Does anyone know what kind of G-forces a DF experiences? I'm assuming that pulling up out of a pretty high dive is where the g-forces would be the highest.

Just wondering so I know how much to beef up the wing mounts for my next scratch build.

Thanks

Russ

Most of my Gs are pulled in the landing phase of flight

Seriously, with a ducted fan I wouldn't think you could pull more than 6-7 Gs and that would bleed off fast since you don't have the thrust to sustain it for more than an instant. A level sixty degree banked turn requires 2 Gs as a guide.

That's not to say that all level 60 degree banked turns are created equal.
2G may be the minimum force required to HOLD that banked turn, but it isn't the maximum

I-Nav is probably right about not sustaining high G's for long, but I think his estimate
may be a little low. There have been RC gliders said to have pulled over 20 (30?) G's.

The new Jetcat GPS unit will give this data. I will be installing it in my Bobcat this weekend and will report the results of highest G-force recorded...... This is under turbine power of course

Todd

Maybe one of our aero guys can speak better on this, but the G loads are a function of the combination of the radius of turn, bank angle and speed as I remember it.

Todd, didnt you put one in your Eurofighter? What did it say?

G's pulled in turns are the least of your worries...
if you're going to be doing 400 foot powerdives and pulling out at 5 feet, you'll have
transitory G loads of far greater than the engine alone can provide.

John,
The G force menu is only available on the latest ECU software version 4.0L which I have not upgraded to yet on the Eurofighter. I do have this version on my Bobcat so I will be able to read the G force readings from that plane. This is the only feature in the GPS menu that is not included in the 4.0H software revision .

Todd

Ah, I see, Thanks.
Cant wait to see what it says in the BC And speed.

Thanks for all the input guys. I'm interested in seeing what Todd has to report after this weekend.

Russ

Why would the angle of bank matter (other than changing the direction that the G force is applied in) ?

Gordon

I remember something from class....

Velocity^2/radius this gives acceleration for a circular path.
Now if you do it vertiacally you have to add/subtract gravity at some point....I dont feel like thinking.

You would have to change your speed in MPH to Ft/s. Always ft/s! and your radius in ft aswell

Can't wait to find out how many g's you guys are pulling! Hurry up todd.....!
Dustin

A level turn with 60 degrees of bank, not gaining or losing height, is 2G end of story. Regardless of microlight or Concorde, weight, speed or anything else, 60 degrees banked level turn is 2G. It is determined by the geometry of the inclined lift vector, and nothing else. The speed at which you are travelling then determines the diameter of the turn and the rate of heading change.

Harry

Ok... you're saying that there's no difference in G-force between doing mach1 and entering the turn,
and travelling 60mph and entering the turn. Right? I'm no expert, but this sounds far-fetched to me.

It may be true if you ALLOW the speed to determine the diameter of the turn, but what if you're using
thrust vectoring and large control surfaces to maintain the turn at a smaller diameter?
There are different diameter turns you can make, given a certain speed, you know. Not all equal

Harry C is right on. a 60 degree bank, level turn produces a loading of 2 g's whether it is done in a J3 or the Concorde. However how many G's can be pulled is a more complex problem. Full size aircraft use what is called a VN diagram. Without having one of these tho, we're looking at basically how much lift the wing can develope and how much authority the tail has to move the angle of attack to it's angle for maximum lift. even tho there may not be enough power to ofset the high induced drag at high angles of attack inertia alone would allow a momentary peak in g loading. The best way to find this out would be to put a recording G meter on your plane and then go wring it out.

Cwatkins, using thrust vectoring is not a normal turn!

The vertical component of the lift vector must equal weight in order for level flight to be maintained. When the aircraft is upright the lift vector and weight vector are at 180 degrees to one another and therefore the lift needs the same value as weight to balance it. Once the lift is inclined due to banking, the vertical component of the lift is reduced, until ultimately at 90 degrees of bank there is no vertical component of lift from the wing, no matter how much lift it produces. If the plane is banked and no increase of lift is made, the plane will descend, since although the lift produced by the wing remains the same, the vertical component is less, weight now exceeds lift and the plane descends. The force available to turn the plane is the horizontal component of the wing lift. In order to maintain height, the wing lift is increased by increasing the angle of attack, by pulling back on the elevator. That way the greater lift, although inlcined, now comes back up to the same vertical component required to balance the weight. The G force is equal to the multiple of lift force divided by weight required to get the correct vertical component to balance weight. Thus the lift, or G force, is simply found by lift = weight / cosine angle of bank. At 30 degrees bank the lift or G is 1.15G, at 70 degrees bank it is 3G, at 80 degrees it is 6G and by 90 degrees it is infinite.

