Control of Vmc

[WLIU]

Vmc is a factor of two forces, both of which act around the CG. You can best visualize and calculate the forces looking down at the aircraft.

First, the engines at 100% power (sea-level is the worst case) produce a specified thrust. That thrust works through the arm over to the CG. We locate the center of thrust slightly away from the center if the propeller to account for P-factor. This is why for Lyco's and Continental installations that are not counter-rotating, the left engine is critical. The thrust force acting at the end of the arm that originates at the CG creates a torque force around the CG. We can calculate this number.

Second, the vertical stab and rudder create a lift force in the horizontal plane. At full rudder deflection we can calculate this lift force vs airspeed. Its a wing turned sideways so you can figure out its coefficients of lift, drag, etc. You can spend a few hours with a program like XFLR5, OpenFOAM, etc figuring out lift vs airspeed graph for the airfoil shaped by the vertical stab and rudder at full deflection.

The vertical surface generates lift force that works around the CG just like the engine thrust does. So you can convert the graph of horizontal lift vs speed to a graph of the torque provided by the force of the vertical surface acting on the arm over to the CG.

Once you have this info, you just take the torque around the CG that you calculated the engine creates, and move across the graph you just plotted for the vertical stab/rudder until you find the same value. From that point go to the airspeed axis of that graph and you have your calculated Vmc.

Flight test will confirm your calculation.

The above is the Cliff Notes version of the calculation. There are more steps that add to the accuracy/correctness of the calculated result.

Best of luck,

Wes

[martymayes]

Yes, I have looked at Angel Aircraft. Not the "prettiest" looking plane.

I bet one of those paint jobs with "swoopy lines" would change your mind......

[Matt Gonitzke]

Airplane Performance, Stability, and Control by Perkins and Hage should have the required calculations. It has been out of print for awhile, but used copies can be found. Anyone designing an airplane should have one.

[Ron Blum]

There are many Vmc numbers. Yes, an airplane that stalls before you lose control would be good. Vmcg is used when the airplane is on the ground (the yaw pivot axis is at the main wheels). Vmca is used when airborne (and may be different for each flap/gear configuration). Reading the regulations, available free online at www.faa.gov, (14CFR23, subpart B, Flight) will tell you the relationship between all the takeoff speeds. Be safe and remember to start on the fast side ... especially with Vmcg.

[Wylbur]

Sorry for taking a while to get back to this. I've been a bit busy... Until I can find compression ignition engines that meet my design specs, this project is on hold.

[Frank Giger]

So long as you're kicking around the ideas and looking for what fits and what doesn't, the project isn't on hold.

It's just in pre-planning stages.

[Byron J. Covey]

[Q

UOTE=FlyingRon;40910]The FAA gives leeway in the experimental, though it would behoove you to meet the loading requirements of the intended use. Transport is +2.5/-1, Normal category is +3.8/-1.5. There's more than banking that's an issue. Lowering the G limits also greatly lowers maneuvering speed, for example.

[/QUOTE]

Both Transport and Normal category aircraft have been tested to the load factors. They also have consistent quality control to ensure that they actually achieve the intended load capability. That may or may not exist in homebuilts - especially composite structure homebuilts.

[CDS]

I've done some searches and VMC is not found in EAA forums. I would assume that's because no one seems to want to tackle a twin (except for the Twin JAG).

For those of you who have aircraft engineering experience, or multi-engine piston a/c time, I have a question. How does one project what the Vmc would be?

And why is Vmc where it is for a particular a/c? Example: For a Seminole, Vmc is less than Vso. For another plane, Vmc is > 80KIAS and Vr is ~65KIAS (so pilots accelerate to Vsse and then rotate).

If you used pusher props with wing mounted engines, would you even have Vmc or would it be well below Vr?

Do jets have Vmc? I've never heard of such for a jet, just V1, where if you lose an engine by that point, you chop power and stop.

If your engines are mounted on the sides of the fuselage, does that prevent there being Vmc, and does the FAA consider this an inline thrust situation? I would bet that you would still need rudder to control yaw in that case.

Just someone with an inquiring mind trying to figure out a multi- design.

One last question: Does an E-AB have specific G loads to meet for the wings? Could one design around "transport" category (+2/-1 G I think is the requirement)? After all, if you design and build a plane for 6-8 people, would you really want to do 90 degree banks on purpose?

Regards,
Wylbur

Wylbur - based on your posts in this thread, it occurred to me that a "design study" might be helpful to you. That is, compare existing aircraft that are comparable to your requirements and you'll find a range of solutions; wing area, aspect ration, power, power loading, empty and max weights, tail volume (square footage of the vertical stabilizer and rudder multiplied by the distance from the aft edge of the C.G. range). Again, based on your posts, it seems that your thoughts are akin to the Navajo series and Cessna 402s.

Once you've digested that information, when you design your own twin, if you come up with answers that are significantly different than the design study aircraft, you'd best rethink those answers. That is, there's a reason existing aircraft are set up the way they are (and in the end, you might discover that it's cheaper to buy a twin than to build one - some twins are relatively cheap right now (who wants to pay the fuel and insurance?)).