• Welcome aboard HomebuiltAirplanes.com, your destination for connecting with a thriving community of more than 10,000 active members, all passionate about home-built aviation. Dive into our comprehensive repository of knowledge, exchange technical insights, arrange get-togethers, and trade aircrafts/parts with like-minded enthusiasts. Unearth a wide-ranging collection of general and kit plane aviation subjects, enriched with engaging imagery, in-depth technical manuals, and rare archives.

    For a nominal fee of $99.99/year or $12.99/month, you can immerse yourself in this dynamic community and unparalleled treasure-trove of aviation knowledge.

    Embark on your journey now!

    Click Here to Become a Premium Member and Experience Homebuilt Airplanes to the Fullest!

Expect the Unexpected! (Flight testing)

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
T

Tinbuzzard

Many of us are building popular, thoroughly engineered and tested kit designs. When ready for flight testing, we have a good idea of the performance and flight characteristics to expect from them. Others of us will modify a design, or build something uncommon or unique, that might not have much or any information available about how it will fly.

In the latter case, we have to be careful of possible hidden gotchas that might surprise us. Minor seeming changes to an established design can make its behavior drastically different, and new designs are complete unknowns until you test them.

Here is a cautionary tale from the second test flight of my BD-5. This is a high performance aircraft that has bitten many pilots who have fiddled with the design. In my case, the fiddling was raising the stabilator three inches on the fuselage to make replacing the cable control system with a pushrod one easier. This seemed a minor change, and dosn't have any effect on how the plane handles unless you stall it!

On this flight, I was mostly concerned with engine performance and cooling, and in establishing the stall speed so I could set a normal approach speed. After takeoff, I orbited above the field somewhat higher than pattern altitude as I was limited by class C airspace above.
The engine was doing its job without a hint of the vaporlock problem on my first flight. The only problem was a minor one, the gear wouldn't lock up. The gear on the BD is manually operated, so I just kept holding the lever to keep them in the wells. After several trips around the field, I decided to throttle back and slowly decelerate to determine at what speed the plane was about to stall. (remember, I had to let go of the gear handle to work the throttle, and then pull the partially deployed gear back up) At about 82 mph indicated, I started to feel a slight buffeting through the sidestick. I was expecting the stall to occur at about 75, so I continued to slow. At 80 mph, the plane pitched over smoothly, but shockingly fast to 90 deg nose down. I went from slightly nose high above the horizon, to a straight down 90 deg attitude in about one second! There was NO discernable stall break, but I was suddenly staring at a lovely diagram of the airport. THE URGE TO PULL BACK ON THE STICK WAS ABSOLUTLY OVERWHELMING! I froze for an instant, and then let go of everything. I knew that pulling aft stick was the one thing you can't do when stalled, but letting go was all I could command my hand to do at that moment. After an eternity (maybe another second) I batted the stick forward a little with the palm of my hand, then grabbed it and started gingerly pulling out of the dive. I was already going through 140, and accelerating fast as the BD-5 is very clean aerodynamically. The only thing that kept me from overspeeding was the gear popping out when I let go of the controls. After recovering, I flew around the pattern a few more times as I was too rattled to attempt a landing just yet. I finally made it down and fixed the gear problem with a simple turnbuckle adjustment.

I later found out from a very experienced BD pilot that what had happened was a stabilator stall caused by it being mounted high enough on the fuselage to be in the partially stalled wake of the wing root.

Lessons learned from all of this:

1. Don't continue the flight if a problem crops up. Holding the gear lever up distracted me and kept a hand occupied that could have been doing other things.

2. Don't assume that the plane will act like others of its type if you have made changes. It stalled at a higher than expected speed with a sharp negative G pitch down; although the recovery was normal.

3. Have plenty of altitude before doing any stall tests. I only had about 2,000 ft. and used a lot of it.

4. Fly scared! Expect things to happen during flight testing. That's why we admire test pilots! Don't ever get complacent.

Jeff
 
Back
Top