Inadvertently Exploring Flight Envelope Corners

I'm not sure if anyone has actually tested this corner of the RV-7 flight envelope, and I sure hadn't until today, and subsequently learned something new!

I was out for a short flight today, cleared out from under the Puget Sound overcast and found a nice big blue hole south of Enumclaw (south east of Seattle) to climb up and do some light acro at about 4000?. I lined up for a nice easy barrel roll to the left, got the nose up and let myself fall out of the maneuver. I recovered with further roll to the left with nose down about 20 degrees, which in itself isn't a big deal. I have a coolie hat trim switch on the top of the stick, and sometime during the recovery, I inadvertently pushed the trim full nose down. Speed built up relatively quickly to about 220 MPH and with the nose down, high speed and full nose down trim, the stick forces were very high! Stick forces were high enough to actually give an impression that something could be jammed, but that wasn?t the case. This all happened fairly quickly and as soon as I noted the high airspeed, throttle went to idle and prop forward to assist in getting slowed down, all while pulling the stick. I was trying not to pull too hard in order to keep from over-G condition, but pull hard enough to get some airspeed bled off, arrest the dive, and avoid a close encounter with some big trees. I finally got the airplane returned to level from the nose down pitch around 1500? or so, and then I noticed the trim indicator in the full down position, which I was happy to see, and ruled out some sort of linkage issue. The airplane was never out of limits, but the trim condition made a simple recovery into something a bit more difficult.

In short, I?m not sure I?d want to see an airspeed any higher with full nose down trim, or one might not be able to apply enough back pressure to effect a recovery. For those with trim on the stick, just be aware something like this can happen when you least expect it.

Be very careful with the prop levers...

...ask Kevin H about engine overspeeds. The prop automatically goes flat with sudden throttle reduction, so there's really no need to mess with it.

Best,

Install a safety trim!!!

Thanks for sharing the incident Jim!

Just curious - what stick grip? I find it takes a very positive effort to get to the coolie hat on my infinity from where I naturally hold the stick - but that could be the unique shape of my hand. If an airplane had a stick where your thumb naturally rests on the trim, I could see this as a more likely problem.

Paul

I have one of the wood teak grips (http://adap.com/rv7/IMG_1495.JPG) and if I hold the stick where one would normally hold the stick, it probably wouldn't be an issue, but I find that I sometimes tend to hold the stick somewhat on the top (I know, it's different), which did it.

This is a subject where I did some research before selecting my stick grip. I looked at what grips were available, and I looked very carefully at the trim switches and their respective coolie hats. My choice was a copy of the stick used in the T-38 and F-4 (don't have my notes just now to say what the model number is, or where I purchased it). The main reason I selected it over grips such as the Infinity grip, is that the coolie hat fits closely around its edges. That keeps dirt, clothing, or the edge of a glove from getting under the edge of the coolie hat and activating the trim when you don't want it activated.

If you have the wooden grip with the small coolie hat, that switch is the little brother of the actual trim switches used in military grips. The larger switch is expensive, but highly reliable. The small one should also be very reliable, and you can see that it is well sealed throughout its movement. I took my grip apart and thought "how could I stuff one of those expensive switches in there?" But the urge to fly was stronger than the urge to modify.

On a close subject to this one, because we are discussing inadvertent trim actuation, is how to wire the trim system with two grips. I decided to have a separate switch so that the trim can be activated from either the front or the back of my RV-8, but not both. If I have an unqualified observer in the rear seat, I don't want them inadvertently activating the trim. Some years ago, a USAF C-135 was lost in western Maryland on an orientation flight. The person in the right seat was a guest, and not pilot qualified. To activate the intercom, it was necessary to activate the top of a rocker switch on the forward side of the right horn of the yoke (on the right yoke). The trim switch was located on the aft side of the same horn of the yoke. The instructor in the left seat was trying to get the observer to key the intercom, but instead the observer pressed and held the trim in the nose down position and the airplane quickly became unrecoverable.

Electric trim is a great thing, but like anything else, it has to be well thought out and highly respected.

Some interesting points, thanks for posting. I would second the recommendation for Safety Trim. Also consider the spring rating of the trim switch - the military aircraft I have worked on have required reasonably high pressure to operate the trim switch (so have strong springs in the switches) to prevent inadvertent trim operation. I have heard of people operating the trim by laying a map on some stick top switches.

Pete

Glad you are out practicing for formation

Diving on Eniumclaw??? and there is not even an IRS building over there.
Glad you figured it out and are comfortable again. Watch the airspace over there there are victor routes that make the practice window small.

Good to have you stop by Sat? Crest was closed Sun for a photo shoot with 20-30 Lambos and Ferraris. One hit 215mph by the end of the runway and the other owners accused him of having the speedometer in kph. It was funny watching them. I think these guys are friends of yours from Redmond.

Safety-Trim

A few others already mentioned it, but here are the details:

Safety-Trim: 3 second max travel time, 2 speed trim control (airspeed based switching), very simple wiring (get rid of all the relay decks)

visit www.tcwtech.com for all the details,

available at all of the typical places and our site.

Thanks,
Bob Newman

I can vouch for the effectiveness of Safety Trim. In addition to helping to prevent a runaway trim condition, the two trim speeds (higher trim speed for lower airspeeds, lower trim speed for higher airspeeds) really help out when you adjust trim at varying airspeeds. My experience with TCW Tech has been outstanding. Their products are superb, and their customer support is second to none. In my opinion you can't go wrong with this or any of their products.