As you can see, these calculations need know nothing about the actual speed or design or weight of the plane. They are true for every aeroplane. If a microlight and Concorde at mach2 both bank 30 degrees they will both experience a 1.15G turn. Both will also experience the same acceleration towards the centre of the turn. But because one is travelling much faster than the other, it will take far longer for its speed in the original direction (i.e. its velocity) to reach zero, which means it has turned 90 degrees. That is why fast aircraft like fighters always bank hard over, their diameter of turn and time to turn would be too great if they allowed 1.15G to turn them. But the G load and the increase in stall speed in a level turn are related entirely and only to the angle of bank are are the same for every aeroplane.

Harry

Should get out this weekend with the GPS in the BC, weather permitting .... I'll post results of all the readings provided from the system.

Todd

Harry is right on. I flew a full-scale S1-T Pitts in IAC for several years. The standard aerobatic turn is one of at least 60 deg bank during a pattern sequence. Hauling it over to 60, pulling to maintain altitude, at full power setting yielded 2G's on the meter every time.

Radius of turn depends on airspeed on entry at a constant bank angle. That's why an SR-71 doesn't do standard rate turns (3 deg/sec) on an instrument approach, like you do in a Skyhawk. You'd end up in the next county.

Speed decays rapidly due to induced drag, but will reach an equilibrium based on power, weight and wing design. That equilibrated state is what you strive for when the FAA examiner has you do constant airspeed, constant altitude, 720 deg steep turns at 45 & 60 deg bank. Round and round you go till you puke! And then you reverse from R-L to L-R and get the puke going the other way!
It get's more exciting, the steeper the bank of course. If you let the nose drop just a little at 75 deg bank, the vectors tilt, and you enter the infamous graveyard spiral in a heart beat in high performance planes, like Bonanzas, and our jets for instance. Guys like to call it "Tucking" in the turn. The steeper the bank, the faster it happens, especially in planes with adverse tip stall characteristics.

Now if you change the flight plane from horizontal to vertical, then look out. A moderate pull-up to a loop at full power in a Pitts gets you 4G with only a light pull, and 8G with a hard pull. A vertical loop is similar to a horizontal turn with 90 deg bank---not exactly the same, but aerodynamically close.

By the way, Tony Tehan of Mini-Hobbies had a neat little recording G-meter at Superman 2002 for those who don't want to spring for the JetCat GPS.

Tom

Turn Radius = Velocity^2 (ft/sec) / (Radial g x 32.2 ft/sec^2)

Radial g = g's in the plane of the turn which would be the same as G Meter g's at 90 degrees of bank.

At 200 mph (293 ft/sec) and a turn radius of 200 feet: Radial g's = (293^2) / (200 x 32.2) = 13.4 g's

A 6 g level turn at ~81 degrees of bank at 200 mph gives a turn radius of ~445 feet. The fast jets are encountering some pretty good loads.

The dymanic soaring guys are hitting well over 40 g's (and breaking wings) with their sailplanes when they get their speeds up over 150 mph.

Somebody check the math :tired:

Most folks are unaware of just how much the G load rockets up as the speed increases. To maintain the same diameter of turn, if you double the speed the G load quadruples, since the equation is related to velocity squared. So if you are in a 2G manouevre and then double the speed while pulling ever harder back on the stick to keep the manouevre the same size, the G load will hit 8G! If you quadruple the speed, the G load will go up 16 times, to 32G. Typical sports models do 50mph. Jets can achieve 100 or sometimes 200mph. I have seen a well known jet pilot try to crank a big jet around tight curves like a sports model, he doesn't seem to realise he is not getting say 4G but perhaps 30G. A few weeks after I saw him do that, the model blew apart in the air, no wonder! So if you double the speed, but want to maintain the same G load as before, the diameter of the manouever must increase by a factor of 4, not 2. If you treble the speed, say from a 50mph sports model to a 150mph jet, the diameter of the manouevre must increase by a factor of 9 to maintain the same G load. Hence gliders at low speed do tiny loops at 4G, aircraft at several hundred mph do huge loops at 4G.

