Planespotta

Hangar chatting at the airport today and the question of whether a single engine airplane has a Vmc speed like a multi came up. Any comments/thoughts on the issue? I'm thinking that if you get really slow and cross-controlled, it's possible for the nose to start moving regardless of how much rudder input you have and thus lose directional control, thereby "attaining" Vmc, but does anyone have a more definite answer?

thrustset100

Single engine airplanes do not have a "Vmc." When you get cross controlled and stall/spin in a single, you just happened to reach Vs or Vso and when yaw is introduced, one wing is more stalled than the other inducing a spin.

Vmc is defined as minimum directional control speed with the critical engine inoperative. The critical engine being the one, where if it fails, causes the most heartache..I mean, biggest change in performance, handing, directional control. It only applies to Multi-Engine planes

Airhoss

Yeah, but in a single engine aircraft it's called Vso or Vsi.

Of course if you've ever flown a super high power single like a P-51 or an F4-U Corsair or maybe an F8-F Bearcat you'll find that they definitely have a Vmcg issue at low speed high power settings on take off.

Any of those birds need 60 or so KIAS over the tail before you can go to full power or you'll find yourself in the weeds real quick as the rudder is unable to overcome the torque of the engine and prop at less than about 60 KIAS.

So the procedure is to initially go to a reduced power setting until obtaining 60 KIAS then you can feed in max power for takeoff.

Same difference as Vmc in a twin sorta kinda.

Planespotta

And we already have a disagreement You can imagine how it got out at the airport. Thanks for the replies, hope to get a few more

Cubdriver

Are you asking how the vertical tail and rudder are designed on ASELs? I am not sure what the question is exactly. All the flight controls must be sized, selected for aerodynamic lift, given hinge moments, assigned deflection ranges, analyzed for aeroelastic response, set up for stress loads, and adjusted through many design cycles in an iterative process when there is a need for a fresh airplane. Then the controls must be tested either using approved expansion calculation codes or flight tested. If you want to explore aircraft design it is a fascinating topic that can absorb your mind for years. Raymer makes some decent aircraft design books, or you could probably get something simpler aimed at making model airplanes. My school used the popular ten volume series by Jan Roskam titled "Aircraft Design" which is probably overkill for most hobbyists.

f16jetmech

Where you teaching at now planespotta? Havnt seen you since your checkride with Ellis

Mason32

Vmc requires an airplane with more than one engine, not centerline thrust. In layman terms, it's the slowest speed where the flight controls can still maintain directional control under the worst conditions (critical engine, worst CG, max power....) The airplane is not stalled, it's directional control is lost.

Big enormous difference than Vs in a single. It also does not correlate at all with p-factor taking a powerful single off the taxiway due to poor technique.

Planespotta

Hey man, check your PMs. What are you up to these days?

Cubdriver

I think he wishes to pose the question could there be a flight regime where aileron opposing rudder results in one prevailing over the other. The answer is sure, and if such an airplane does not exist we can design one. Sort of like the Skycatcher before it crashed (twice), it had an undersize rudder and they realized it was too small and made it a bigger. The problem is more rudder means more weight, drag, and structures and there is no getting around some tradeoffs.

The basic engineering idea is that knowing how an airplane will behave is dealt with by flight dynamics. You can develop an accurate numerical model of an airplane to see what it will do. The engineers are called stability and control engineers. There are a bunch of design coefficients you have to know, and some of that is guesswork, but you can see what a design will do or not do this way. This is also what drives flight simulators although it is usually called a flight engine there.

Back to the question. If you have too small a rudder you can get an airplane that has excessive aileron authority and not enough rudder authority. Ailerons roll, rudders yaw and to a lesser extent rudder also affects roll. An exercise I sometimes have my students do is to take their hands off the yoke and fly the airplane back to the airport using only rudder pedals and thrust. It certainly can be done. Rudder affects bank secondarily, and thrust as you know controls pitch.

Think about the basic physics some. What are we doing here? We are after moving the airplane CG using control forces. What does the aileron-wing combination do? It shoves the CG sideways horizontally and holds up the airplane CG. That is linear force. What does the rudder do? It rotates the CG about itself. That is rotational force. Which is harder to do? There are two ailerons on a massive huge wing on the one hand, yet the rudder is a single flying plane mounted on a lightweight tail box. Think about it. The rudder has the far easier job here.

As far as certification goes, I do not know of any rules that apply to an opposition situation between rudder and ailerons. There may be one, I am not the be-all on this. Quiet a few aircraft even have coupled ailerons and rudder to prevent cross-controlling. Ercoupes and Stationairs come to mind, but there are many. Of course there are a lot of rules concerning spin recovery, crosswind authority, stall speed etc. for ASELs and in theory we can design any type of ASEL we want, including one with the ability to maintain rudder authority throughout any flight regime.

If the rudder is weak it isn't because rudders are impossible to make strong enough, it's rather easy to do. The more serious question is how will the tail deal with all the stress placed on it by this huge flight control hanging out there on a thin aluminum box. In truth it is not hard to make a tail that is strong enough to withstand just about any reasonable rudder force. The wing is what fails first. The Airbus Flight 587 crash was a special case involving composite construction and dynamic loading.

Getting warm, Spotta?

Planespotta

Thanks for your response, Cubdriver. Do you know of any specific aircraft types that have one engine but can still experience a loss of directional control?