Vd Speed Question
Fleet Manager
Joined: Aug 2006
Aviation Qualifications: CPL
Posts: 7,089
Likes: 2,952
From: Ontario, Canada
Seeing as we thread drifted into resonance, flutter, and flying through it, it is perhaps relevent that I relate one of my flight test experiences:
I was flight testing a Bellanca Scout with a Hoffmann four blade prop of reduced diameter (for quiet) replacing the big two blade metal neighbour annoyer.
While flying the aircraft from the front seat, things were fine. When I went for a ride in the back seat for a glider tow, I could feel a buzz in my butt. I looked all around, and noticed that the horizontal stab was either fluttering or resonating. I more think it was the latter, as we were flying at glider towing speed, (not even close to Vd!). This turned out to be to some degree associated with the prop, and it's different mass distribution. When we flew at lower or higher speeds, it was much better. Power setting did not affect it as much. When I flew the same aircraft with the original metal prop, it was fine. I found a CAA flight test report for a Citabria which presented the same observations, when a similar prop was flown. There are harmonics at work in that airframe, that go beyond a basic understanding.
Needless to say, I did not approve that Propeller on the Scout. Hoffmann later sent a solid (fixed pitch) propeller with the same geometery. When I cautiously test flew it, there was a marked improvement, and I did approve that installation.
Here's a video clip of the tail...
Video of Scout tail resonance - Photobucket - Video and Image Hosting
As for the original topic, Transport Canada will only allow me to approve Vne as 90% of the IAS to which I have flown (Vd IAS = Vne IAS x 1.1). When I test flew a Twin Otter for a nose boom installation, the 170 published Vne corresponded to a 187 KIAS Vd, which what I flew, with the shaker going at the front of the boom.
Pilot DAR
I was flight testing a Bellanca Scout with a Hoffmann four blade prop of reduced diameter (for quiet) replacing the big two blade metal neighbour annoyer.
While flying the aircraft from the front seat, things were fine. When I went for a ride in the back seat for a glider tow, I could feel a buzz in my butt. I looked all around, and noticed that the horizontal stab was either fluttering or resonating. I more think it was the latter, as we were flying at glider towing speed, (not even close to Vd!). This turned out to be to some degree associated with the prop, and it's different mass distribution. When we flew at lower or higher speeds, it was much better. Power setting did not affect it as much. When I flew the same aircraft with the original metal prop, it was fine. I found a CAA flight test report for a Citabria which presented the same observations, when a similar prop was flown. There are harmonics at work in that airframe, that go beyond a basic understanding.
Needless to say, I did not approve that Propeller on the Scout. Hoffmann later sent a solid (fixed pitch) propeller with the same geometery. When I cautiously test flew it, there was a marked improvement, and I did approve that installation.
Here's a video clip of the tail...
Video of Scout tail resonance - Photobucket - Video and Image Hosting
As for the original topic, Transport Canada will only allow me to approve Vne as 90% of the IAS to which I have flown (Vd IAS = Vne IAS x 1.1). When I test flew a Twin Otter for a nose boom installation, the 170 published Vne corresponded to a 187 KIAS Vd, which what I flew, with the shaker going at the front of the boom.
Pilot DAR