Loops present interesting G load variations. Assume a model flies a loop that is perfectly circular and at a constant speed all the way around. At the bottom the load is 3G. What is the load at the top? 1G. The load in level flight is 1G to start with, so the G meter reads 1 even before you start the loop. If it reads 3G at the bottom, then 1G is normal gravity and 2G is the loop. At the top, you still have 2G from the loop, but now the 1G of mother earth is reversed, so is deducted not added to the loop G. Hence, 3G at the bottom, 1G at the top.

What if the model flies a perfectly circular loop but the speed at the top is half the speed at the bottom? At the bottom, it is 3G. what is the G at the top? Would you believe it is minus 1/2G! Again, G due to loop is 2 at the bottom. The speed has fallen to 1/2, and the speed factor is squared in the calculation, so the load has fallen to 1/4 what it was at the bottom. Therefore the load due to loop is now just 1/2G. But the model is inverted, deducting 1G, the total load is minus 1/2G. So now the load on the plane varies from +3G to -1/2G in a perfectly circular loop where speed at the top is half what it is at the bottom.

In level flight, the wing produces an acceleration of +1G. This is set by altering the elevator, too much produces more than 1G and a pitch up, too little produces less than 1G and a pitch down. Push far enough forward and the wing will produce an acceleration of minus 1G. Roll inverted and the elevator must now be set to produce an acceleration from the wing of minus 1G to maintain level flight, funnily enough being the same setting as it was to produce minus 1G when upright. At a constant speed, the ele setting determines G load whichever way up you are. Now fly that perfectly circular loop at constant speed. At the bottom the ele must be set to produce 3G. At the top it must be set to produce 1G, which is the same as level flight, i.e. not pulling back at all, but at neutral. If you have lost speed and now need less than 1G to maintain circularity, the stick might actually be in the down position at the top of the loop. A lower speed will usually let the nose drop anyway, so the ele will be set at the eqivalent of whatever it would be in upright flight to produce minus 1/2G at the lower speed. Watch most model fliers do a loop, and you will see they hold the stick back a constant amount all the way around. That's why their loops are egg shaped. Now you know what to do, you can ease off the elevator as you go over the top and let your loops become a lot more circular. Don't forget to start pulling back again on the downline though!

Harry

Harry's comments again are right on. Rule #1 in basic aerobatics is to never split-S out of a busted slow roll or inverted flight practice. You're usually at cruise or full power and at cruise speed or faster entering the maneuver.

With the exception of the planes stressed to more than 10 G's and high Vne like the Extra's and Sukhoi's, most planes will experience a structural failure on the pullout, due to G or airspeed overload, or both-----if that fortunately doesn't happen, G-LOC is waiting to do the final number on you, especially after prolonged inverted flight practice, all due to the velocity/G relationship being a square function.

It also relates to airspeed in turbulence, with gust G-loading running up the G's to the structural failure point, almost instanly, one of the significant contributing factors in thunderstorm penetration accidents.

Fortunately, our high performance sport jets like the HotSpot, Bandit, etc. are really built to take the G's. I really am interested in how many G's we really are pulling. In addition to the JetCat GPS unit, Tony Tehan of Mini-Hobbies, had a tiny recording G-meter at Superman this year for sale.

Tom

LOL..... everybodies an aeronautical engineer now

I AM one, but I don't remember any of that performance or stability & control stuff! I just remember that a level, coordinated, steady state 60 deg banked turn is indeed a 2G turn, by definition.

I just got through flying T-38's and NEVER got so wrapped around how,where, when, and the why's of G's. You pull hard,they come quick, you use them to your benifit, or your dismay. Pos G's feel alot better than Neg G's. If you dont strain...goodnight. And the all important, dont OVER-G the aircraft Everythings got it's limits.

Im am curious to find out about how many g's were pulling on these aircraft. As many of you know, the amount of G's is only restricted to human tolerance, not aircraft structual stability.

The amount of Gs you are limited to is restricted by human tolerance AND aircraft structural stability. Its just that usually, the pilot is the wimpier of the two!