SPRITE: PART 2

Sprite gets airborne…

Tony Barber looks at the Practavia Sprite, part two – the problem with gravity…

eight and balance turned out to be

W‘interesting’ with this aircraft and, in hindsight, we should have weighed it before it was dismantled and resprayed… but we didn’t! The weight and balance schedule from 1991 was rather tight for a two-seat aircraft – probably two super light people could take off and land with about one hour of fl ight – but when we came to the weighing after the rebuild, odd looks were exchanged.

Unexpectedly heavy

We weighed ‘VF with empty tanks and the results were unexpected and more than a little disappointing; she had gained 72lb (32kg), that’s equivalent to the weight of 45 litres of fuel. We were expecting the replacement Above Tony says landing at the Rally with his son, was a great experience.

Photo: Nigel Hitchman

Continental O300-D to burn around 33 litres an hour, so that’s about 1.5 hours of fl ying gone. The newer O300-D engine was supposedly 11lb heavier than the C125, plus around 4lb for the addition of the vacuum pump. We had replaced a lot of DIY hardware (nuts and bolts) with aviation grade which may have accounted for a couple of pounds in weight. We had removed three layers of paint and added an undercoat and two-pack topcoat, theoretically not adding any weight. The prop should have been very similar and the prop extender slightly lighter. A new radio was, if anything, lighter than the very old one we removed and other than that, there were no differences, so the jump in weight was something of a mystery. We concluded that apart from the old weights being erroneous, she had added a few pounds over the years.

Weight loss ideas

Removing weight from an aircraft is notoriously diffi cult. The options as I saw it were: • Change from metal to wooden prop (£1,700 + vat). • Change the 14lb generator to a 3lb lightweight alternator (approx. £1,000 incl. fi tting). • Replace the heavy original starter with a lightweight model such as from B&C (about £1,000). • Investigate an as yet unapproved lithium battery (approx. £1,000). My wallet didn’t like any of these options so, during March 2017, I became an expert at spreadsheets as I looked at all sorts of options. I had also spoken to LAA Engineering about the weight differences hoping we would be able to revisit this later, with them showing some interest in test fl ying this Sprite to evaluate the longitudinal stability compared to the other fl ying Sprite.

Different weight limits

One other oddity I found, which didn’t have any explanation, was a different maximum landing weight to MAUW for take-off. There was a 20-year-old letter in the notes where the then PFA engineering team had granted an extension to the MTOM but not to the maximum landing mass (MLM). However, G-BCWH, the other fl ying example, actually had a far higher MAUW and no landing restrictions. It did have a different undercarriage though. With its current limitations, ‘VF was not really a two-seat aircraft. I could probably take-off with two superlight people (which is not me) and limited fuel, but I could not land. Posing the question to Engineering, I found another unexpected answer. Francis’ encyclopaedic memory recalled that the other example had been granted a MAUW increase because it also had previously been fl ying with undeclared weight. It also had a modifi cation to improve the longitudinal stability following an extension of the rearward CofG. I wondered if we could do the same with ‘VF. Calculations on VF’s new schedule showed that she was sensitive to loading, and easy to fi nd yourself exceeding the rearward CofG with two up and low fuel (ignoring for a moment that you could not legally land back). Furthermore, the forward CofG could be (only just) exceeded with a light solo pilot and full fuel. But for now, that would have to wait. She needed to fl y. In early 2017 I at last had the chance to sit in her and fi re the engine up myself, it started easily and ran smoothly. Ground runs showed a mag drop and given all we had previously found, I suppose we should not have been surprised to fi nd that the key was wired the wrong way around. Once this was resolved a faulty plug was diagnosed, which was the best possible problem really. A new plug resulted in the engine running super smoothly and sounding lovely. It is said in building circles that the aircraft won’t fl y until the weight of paperwork matches the weight of the aircraft. Fortunately for me, given the weight of the Sprite, this wasn’t true, or we would have been guilty of deforestation to an extreme degree!

The Sprite fl ies!

