thus preventing the seepage of oil from the tank to the engine sump, and past the rings and into the lower cylinders. Next, the electric scavenge pump is run for a minute or two to pump any remaining oil in the engine sump back into the oil tank. Then, upon exiting the aircraft (don’t forget to shut off all switches and close the main fuel valve), the quick-drain on the drain manifold connected to the lower three engine cylinders is opened, in order to prevent oil from accumulating in the intake manifold pipes connected to those cylinders.

I hope you enjoyed this look at a flight in the Radial Rocket – It is a distinctive aircraft, a superb performer, and a lot of fun to fly. As I walk across the ramp after a flight I can never resist the urge to turn around and take another look at this awesome aircraft!

Radial Rocket TD Pilot Report #2: Cruising at Altitude – The High Teens.

By Jeff Ackland
A question pilots ask me fairly often is: “What is the service ceiling of the Radial Rocket?” My reply to date has been: ” 20,000 feet MSL or better”, based upon how the Radial Rocket performs at lower altitudes. On various cross country flights from our home base in the Kansas City area to Florida, Oshkosh, and other locations, we have routinely cruised at 10,000-13,000 ft MSL altitudes with plenty of performance to spare, as well as excellent fuel economy.

Knowing that we will be taking the Radial Rocket to the West coast at some point in the future, I was curious to see how the plane performed at altitudes up to 17,500 feet MSL – plenty high enough to clear any mountain in the continental US, and as high as you can fly VFR. Specifically, I wanted to gather data with regard to speed and economy at various altitudes between 12,000 and 17,500 feet MSL

Another item I was curious about was the performance of the auto-mixture feature of the carb on the M-14P, 360 horsepower engine installed in our Radial Rocket – would it perform properly and keep the EGT’s at a satisfactory level? In addition I wanted to see if the un-pressurized mags would function properly at altitude. The mags on this engine throw a very hot spark, so I wanted to check for any tendancy to cross-fire at altitude.

With these thoughts in mind it was time to load up the portable oxygen bottle and head upstairs! My plan was to climb directly to 17,500 feet MSL, noting temps, pressures, climb rates and fuel flow along the way. On the descent, I would level off at various altitudes and note cruise performance numbers.

The flight was performed mid-morning in early May. On this particular day, density altitude here at KIXD was about 2300 ft (field elevation: 1050 feet). Take-off weight with pilot, fuel, and miscellaneous baggage items was 2300 pounds (gross weight is 2550 pounds).

As opposed to a max performance climb, hanging on the prop, I wanted to simulate the typical cross country departure and climb to altitude, so after lift-off I retracted take-off flaps, set power to full throttle and 2450 RPM (max continuous), and began the climb at 135 MPH IAS (a mid-range cruise climb – best rate speed is more like 120 MPH). At this speed, the cowl flaps were closed, fuel flow was 23 GPH, and climb rate was 2100 FPM (max rate of climb available at Vy easily exceeded 3000 fpm). As the climb progressed, I gradually reduced speed to arrive at 17,500 feet MSL at a speed of 110 MPH IAS. Time to climb, from field elevation to 17,500 feet MSL, under these conditions, was right at 15 minutes. Observed manifold pressure at level-off was 18″ Hg, with a corresponding fuel flow of 8.8 GPH. Due to warmer than standard temps at altitude, 17,500 feet MSL was actually 18,500 feet density altitude. Rate of climb at this altitude was still a healthy 700 FPM.

Leveling off for cruise at a density altitude of 18,500 feet resulted in a TAS of 210 MPH (183 KTS) on the aforementioned fuel flow of 8.8 GPH. Not Bad!

Time to descend and check some more cruise speeds:

At 15,000 feet MSL, full throttle (20″ Hg) and 2450 RPM resulted in a TAS of 222 MPH (193 KTS) on 15.6 GPH fuel flow.

At 12,500 feet MSL, full throttle and 2450 RPM resulted in a TAS of 226 MPH (197 KTS) on 15.8 GPH. Pulling the prop back to 2050 RPM at this altitude gave 214 MPH (186 KTS) on 12.4 GPH fuel flow.

Conclusions:

Observed EGT’s for the flight were in the 1300-1400 degrees F range. The engine ran smoothly with no hint of carb or ignition issues. The cowl flap remained closed for the entire flight, with all temps and pressures comfortably in the green at the top of the climb. The Radial Rocket undoubtedly has excellent altitude capability with the 360 horsepower engine – service ceiling with this powerplant is well above 20,000 feet. The 400 horsepower “PF” version of engine would provide even more climb and cruise performance at altitude!

The aerodynamically clean Radial Rocket airframe combines excellent load carrying capability and comfort with very economical high speed performance, especially at altitude.