Warbird Information Exchange

A thrust line offset is often lateral, a cant left or right but can be up or down as well. The cant is fixed but the forces on the airplane change with thrust; the resulting changes in torque/gyroscopic effect, spiral slipstream, and p-factor. Randy explained the effect of speed on pitch trim. Another reason for change in pitch trim is change of center of gravity. Wartime Mustangs had different pitch characteristics when the fuselage tank contained fuel; that's the large example. A more subtle example would be any tandem airplane pilot noting different trim settings, stick forces, or simply turning around and visually noting a slight change in elevator position when there is a rear seat passenger or change in load as compared to an empty airplane. Another reason is configuration change; extending flaps. Either way, the pilot finds a "new neutral" given the speed, configuration, and CG that day. For that reason I don't think any thrust line cant (fixed) could cause a greater effect than already provided for such as a common CG or configuration shift (dynamic).

Designers take all this into account when establishing the pitch trim range and pitch trim neutral. In general, wings provide lift and horizontal tails provide down force. The tails "negative" lift generates induced drag of its own - made greater when out of streamline. So,for efficiency, designers ideally install the horizontal tail with an angle of incidence to allow for streamlined elevators and trim in the heart of the operating envelope. Seems like this was one reason the handling of the F-86 improved so greatly when the flying horizontal tail was introduced; the tail could be positioned optimally for varying speed regimes. When we talk about the large forces involved with trimming a Reno racer nose down, that illustrates how the course speed differs greatly with the normal envelope of a stock Mustang. Without a flying tail, neutral trim will only be ideal in one speed/CG band; it will take unwanted deflection to go either really slow or really fast.

I'm on the record as being against speculation but a technical discussion is certainly fine with me whether it's on aerodynamics or paint. This thread has become too large to focus on just one thing, but I do hope that our top concern remains keeping this particularly respectful given the trauma that so many people are still going through.

Ken

P-51 doesn't have a L or R offset to the engine. The prop shaft is canted down 1 degree 45 sec on a stock P-51B,C,D or K when if left the factory.
Horizontal has 2 versions, early has a 2 degree positive angle and the late has a 1/2 degree positive angle.
I don't know how GG was set up so I'm not speculating on their setup.
If flying with a 2 degree horizontal then the down thrust line combined with an upward angle of the horizontal (lifting the tail) would be designed to lower the nose to fight the rising nose of an accelerating aircraft, at least to some degree. Because this relationship is fixed then the influence of the tail lifting force will vary as the speed varies I would guess.
One method of determining how these different racers were set up would be to examine the photos of them flying the straightaways on the course and look to see what trim position they have.
Racers have been known to run more toward the rear of the CG range as the weight the tail carries is on a smaller surface and induces less drag.

Just seen a post on the Furias FB page that claims someone clocked GG at 497mph when it passed RareBear, but on previous passes had been 480mph.

Just seen a post on the Furias FB page that claims someone clocked GG at 497mph when it passed RareBear, but on previous passes had been 480mph.

Since a growling stomach half an hour earlier was the only reason I wasn't in a position to get footage like that, I have asked myself what I think I'd have done. I'd like to think I'd have kept shooting. What's the point in running when you only have time for half a step?

August

To you guys that were thrust into this horror take care of yourselves and your buddies. And remember there is NO harm in seeking help to cope if you need it!

The problem with that, James, is that at speed an aircraft has nose DOWN trim, not nose UP. Speed increases, nose down trim increases.

If the racers were set up for "race speed trim" as they were entering the course, that would require putting back pressure on the stick to keep it in level flight, with the back pressure decreasing as speed increased...not the other way around, as indicated in this passage.

I understand the idea of a little back pressure trim to assist with the control forces while making the turns, but this, too, would be a comparatively nose down trim condition while they were forming up for the start.

For the airplane to be somewhat manageable in the straight stretches when not in a bank, it would still require a significant amount of nose down trim when compared to "neutral".

I am going to change the topic for a minute.

It is clear that several posting here were witness to this event and the carnage that followed. I have been a paramedic for more than 20 years and while I am trained to deal with emergencies and even mass casualty events I can think of few things I have faced that would have come to this level. We are trained and we expect to see what we see. In this case it was not expected and people were not prepared.

That being said I am sure many who were involved and might not have been injured are finding it may be difficult to cope with what they experienced. I would suggest if you are having these issues to read up on post traumatic stress disorder or PTSD. If you have access to a employee assistance program take advantage of it. If not ask around to see if Fire/EMS or even the race organizers are offering any critical incident stress management. If so take advantage of that.

I know in EMS we all like to think of ourselves as impervious to what we see and what we deal with day to day but the reality is that we are all human and there are just somethings we are faced with that go well beyond our coping mechanisms. There is nothing wrong with seeking help to cope with this event. Sometimes just sitting down and talking it out does worlds of good and sometimes you just need more than that.

To you guys that were thrust into this horror take care of yourselves and your buddies. And remember there is NO harm in seeking help to cope if you need it!

I can remember reading an interview with Kevin Eldridge (SP?) who had to bail out of the Super Corsair, IIRC he said that the Super Corsair had its non adjustable trim set to run level at 450 mph, letting go of the stick when trying to get out at 200mph the Corsair just wanted to nose down, after he got out, the Corsair went into a dive and as the speed increased it started to 'pull up' by its self.

From a print section-

But after the abrupt pitch up, the plane went ballistic... and inverted

I am only in my 20s but someday I will fly it at airshows. I am getting rich really fast writing software and so I can afford to do really stupid things like put all my money into warbirds.