With power plant modifi cation forms approved, it was time to test fl y and my Inspector, Carl, was keen to be the fi rst to test his work. Given my previous experience with an emergency landing and all the changes we had made, I was also very keen for him to do the fi rst fl ight. Come the big day we almost didn’t make it. The fl ap lever proved troublesome with a nylon bush in the mechanism sticking in the outer lever, meaning the fl aps could not be locked into position consistently. A couple of hours’ work to remove it and a rub down with some wet and dry emery cloth did the trick. All this meant it was late in the day, but there was still time, and armed with a bit of ballast to bring the CofG into range, I watched as Carl taxied off to the end of the runway to do the power checks. With the video camera in hand, we then watched as he took off into the blue and white evening sky. It was a fairly nervous time but after 20 minutes he returned safely, the only minor concern being the oil temperature gauge showing around 100° and just off the scale. I knew this was not a problem as the scale ended at 100 and the previous owner had told me it was linear after that point, with 105° being the usual temperature. However, for the next few fl ights we kept a cautious eye on the temperature, but it never rose above 105°, well below the red line. A few weeks later I fi nally got to fl y the new look ‘VF myself. I remember the hesitancy lining up on the runway in an aircraft still new to me, with a chequered history and a newly overhauled engine that had seen little use in the last few years. But I had nothing to worry about because she fl ew very nicely, and I returned with a broad smile on my face.

Above Emptying the tanks prior to the weighing.

Resolving the W&B issues

So, back to the next phase, fi nally resolving the weight and balance. It was quite clear that as it stood this was legally only a single seat aircraft. What could be done? It was time to revisit the changes made to the other Sprite and see if ‘VF could benefi t from them. The LAA’s modifi cation principle often works on the premise that if something has been done successfully before, then there is a good chance the change might be approved for other examples of the type. This entailed fi nding out more information about the modifi cations approved for G-BCWH and submitting a MOD proposal seeking the same weight and CofG limits that she had. Naively I thought this bit might be easy, based on the premise of proven changes, but it was not to be… The process of change was initiated by a MOD proposal request, which resulted in a visit to LAA HQ for a discussion of the problem, which led to more application

forms and eventually a certifi cate to test fl y at a higher weight and increased forward CofG. The test fl ying was to be carried out by a combination of my Inspector and myself, but it was over a frustratingly long period of time. A mixture of weather, availability and crucially, aircraft airworthiness and permit lapses, conspired to draw the process out to over a year, but we got there in the end. The results, I thought, were fi ne other than the stall speed, which was slightly higher than I would have liked. The stall characteristics also displayed very little pre-stall buffet and a slight wing drop.

Vortex generators

Chatting this through with Francis, we thought it would be worth seeing if vortex generators would decrease the stall speed. She already had stall strips at the wing roots, which are supposed to improve approaching stall awareness and I subsequently added a set of Stolspeed wing vortex generators. We then reran all of the fl ight tests, and the results were positive. The stall speed was reduced by around 2-3kt but, more interestingly, I felt that the slow speed handling had become more ‘docile’, the VGs doing exactly what they are advertised to do. At this stage I thought the results were acceptable and not enough to warrant major concern from the LAA design team, after all the concept was proven by G-BCWH, and our engine had an extra 20hp than her. However, our LAA team was very measured and cautious in its response, being rightly concerned about making changes to the aircraft’s operating envelope, especially in light of the history and the stall characteristics I had reported. After a few months of ponderous consideration and a bit of prompting, I was invited to fi t a stall warning device and to have the aircraft evaluated by the LAA Chief Test Pilot, Dan Griffi th.

Dan fl ies the Sprite

This entailed more hoops to jump through, but they made sense and I did feel that it would be a very positive move to get a thorough evaluation by a pilot vastly more experienced than myself. The stall warner was quickly fi tted, and contact made with Dan. The fi rst test fl ight was carried out in October 2020, just before the November Covid lockdown. You know what’s coming now – it took another four and half months before we got through the 2021 lockdown and Dan was able to match his schedule with mine. And so it was that on one sunny day in April 2021, after endless delays caused by the usual combination of weather, work, life, family, holidays, pilot availability, aircraft serviceability, and ultimately the pandemic, we gathered at Spanhoe Airfi eld to watch Dan methodically go through his checks and fl ight plan. Engineering had requested a handling fl ight test to verify our request for increased MAUW and a onecentimetre extension of the aft centre of gravity limit. Dan had fl own a general handling test at the forward CofG in 2020, but for the above reasons had not been able to complete the full test schedule. So here we were, a 10kg lead weight attached to the rear tie-down, about to send our intrepid test pilot off to investigate incipient spins in an aircraft that is not cleared for said manoeuvres. I can tell you that watching Dan depart into the blue yonder was one of the most nerve-racking moments of my fl ying life. I knew my pride and joy could not have been in better hands and I was buoyed by the knowledge that he was wearing his parachute. Spins in an aircraft, which has not been previously spun, could be fraught with danger. Seeing ‘VF returning to the fi eld was a massive relief. Subconsciously I had this vision of her returning like a shot-up Hurricane after a Battle of Britain dogfi ght with bits hanging off, but thankfully nothing was further from the truth.

