Andy Sephton concludes his flight test report on the Razorback Mustang from VQ Models, the assembly of which was described in the July issue.
The story continues; in the first part of this review the VQ Mustang had been successfully flown and photographed. It has a solid feel in the air, and I found it easy to fly.
However, the route to success was a challenging one, with few of the issues directly related to the model itself. I’ll start at the beginning…
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POST LOCKDOWN CHECKS
With lockdown over, a good strip available with the Ivel Model Club in Bedfordshire, a photographer in the form of Kevin Wallace and a good weather forecast, it was time to go flying. A quick check over of the model produced the first stopper; some of the servos had come loose. I tried to fix the problem with cyano in the screw holes to stiffen up the wood, followed by plugging the holes and re-drilling them, but neither worked. In the end I opted to glue 1.5mm ply plates at each end of the servo mounting holes and screw the servos down on to those. Several flying sessions later the fix has held well.
I also double checked the Centre of Gravity (C of G) and control throws. All were set as recommended in the instructions. In order to get the correct C of G, I had to move the LiPo back to its most rearward position. This put it close to the servos, so I glued a bulkhead across the fuselage between the servos and the LiPo to prevent any rearward movement of the battery pack.
The only other minor issue I had with preparing the model for flight was access to the wing mounting bolts. The hole positions moulded into the radiator fairing for the wing bolts didn’t line up. In the end, I had to expand them forwards by about 2cm.
It was now time to fly. On the chosen day the weather was just about perfect, with 10 knots of wind down the strip and puffy clouds at about 2000ft. The model was assembled at the strip and range and failsafe checks were carried out satisfactorily. Control throws were also checked and confirmed to be those recommended in the instructions.
Taxy tests came next. Power was limited to about a third throttle and the elevator was set fully up. The tail wheel has a lot of play in it, but the model could be steered easily and adequately on the ground with no hint of nosing over.
I walked the model out onto the strip and, standing behind it, lined it up into wind. The throttle was opened slowly to full. There was a slight swing to the left as the tail came off the ground, but it was easily contained by rudder. The climb out was straight, and the model was just about in trim. On throttling back all it needed was slight right rudder and slight up elevator. Control in pitch and yaw was good but I felt the ailerons were a bit sensitive. Nevertheless, I left the throws as set for the remainder of the test flights.
Having now got used to the set-up, I am of the opinion that all the controls are well set and well harmonised. The only change I have made is to move the C of G about 1cm forward from the design position; see later for an explanation. I then made a check of the stability by putting in small control deflections and letting go of the sticks to see how the model behaved. In all cases, it appeared that the stability was satisfactory.
With the basic handling tests successfully completed, I decided to try the stall. Again, there were no surprises. With the throttle at idle, the nose dropped cleanly at the point of stall, with no hint of a wing drop. With this knowledge in hand, I was then ready to try an approach.
I flew the model onto finals and lined her up nicely with the strip. The approach was a little fast, so I held off to let the speed drop and eventually carried out a light touch down for a tail-down wheeler landing. However, the undercarriage collapsed immediately, and the model came to rest on its belly. The steel pins connecting the undercarriage retract mechanism to the undercarriage legs had bent back; it would appear that they were too soft.
A quick report to MacGregor was sent via our Editor and by return of post two sets of new pins arrived in the post. In the event, they proved to be of too large a diameter to fit the undercarriage, so I substituted home-made pins from some good quality spring steel. I’d left the undercarriage down for Flight One just in case either the take-off or landing altered the geometry of the system, which may prevent retraction. Obviously, I couldn’t get any sensible data from that flight, so a repeat with the new pins was required on Flight Two.
There then followed what I would call a rooky error. We arrived at the strip, the weather was good, the model assembled, and the controls checked. The port aileron didn’t work! I’d failed to put a keeper on the extension lead from the servo, which was now inside the wing. The session was cancelled, and the problem rectified in the workshop.
As supplied the model has threads in the wings for pulling the servo leads through. These were now lost, so I had to figure out another method for getting the servo lead through the wing. In the event, a small weight on the end of a length of thread proved to be just the ticket. The holes in the ribs were more than adequate to give a free path through for the weighted thread and, subsequently, the leads.
We were now ready for attempt two at Flight Two. Again, we had good weather and a 10-knot wind, but this time it was from the north, putting it across our strip. The pre-flight checks were carried out and, again, were satisfactory.
The taxy and take off were fine and there were no surprises with the stability of the model. It was time to try out a landing and that is where disaster struck. While turning onto the final approach the model dropped a wing and appeared to spin in. Colleagues who witnessed the event said that it looked just like classic radio failure. But having had only two other failures in nearly 50 years of R/C flying I was reluctant to agree.
On the other hand, I had had plenty of speed in hand and the stall characteristics were proven to be benign, with no hint of a wing drop.
There could have been some curl over from the hedge just short of the strip, but that was all. There was no aerodynamic explanation for the departure, so maybe it was the radio after all.
Just to be sure for the next flight, I installed my trusty Futaba T7c with a 7ch receiver. I’d lost the telemetry from the previous installation, but I was now confident that the R/C gear was reliable.
The damage proved to be light and was limited to a broken prop, bent undercarriage (again!), several holes in the covering and slight damage to one side of the tailplane. The damage was repaired, the undercarriage pins replaced and the spare sheets of covering material supplied with the model were put to good use. We were now ready for Flight Three.
Thinking about Flights One and Two, I perceived a slight sensitivity in pitch. The standard fix would be to move the C of G forwards, so that’s what I did. For Flight Three, I moved the LiPo to its most forward position, which moved the C of G about 1cm forwards.
We now waited for the weather, my availability (I’d recently gone back to work) and my photographer’s availability (he’d recently gone back to work, too). The day arrived, the sun was shining, the wind was light and variable, and there was hardly a cloud in the sky.
Two successful flights were carried out, with satisfactory undercarriage retraction and extension in flight. Following the repairs, the aircraft was still in trim, with slight up elevator and slight right rudder required on the trims. The aileron sensitivity wasn’t noticeable. Whether or not the C of G changed worked is a moot point, but the pitch handling appeared to be a lot better. Both landings were on three points and both rollouts were straight. During the post flight inspection, however, the undercarriage appeared to be slightly collapsed.
Overall, the model proved to be easy to assemble, finish, set-up, fly, trim and land. The aircraft looks pretty good in the air, too. The issues I had seen have been mainly due to the retractable undercarriage system chosen and the radio system first fitted.
The aim with the model was to enter it in BMFA Light Scale. I still intend to do this, but I write this article the day before the final BMFA Scale competition of 2021 and, in this case, I’m judging rather than flying.
Also, the model needs some work before it is competition fit.
Winter work will be centred on finding a better solution for the retract system. The first pass will be to drill out the retract actuators and the undercarriage legs to receive a thicker, and hopefully stronger, joining pin. If that doesn’t work, then I’ll need to research a new retract system.
Either way, it’ll give me something to add to my winter build programme. I also need to bottom out the radio issue and more research is required in that area.
Having said that, I believe the model will still score well in BMFA Scale competition as it is a real picture in the air.
Name: P-51B Mustang ‘Berlin Express’
Model Type: WWII warbird
Manufactured by: VQ Models
Distributed by: MacGregor Industries
RRP: Options start at £199.95
Wingspan: 60in (1524mm)
Length: 46.8in (1189mm)
Weight: 2.72 – 2.94kg (6 – 6.5lbs)
Power system: Electrospeed Boost 60 Power Pack
Functions (servos): Ailerons (2), rudder (1), elevator (1), throttle (1 or ESC)
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