Blow n Go

Thanks cltom. I instructed for years, so try to reduce statements to the shortest, simplest form for the average person using this forum. Maybe to simple, as I frequently find myself defending obvious statements like "the sky is blue". I can "talk engineering" if I need to, but we'll just leave the poor guy asking the question out in the cold and while serving no purpose......like in this thread!?!

You guys have a happy Thanksgiving..........You too Ben

CJ

rmh

We need more forums!
Question
Does anyone think that the reason there is not a lot of aerodynamic discussion on model airplanes , may be that the discussions often quickly evolve into full scale practices and formulas?
Am I being too picky?
Frankly I enjoy the BS sessions about how the B17 really got started.
However -does that fit the intent of the forum?
Maybe we need a Aerodynamics , Full Scale Chat n Stuff
And Aerodynamics , Model Aircraft Questions and Answers.
Of course -this is from someone who proposes that if the wingloading is low enough - you can almost disregard the CG--
and prove it.

Ben Lanterman

"I've met the guy that flew the straight tail (and wing) in that full size wind tunnel. The plane John's Dad finally tooled was fair for its day, but it had this nasty habit of losing control violently in a fight. They worked some of these things out in tunnels after the pilots told them the plane was unflyable in a fight."

This is an example of reducing statements to the shortest, simplest form??? Analyze what you said - 3 things that weren't true -

The straight tail F-4 never flew.

It doesn't lose control unless it is flown outside of the recommended flight envelope or is damaged.

How could the pilots tell them that the straight tail was unflyable in a fight if the airplane never flew???

Shortest, simplest form should also be accurate and have the facts straight.


Dick the only difference between model airplanes and real airplanes is a Reynolds number and scale effect. I find that they aren't mutually exclusive. I have been doing full scale aero since 1965 and modeling since 1950 and find the two get along together very nicely. It is nice to know both what happens and why it happens.

Regardless of wing loading the only thing CG effects is the stability of the system. I flew one of my little F-4 foamys with the CG too far aft and found that I had to continually input elevator control to stop the nose from diverging either up or down. It was about 1 or 2 percent unstable (a small amount). After a minute it was pretty tiring and I throttled back and landed (power effects make the airplane more stable in low power than high power). With the CG moved a few percent forward it was quite relaxing and still very maneuverable. In my experience we still have to watch where the CG aft limit is.

Doing a whoopsie with a foamy is certainly less of a ripping out the heart than doing one with a 30% scale machine - that in itself would make them easier to fly.

Blow n Go

"I have been doing full scale aero since 1965 "
"The first flight of the F-4 was on May 27, 1958 "
"wing tip dihedral was determined in the wind tunnel but wasn't to balance or counteract the anhedral in the tail, but because an increase in roll stability "

"the Phantom certainly came from guys with the Demon influence - both good and bad"
"How could the pilots tell them that the straight tail was unflyable in a fight if the airplane never flew??? "
"Both things were found out in the wind tunnel before flight test. "
"the F-4 in my mind is a prime example of tweeking a design in the wind tunnel."

"The F-4 has impeccable flying characteristics "
"McDonnell that caught it, not the Air Force Academy Cadets"

"don't like random unchecked statements. "

Cdallas2

Well I guess that since I'm not an engineer I couldn't possibly understand aerodynamics.

Guess I'll just give up and fly someone else's design.

I thought I was going to learn something from this thread. Too bad it's become an unathourized history lesson of the F-4. A direct evolution of the AVRO ARROW.

Ben Lanterman

Blow n Go,

I worked with the folks that designed the F-4, I worked with the F-4 wind tunnel models, flight test and analysis in the advanced F-4 design group at Mcair. I worked on the derivatives of the F-4 we called the F-4FV and the F-4FVS. We always tried to go to a straight tail but came back to the anhedral version each time. I worked on the losing Mcair entry for the F-14 (which had a straight tail). Yes we lost often but learned from it. Witness the F-15 from the same guys (no anhedral tail). Having worked at the airplane factory that made the airplane in question, in the aerodynamics group that designed it, I felt I had something to offer.

Tell me again where you heard your information about a pilot flying a straight tailed F-4?? What am I missing in your responses about this??

Present some real facts to back up your outrageous claims. This isn't engineering class but the folks that come here deserve accuracy and don't need the information to be watered down for them.

I would like to discuss aerodynamics and airplanes with you. Airplanes have been a big factor in my life and I like talking about them with anyone and everyone at anytime. But there are "old wives tails" that kick around the hangar that seem to become fact with the retailing.

Let's try to keep it civil and accurate - but, if you make a claim be prepared to back it up.

But I would like to know the name of the pilot who flew the straight tailed F-4.

Blow n Go

Ben, I'm sorry, but that was 27 years ago, and I just don't remember a name. He instructed aero at the Air Force Academy and was a test pilot before that.

Please realize that just because you have not heard about something, it does not mean it did not happen. A lot went on at Edwards that I do not think you, or the rest of us knew about. There was a cold war going on and a lot of this stuff was not open to the public......or employees that were not directly working on the project. If that were not true, then I fear for the safety of this country's military.

The fact is that the location of the horizontal pivot point you worked on with the F4 is in the worst possible spot for positive "G" maneuvering. If it were straight, then it would be completely blanketed by the turbulence when the wing stalled. This is the worst possible situation, because you then have no control to reduce the angle of attack and break the stall. The angle your guys set the horizontal to is not efficient for pitch or yaw, and would not have been done unless it was absolutely necessary. The location was most likely designed there because that is the usual location every fighter used in WWII. The difference, of course, is that the WWII fighters had 2,000+ HP with props that blew air over the tail surfaces........even in a "deep" stall.

