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I was asked an interesting question on the VAF WWW Forum:

Kevin,I'm building an RV-8 also. I was perusing your site the other day when I found your POH. I downloaded it, and then I noticed that you had increased the max T/O wt. to 1900 from 1800. I pondered this and wondered if maybe the MOT would have something to say about it. Obviously you have though this through so I wondered if you could elucidate on this point a little for the unwashed.Thanks, Pete

I decided to look into a higher declared gross weight as I will have a heavish aircraft. I've got an RV-8, with IO-360-A1B6, Hartzell prop and an IFR panel. Van's RV-8A demonstator, with a similar engine and prop, and a VFR panel, has an empty weight of 1127 lb (Ref CAFE APR on the RV-8A). The RV-8 is reputed to be heavier than the -8A, due to those heavy gear legs, and the landing gear boxes. I've also got a Christen inverted oil system and an IFR panel. So, I'm guessing my aircraft will be around 1140-1150 lb empty. I weigh about 210 with flying gear, and I've got a good friend who weighs a fair bit more than I do. Add full fuel for a cross country, a travelling tool kit, etc, and the 1800 lb gross weight just won't cut it.

Pete raises an interesting point on how Transport Canada may view my planned declared gross weight.

There are several aspects that I will need to discuss with the MDRA inspector. First, there is the question of structural strength. If I had designed the aircraft myself, I could declare any weight I wanted, without having to justify it, as Airworthiness Manual Chapter 549, Amateur-Built Aircraft does not require a structural substantiation. The mandatory inspections are supposed to be for ... workmanship and general serviceability ... (AWM 549.19(a)). If the inspector goes beyond his official mandate to interrogate me about the gross weight being higher than Van's recommendation, I'll explain how I've limited the load factor to ensure the wing bending moment is less than it would be at 6g at Van's recommended aerobatic gross weight. I'll also explain that I've limited operations above 1800 lb to smooth, hard surface runways. That should ensure that landing gear loads are less than they would be than from operations at 1800 lb on grass strips, which Van allows. The only area that I can't easily address is loads on the flaps, as there is nothing else I can limit to trade off against the higher gross weight. So, I'll pay particular attention to the flaps, flap pushrods, torque tube and flap actuator attachments when I do inspections.

I'll do flight testing to determine whether the normal CG limits are acceptable at 1900 lb.

There is one interesting aspect that is uniquely Canadian though - AWM 549.103 has wing loading limits. If the specified wing loading is exceeded, the aircraft is classified as a high performance amateur-built aeroplane. There is a complicated formula for the max allowable wing loading, which uses flap span ratio, flap chord ratio and max flap angle. I measured my flaps, and I come up with a weight of 1808 lb. If the gross weight exceeds 1808 lb, then if jumps into the high performance class. Cracking the threshold for high performance has licensing issues. The fact that the aircraft will be classified as high performance means I'll need to get a Type Rating for the RV-8. This should be a very entertaining trip around the Transport Canada bureaucracy, as no such type rating currently exists. I'll have to ask the manufacturer (i.e. me) to give me some ground training, and I'll need to conduct a qualifying flight, whatever that is. I suspect I'll end up using an 1808 lb gross weight for some time, while the bureaucracy sorts out how to deal with the rules they wrote. I'll post the results on the site, whenever I get to the end of the process.

Note: the threshold of 1808 lb is based on the following data:

  • Wing span of 23 ft (Van's data)
  • Wing area of 110 sq ft (Van's data)
  • Flap span of 57.5 inches (measured)
  • Flap chord of 11.25 inches (measured)
  • Flap travel of 40 degrees (Van's data)

The above data is with the traditional Hoerner style wing tips. Newer aircraft have different shape wing tips, which gives a span of 24 ft and a wing area of 116 sq ft (both numbers from Van's). A gross weight of 1888 lb or higher would make it a high performance aircraft.


Update - 2 May 2005 - I finally sorted this out with a local TC inspector. The latest info is in anew posting.

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