Above The O-300 engine, slightly heavier but more powerful.

Success at last

Dan landed and all was good. Behaviour in both the stall and incipient spin was faultless and she did not even need full anti-spin control to stop the autorotation, just a centring of the stick. “Excellent, I do hope this will keep Engineering happy,” I thought – and indeed it did. The design team was satisfi ed with the results, at least suffi ciently to grant ‘VF a new set of limitations, increasing her MAUW to match those of the other fl ying Sprite. The request for slightly extended CofG forward and aft were considered safe by the fl ight testing and subsequent calculations and ‘VF is now cleared to take-off and (importantly) land at 794kg. She can now legally carry two reasonably average adults and enough fuel for two hours, or one person for 3.5 hours. I consider that we have come away with a good result. I am now the proud owner of a very rare British designed aircraft, one that I have brought back to life again. Knowing that there are currently only two of them fl ying in the UK makes me feel quite protective of the old girl.

I try now to keep it hangared all year round and have had some great covers made by Dave’s Custom Covers, which I can highly recommend.

Satisfactory performance

How does she fl y? It’s not a STOL aircraft for sure, and you need to be aware of TODA, temperature, loading and headwind. With no wind at MAUW we try to keep to 700m of tarmac and 800m of grass. Once airborne though, the rate of climb is around 500fpm at MAUW, which is fairly standard for the class – the Robins that I regularly instruct in struggle to make this on warm days two up. Cruise is around 100kt, although she looks like she should be faster – the wide cockpit and spatless fi xed undercarriage probably play a part in that. With small wings and a high-wing loading you can feel the lumps and bumps on a thermic day, but that is no different to the RV range. I would consider her sensitive in pitch, fairly sensitive in yaw and on the heavy side in roll, but she has no real vices. Visibility out of the large domed canopy is excellent and the cockpit is the widest I have ever seen in a two-seat side-by-side aircraft. Landing is fairly normal, though you do need to keep some power right to the end as she will drop quickly if power is reduced too early. All in all, she is very acceptable and conventional.

Would I do it all again?

It has defi nitely been a hugely interesting and rewarding project, I’ve met some great people and spent too much money. I have also learned so much about airworthiness and light aviation. So yes, I probably would do it again for the experience, but I admit there were days when I thought what the heck am I doing, especially as the bills rolled in. As my Inspector said to me, ‘aviation costs are inversely proportional to the size of the component’. I can vouch for that, as tiny packages arrived from suppliers following huge invoices.

Above Back after a successful fi rst fl ight.

Below 10kg ballast for Dan Griffi th to test permissible rearward CoG increase.

Advice to consider

What advice would I give to others? The best advice I can offer anyone tempted to take on a fi rst major rebuild project is to get the thing inspected before you take it on. I did and went into it with my eyes mostly open, however, I underestimated the work required and the challenges of even the small issues. Don’t assume that everyone else will have the same high standards as yourself. I know that the LAA permit scheme is about affordable innovation but some of the things we found were questionable, so do a full renovation and then you will know what you are sitting behind, and you will have a greater level of trust in it. Finally, get a great Inspector or engineer, this relationship is critical. I knew I had found the right person when I fi rst told Carl what the aeroplane was. One of the fi rst things he said to me was… ‘that’s the one with a Jet Provost canopy isn’t it?’. Remember this was an aircraft designed in the 1970s and there are only two that are fl ying, with maybe three others having seen fl ight in the UK. None had been built since 1978, so how many people would have known that! If you do take a project on, take a deep breath, put your hand deep into your pocket and bring back to life something that deserves to be cared for. Good luck. ■