Adjust Low Thrust Line closer to Centerline thrust on RAF Sparrowhawk?

Eric S

Junior Member
I have an RAF Sparrowhawk with the standard low thrust line (LTL) and I'm curious if anyone has made adjustments to bring it closer to centerline thrust (CLT).

My understanding is it was designed to be CLT at full gross. The problem is I fly 95% of the time by myself. When I add or reduce throttle, I end up chasing it with trim since otherwise the stick forces are very heavy. The thrust line is supposedly around 2" low when flying solo. I was thinking of raising it 1" to be 1" LTL solo and 1" HTL at gross.

Has anyone raised their engine or maybe tilted the engine up to bring it closer to CLT?



FW and Gyros
I think the "chasing it with trim" is an effect of the cruciform tail, which causes the gyro to "hunt" as the pressures build and release unevenly from each quadrant.
I.e., like gimbal thrust.

Thus, I doubt that changing its vertical cg will make any difference.


Doug Riley

Platinum Member
A cruciform tail unit does not lead to "hunting" or a need to "chase" trim, in my experience. I taught in a Dominator tandem for several years. This aircraft, of course, has a cruciform tail and LTL. The low thrustline does make it necessary to adjust trim with significant throttle changes -- but once you adjust it, the machine flies on rails. In practice, I used essentially full forward trim at takeoff, and full aft trim during an engine-idle approach. Cruise naturally was somewhere in between, the precise position governed by one's selected cruise speed.

A wide, blunt body pod with insufficient tapered streamlining back to the tail could result in the tail operating in the body's wake turbulence. But I don't think that was the question.

I'd start by examining the incidence of the H-stab. If, in the aircraft's normal flight stance, the H-stab is down-loaded (leading edge lower than trailing, with respect to the slipstream), then adjust the H-stab's incidence to remove the down-load. That ought to be easier than moving the whole cabin, or the engine, up or down. If the H-stab is already at zero, though, and you still have excessive LTL behavior, you're likely stuck with moving the weights or the prop thrustline itself.

The aircraft's CG moves a fraction of the distance that you move a weight -- in the ratio as the moved weight is to the all-up weight. So, in a 1500-pound aircraft, if you move 750 pounds two inches, the aircraft's CG will move one inch, in the same direction.


FW and Gyros
Jean Claude, your photo is of an RAF, but Eric has the Sparrowhawk variant with much lower thrust offset to cg.

Doug, the "hunting" characteristic I mentioned was, as I understand it, more unique to the Sparrowhawk's tail vs. all cruciform tails.
However, since Eric's issue seems related to throttle settings, you may be on the right track.



RAF/Sparrowhawk/SC 2.5 FI

I am a gyro newbie, but have flown many fixed wing aircraft (no jets), from a Boeing B-17 to a Piper Cub in 50 years of flying. All of them exhibit the same phenomena - if you change power setting, you need to adjust the trim to maintain altitude. I believe this is a simple aerodynamic principle.

You might also consider the fact that the prop thrust provides most of the airflow over the h-stab, and not the relative airflow related to airspeed. I am sure there is that some component is affected by the blunt, un-aerodynamic cabin shape, but what is that percentage, and how does it change with airspeed? Prop thrust will ALWAYS be the greater component.

I also think my RAF Sparrowhawk has heavy stick forces compared to most FW aircraft, and the few gyros, that I have flown. Adjusting the h-stab angle of incidence appears problematic, whereas adjusting the prop thrust angle may be done by adjusting the rod ends connected to the engine PSRU plate, and by adding or removing washers on the rear engine mount. Have you performed the double hang test to determine the vertical CG location as a starting point? Of course this location changes with passenger operation. Let us know what you find.


Supreme Allied Gyro CFI
This same issue has cropped up in the investigation of the recent Lion Air Flight 610 Boeing 737 MAX crash. Here's some text from an Aviation Week electronic newsletter about the investigation:

Like all turbofan-powered airliners in which the thrust lines of the engines pass below the center of gravity (CG), any change in thrust on the 737 will result in a change in flightpath angle caused by the vertical component of thrust. This is a moment resulting from the horizontal thrust component caused by a CG offset and a trim stability change.