The design evolution is a natural one, and for the time, you guys did the best you could. And learn you did! The F-15 put the slabs where they belong, so they don't blanket at any angle of attack.......positive or negative. The twin tail wasn't obvious, but became so very quickly for everyone designing twin engine fighters.

I think we can both learn a lot from each other, but there is no need to attack me first and then ask questions.

CJ

Ben Lanterman

"Ben, I'm sorry, but that was 27 years ago, and I just don't remember a name. He instructed aero at the Air Force Academy and was a test pilot before that. "

Then don't report it as fact!

"Please realize that just because you have not heard about something, it does not mean it did not happen. A lot went on at Edwards that I do not think you, or the rest of us knew about. There was a cold war going on and a lot of this stuff was not open to the public......or employees that were not directly working on the project. If that were not true, then I fear for the safety of this country's military. "

Agreed, except that I had access to all of the F-4 flight test data. A straight tail F-4 didn't exist. Don't report it as fact.

"The fact is that the location of the horizontal pivot point you worked on with the F4 is in the worst possible spot for positive "G" maneuvering."

What does the pivot point (normally defined as the axis of rotation of the surface) have to do with anything. It is selected to give loading of the tail to keep the surface from having unstable aerodynamics. If you are talking about the vertical location of the pivot point certainly it was done to keep the mechanics out of the jet exhaust. That configuration of jet exhaust and horizontal tail (regardless of good or bad) was used on several airplanes back then and seemed to be a McDonnell trademark.

"If it were straight, then it would be completely blanketed by the turbulence when the wing stalled. This is the worst possible situation, because you then have no control to reduce the angle of attack and break the stall. The angle your guys set the horizontal to is not efficient for pitch or yaw, and would not have been done unless it was absolutely necessary. The location was most likely designed there because that is the usual location every fighter used in WWII. The difference, of course, is that the WWII fighters had 2,000+ HP with props that blew air over the tail surfaces........even in a "deep" stall. "

That's what the engineers found out in the wind tunnel!! The truth of the aerodynamics isn't in discussion, it was just not found out in flight test. The final horizontal tail is obviously a compromise and is admitted to by everyone that worked on it including one of my bosses who did the actual engineering work and liked to talk about it. Overall though it was more efficient than the straight tail (which we agree didn't work well).

"The design evolution is a natural one, and for the time, you guys did the best you could. And learn you did! The F-15 put the slabs where they belong, so they don't blanket at any angle of attack.......positive or negative. The twin tail wasn't obvious, but became so very quickly for everyone designing twin engine fighters. "

Nothing was too natural, we ran a matrix of the possibilities and picked the one that was the least compromised of the bunch. Even then the anhedral angle was looked at as a solution to high speed directional instability. The final solution was two bigger vertical tails and no anhedral, but indeed it was considered.

"I think we can both learn a lot from each other, but there is no need to attack me first and then ask questions."

CJ, just be accurate. You present things as facts that I know aren't and you will get a response. It isn't an attack, it is a question of accuracy.

cltom

Some flight test data is classified to the extent of whether it exists. Solutions may also be driven by outside forces not immediately apparent to the Engineer. Very few design Engineers have direct contact with the actual test pilots. Even if they do, the test pilots do not control the program. Discussions in regular meetings, if they take place, may still be agenda-biased. Conversations take place, and common knowledge develops inside the customer's culture that may remain undetected by the contractor.

Contractor management also tries to minimize tunnel tests in favor of flight tests, gambling that a problem won't be found that is too difficult to fix. The customer may actually promote this logic as well. This saves program costs, but sometimes that gamble loses.

just some observations - they may not apply in your case of the F-4 ...

Ben Lanterman

Actually my experience working on the F-15 from inception in our advanced design group and for the next 20 years on and off has been that both customer and contractor always perferred to do any problem solving in the configuration area in the wind tunnel and a long time before the flight test program. The net result is that we had very few configuration design changes in the F-15 as a result of flight tests - rounded the wing tip, made a snag on the horizontal tail, and a few others.

Flight test flights are so much more expensive than the wind tunnels at the time that flight test was never considered a time to solve problems, rather it was hoped none would be found - granted wishful thinking but we really thought we had the perfect design when we entered the flight test program.

Certainly I agree with you that the test pilots do not control the program, they are respected for their flying and engineering talent but are just another team member trying to make a good airplane.

I don't know the exact workings of the F-4 groups as I came on at the end of the program and when we were developing advanced versions of it as a follow on effort. However the guys that developed the philosophy of the management and engineering of the F-4 were also instrumental in the F-15 also. I doubt much changed other than the usual hoped for advancements in knowledge and sciences that weren't ignored.

cltom

Makes sense, but a completely new design is likely to be thoroughly tunnel tested - if not part of the actual specification. It may be that the F4H shared enough similarity with the F3H that extensive testing was not thought as necessary. The resulting lower costs may have actually become part of the selling features to the customer.

However, in the case of the F4H, the application of more power and more engines may have caused a problem late in the design. It may have demanded a less than optimal solution: the anhedral tail. I'm not saying that occurred, only that it might have. It has happened on other designs. On the other hand, tunnel testing may have been so economical back then that it wasn't a factor.

These days, tunnel testing is very expensive. That's why CFD is more and more prevalent. It becomes a cost that must be managed (translated: reduced). I agree that fixing a problem discovered in flight testing is much more expensive - but it has happened. Sometimes the flight testing doesn't discover all of the problems, either.

I agree that you were blessed with the F-15. It is a great plane, and may be the best fighter ever built. Even though the F-22 is nearing full production, it may be eclipsed by UAV's well before the record of the F-15 thus far.