Although the Leap 1B is designed to have thrust levels similar to the 737NG’s CFM56-7B, the newer engine is heavier and has a larger fan. Because of its greater size, Boeing had to maintain adequate ground clearance by cantilevering the engine farther forward on a heavier strut, adding to the offset. The Leap 1B’s 18-blade composite fan is 69.4-in. in diameter, compared to 61 in. for the CFM56-7’s 24-blade titanium fan.

Each Leap 1B weighs 6,129 lb., 849 lb. more than a CFM56-7B. For the 737-8, the added structure of heavier struts and nacelles, beefed-up main landing gear and supporting structure add 6,500 lb. to the green aircraft weight, but operating weights are boosted by 7,000 lb. to preserve full-fuel and payload capability.

MAX pilots are therefore trained to know that although the aircraft has natural speed stability through much of its flight envelope, there is also inevitable thrust-versus-pitch coupling at low speeds. The 737-8 has a speed-stability augmentation function that helps compensate for the coupling by automatically trimming the horizontal stabilizer according to indicated speed, thrust-lever position and CG. Boeing advises that pilots still must be aware of the effect of thrust changes on pitching moment and make purposeful control-wheel and pitch-trim inputs to counter it.

Doug Riley

Platinum Member
I'll risk being tagged as a "nanny" liberal. I think that aircraft that are marketed to beginner aviators, as gyros generally are, should be as user-friendly and low-workload as possible. The 737 is flown by professionals. In that context, some pitch reaction traceable to LTL isn't unreasonable. In a recreational craft, OTOH, the less of this sort of thing, the better.

Gyro designers, of course, have been violating this notion since Bensen himself.

Dunc, it's true that the SPEED of the airflow over the H-stab is augmented by the propwash (and a good thing it is), but the DIRECTION of the flow is heavily affected by the direction of the flow INTO the prop disk. In effect, it's garbage in, garbage out -- if the flow into the prop disk is turbulent (because of a blunt pod upstream), the flow OUT will be turbulent, too. This effect accounts for the "growling" noise that pusher props tend to make.

It's fortunate for us that the flow into the prop disk does affect the direction of the propwash, since that way the H-stab still "sees" changes in the aircraft's angle of attack, and produces a restoring moment.

Eric S

Junior Member
Thanks for the helpful replies.

I've been working constantly on 2 main issues on my RAF Sparrowhawk. Heavy control forces and reduction of the Low Thrustline (LTL) offset.

Control forces on my machine have been reduced from very heavy to moderate (subjective). I experimented with modifying all parts of the control system. The 2 adjustments that made the most difference were changing the upper and lower control rod connections at the scissors on the mast (I moved the upper rods connection points closer to the mast) and shortening the rear control horns. These changes give the stick as much leverage as possible while maintaining full rotor head deflection and avoiding binding in the system. This has helped to reduce the need for constant trimming for every minor change of throttle.

Now I'm experimenting to reduce the effect of the LTL. First, by raising &/or tilting the engine in order to aim the thrustline closer to the CG. I used fender washers stacked on the motor mounts to slowly raise the front of the engine while making longer and longer upper support rods that connect the PSRU plate to the frame on top until I could go no further due to reduced prop clearance at the keel. A little tilt seemed to help, but the effect was reduced as I continued to raise the front of the engine (again, subjective). I think the benefit may have come more from raising the engine than from the tilt angle. Also, the ever increasing angle may have been increasing the down force on the horizontal stab. at high power offsetting any benefit? I'm waiting for more hardware and materials from Spruce before I can continue. Second, I adjusted the angle of the T-tail slightly positive which also seems to reduce the effect of power changes which translates to reduced need for constant trimming.

Lots and lots of test flying with each small change. When I'm satisfied with all of this, I'm going to find a way to make the seats more comfortable!



Gyroplane CFI
I have flown several different AAI modified RAFs and in my opinion none had particularly heavy controls.

What are you comparing it to?

I have also flown an RAF with a Sport Copter rotor head and blades and it had a very light stick because it does not use an offset gimbal as the AAI kit did.

It is my understanding that the AAI modification for the RAF was designed to be low thrust line because some feel that gives a more stable response.

My preference is near centerline thrust to minimize power-pitch coupling.

All the best on your adventure.


FW and Gyros
Eric S, bravo on your experimentation and customization!
I hope you continue to inch your way towards improvement, and thanks for sharing the details with us.
Many others can benefit by it.

Safe flying,